Chemistry of LipidsFOR

Fifth LevelBiology department

1435-1436H

Chemistry of LipidsChemistry of Lipids

Definition:

• - Lipids are organic compounds formed mainly from alcohol and fatty acids combined together by ester linkage.

• - Lipids are insoluble in water, but soluble in fat or organic solvents (ether, chloroform, benzene, acetone).

• - Lipids include fats, oils, waxes and related compounds.

• They are widely distributed in nature both in plants and in animals.

Biological Importance of Lipids:

1. They are more palatable and storable to unlimited amount compared to carbohydrates.

2. They have a high-energy value (25% of body needs) and they provide more energy per gram than carbohydrates and proteins but carbohydrates are the preferable source of energy.

3. Supply the essential fatty acids that cannot be synthesized by the body.

4. Supply the body with fat-soluble vitamins (A, D, E and K).5. They are important constituents of the nervous system.6. Tissue fat is an essential constituent of cell membrane and

nervous system. It is mainly phospholipids in nature that are not affected by starvation.

7-Stored lipids “depot fat” is stored in all human cells acts as:

• A store of energy.• A pad for the internal organs to protect them from

outside shocks.• A subcutaneous thermal insulator against loss of body

heat.8-Lipoproteins, which are complex of lipids and proteins,

are important cellular constituents that present both in the cellular and subcellular membranes.

9-Cholesterol enters in membrane structure and is used for synthesis of adrenal cortical hormones, vitamin D3 and bile acids.

10- Lipids provide bases for dealing with diseases such as obesity, atherosclerosis, lipid-storage diseases, essential fatty acid deficiency, respiratory distress syndrome,

Classification of Lipids

1. Simple lipids (Fats & Waxes)

2. Compound or conjugated lipids

3. Derived Lipids

4. Lipid-associating substances

Fatty alcoholsFatty alcohols

1-Glycerol:• It is a trihydric alcohol (i.e., containing three OH groups) and

has the popular name glycerin.

• It is synthesized in the body from glucose.

• It has the following properties:

1. Colorless viscous oily liquid with sweet taste.

2. On heating with sulfuric acid or KHSO4 (dehydration) it gives acrolein that has a bad odor. This reaction is used for detection of free glycerol or any compound containing glycerol.

CH 2 OH

CH

CH 2 OH

HO

CHO

CH

CH 2

2 H2O

Heating, KHSO4

Glycerol Acrolein

3-It combines with three molecules of nitric acid to form trinitroglycerin (TNT) that is used as explosive and vasodilator.

4-On esterification with fatty acids it gives:Monoglyceride or monoacyl-glycerol: one fatty acid +

glycerol.Diglyceride or diacyl-glycerol: two fatty acids + glycerol.

• Triglyceride or triacyl-glycerol: three fatty acids + glycerol.

5-It has a nutritive value by conversion into glucose and enters in structure of phospholipids.

Uses of GlycerolUses of Glycerol::

1. Glycerol enters in pharmaceutical and cosmetic preparations.

2. Reduces brain edema in cerebrovascular disease.

3. Nitroglycerin is used as vasodilator especially for the coronary arteries, thus it is used in treatment of angina pectoris. Also, enters in explosives manufacturing.

4. Glycerol is used in treatment of glaucoma (increased intraocular pressure)due to its ability to dehydrate the tissue from its water content.

2-Sphingosine:2-Sphingosine:

• - It is the alcohol(monohydric) present in sphingolipids.

• - It is synthesized in the body from serine and palmitic acid.

• It is not positive with acrolein test.

CH CH NH 2

CH 2OH

CHCH(CH 2)12CH 3

OH

Sphingosine

Fatty AcidsDefinitionDefinition: • Fatty acids are aliphatic mono-carboxylic acids that are

mostly obtained from the hydrolysis of natural fats and oils.

• Have the general formula R-(CH2)n-COOH and mostly have straight chain (a few exceptions have branched and heterocyclic chains). In this formula "n" is mostly an even number of carbon atoms (2-34) with a few exceptions that have an odd number.

• Fatty acids are classified according to several bases as follows:

I. According to presence or absence of double bonds they are classified into:

• A-Saturated Fatty Acids• they contain no double bonds with 2-24 or more

carbons.

• They are solid at room temperature except if they are short chained.

• They may be even or odd numbered.

• They have the following molecular formula, CnH2n+1COOH.

Saturated fatty acidsSaturated fatty acids (no double )

A-Short chain Saturated F.A. A-Short chain Saturated F.A. (2-10 carbon).(2-10 carbon).

a-Short chain Saturated volatile F.A.(2-6 carbon).

b- Short chain Saturated non volatile F.A.(7-10 carbon).

B-Long chain Saturated F.AB-Long chain Saturated F.A.(more the10 .(more the10 carbon)carbon)

a-Volatile short-chain fatty acids:

• They are liquid in nature and contain (They are liquid in nature and contain (1-6)1-6) carbon atoms.carbon atoms.

• water-soluble and volatile at room water-soluble and volatile at room temperature, e.g., acetic, butyric, and caproic temperature, e.g., acetic, butyric, and caproic acids.acids.

• Acetic F.A. (2C ) CHAcetic F.A. (2C ) CH33-COOH.-COOH.

• Butyric F.A. (4C ) CHButyric F.A. (4C ) CH33-(CH-(CH22))22-COOH.-COOH.

• Caproic F.A. (6C ) CHCaproic F.A. (6C ) CH33-(CH-(CH22))44-COOH.-COOH.

b-Non-volatile short-chain fatty acids:

• They are solids at room temperature and contain They are solids at room temperature and contain 7-107-10 carbon atoms. carbon atoms.

• They are water-soluble and non-volatile at room They are water-soluble and non-volatile at room temperature include caprylic and capric F.A.temperature include caprylic and capric F.A.

• caprylic (8 C ) CH3-(CH2)6-COOH.

• Capric (10 C ) CH3-(CH2)8-COOH.

B-Long-chain fatty acids:

• They contain more than 10 carbon atoms.

• They occur in hydrogenated oils, animal fats, butter and coconut and palm oils.

• They are non-volatile and water-insoluble

• Include palmitic, stearic, and lignoceric F.A.

• palmitic(16C) CH3-(CH2)14-COOH

• stearic (18 C ) CH3-(CH2)16-COOH

• lignoceric (24C ) CH3-(CH2)22-COOH

B-Unsaturated Fatty Acids

They contain double bond

• monounsaturated

they contain one double bonds .

(CnH2n-1 COOH)

• polyunsaturated

they contain more the one double bond

(CnH2n-more than 1 COOH).

1-Monounsaturated fatty acids:

1-Palmitoleic acid :

• It is found in all fats.

• It is C16:1∆9, i.e., has 16 carbons and one double bond located at carbon number 9 and involving carbon 10.

CHCH33-( CH-( CH22 ) )55CH = CH-(CHCH = CH-(CH22))7 7 –COOH–COOH

2-Oleic acid

• Is the most common fatty acid in natural fats. Is the most common fatty acid in natural fats.

• It is It is CC18:118:1∆9∆9, i.e., has 18 carbons and one double , i.e., has 18 carbons and one double

bond located at carbon number 9 and involving bond located at carbon number 9 and involving carbon 10.carbon 10.

CHCH33-(CH-(CH22))77- CH=CH – (CH- CH=CH – (CH22))77-COOH-COOH

3-Nervonic acid

(Unsaturated lignoceric acid).

• It is found in cerebrosides.

• It is C24:115, i.e., has 24 carbons and one double bond located at carbon number 15 and involving carbon 16.

CHCH33 – (CH – (CH22))77 CH= CH – (CH CH= CH – (CH22))1313- COOH- COOH

2-Polyunsaturated fatty acids :

(Essential fatty acids):

• Definition:

• They are essential fatty acids that can not be synthesized in the human body and must be taken in adequate amounts in the diet.

• They are required for normal growth and metabolism

• Source: vegetable oils such as corn oil, linseed oil, peanut oil, olive oil, cottonseed oil, soybean oil and many other plant oils, cod liver oil and animal fats.

• Deficiency: Their deficiency in the diet leads to nutrition deficiency disease.

• Its symptoms include: poor growth and health with susceptibility to infections, dermatitis, decreased capacity to reproduce, impaired transport of lipids, fatty liver, and lowered resistance to stress.

• Function of Essential Fatty Acids: 1. They are useful in the treatment of atherosclerosis by help

transporting blood cholesterol and lowering it and transporting triglycerides.

2. The hormones are synthesized from them.3. They enter in structure of all cellular and subcellular

membranes and the transporting plasma phospholipids.4. They are essential for skin integrity, normal growth and

reproduction.5. They have an important role in blood clotting (intrinsic

factor).6. Important in preventing and treating fatty liver.7. Important role in health of the retina and vision.8. They can be oxidized for energy production.

1-Linoleic:1-Linoleic:

• C18:29, 12.

• It is the most important since other essential fatty acids can be synthesized from it in the body.

CH3-(CH2)4-CH = CH-CH2-CH=CH-(CH2)7-COOH

2-Linolenic acid2-Linolenic acid:

• C18:39, 12, 15,

• in corn, linseed, peanut, olive, cottonseed and soybean oils.

CCHH33-CH-CH22-CH=CH-CH-CH=CH-CH22-CH=CH-CH-CH=CH-CH22-CH=CH-(CH-CH=CH-(CH22))77-COOH-COOH

3-Arachidonic acid:

• C20:45, 8, 11, 14.

• It is an important component of phospholipids in animal and in peanut oil from which prostaglandins are synthesized.

CHCH33-(CH-(CH22))44-CH=CH-CH-CH=CH-CH22-CH=CH-CH-CH=CH-CH22-CH=CH--CH=CH-

CHCH22-CH=CH-(CH-CH=CH-(CH22))33-COOH-COOH

1-Simple Lipids

A-Neutral Fats and oilsA-Neutral Fats and oils (Triglycerides) (Triglycerides)Definition:Definition:• - - They are called neutral because they are uncharged due

to absence of ionizable groups in it. • The neutral fats are the most abundant lipids in nature.

They constitute about 98% of the lipids of adipose tissue, 30% of plasma or liver lipids, less than 10% of erythrocyte lipids.

• They are esters of glycerol with various fatty acids. Since They are esters of glycerol with various fatty acids. Since the 3 hydroxyl groups of glycerol are esterified, the neutral the 3 hydroxyl groups of glycerol are esterified, the neutral fats are also called fats are also called “Triglycerides”.“Triglycerides”.

• Esterification of glycerol with one molecule of fatty acid Esterification of glycerol with one molecule of fatty acid gives gives monoglyceridemonoglyceride, and that with 2 molecules gives , and that with 2 molecules gives diglyceridediglyceride..

H2C O

C HO

H2C

C

C

O C

R1

R3

R2

O

O

O

+

3 H2O

CH 2 OH

C HHO

CH 2 OH

HO C R1

O

HO C R3

O

HO C R2

O

Fatty acids Glycerol Triglycerides(Triacylglycerol)

Types of triglyceridesTypes of triglycerides1-Simple triglycerides1-Simple triglycerides: : If the three fatty acids connected to glycerol are of the same If the three fatty acids connected to glycerol are of the same

type the triglyceride is called simple triglyceride, e.g., type the triglyceride is called simple triglyceride, e.g., tripalmitin.tripalmitin.

2-Mixed triglycerides2-Mixed triglycerides: : if they are of different types, it is called mixed if they are of different types, it is called mixed

triglycerides, e.g., stearo-diolein and palmito-oleo-triglycerides, e.g., stearo-diolein and palmito-oleo-stearin.stearin.

• Natural fats are mixtures of mixed triglycerides with a Natural fats are mixtures of mixed triglycerides with a small amount of simple triglycerides.small amount of simple triglycerides.

CH 2 O

C HO

CH 2

C

C

O C

(CH 2)14

O

O

O

Tripalmitin(simple triacylglycerol)

CH 3

(CH 2)14CH 3

(CH 2)14 CH 3

CH 2 O

C HO

CH 2

C

C

O C

(CH 2)16

O

O

O

1-Stearo-2,3-diolein(mixed triacylglycerol)

CH 3

(CH 2)7CHCH(CH 2)7CH 3

(CH 2)7 CH CH (CH 2)7 CH 3

CH 2 O

C HO

CH 2

C

C

O C

(CH 2)14

O

O

O

1-palmito-2-oleo-3-stearin(mixed triacylglycerol)

CH 3

(CH 2)16 CH 3

(CH 2)7CHCH(CH 2)7CH 3

• The commonest fatty acids in animal fats are The commonest fatty acids in animal fats are palmitic, palmitic, stearic and oleic acids. stearic and oleic acids.

• The main difference between fats and oils is for The main difference between fats and oils is for oils being oils being liquid liquid at room temperature, whereas, at room temperature, whereas, fats are solidsfats are solids..

• This is mainly due to presence of larger percentage of This is mainly due to presence of larger percentage of unsaturatedunsaturated fatty acids in oils than fats that has mostly fatty acids in oils than fats that has mostly saturatedsaturated fatty acids. fatty acids.

Physical properties of fat and oils:Physical properties of fat and oils:1. Freshly prepared fats and oils are colorless,

odorless and tasteless.Any color, or taste is due to association with other foreign substances, e.g., the yellow color of body fat or milk fat is due to carotene pigments(cow milk).

2. Fats have specific gravity less than 1 and, therefore, they float on water.

3. Fats are insoluble in water, but soluble in organic solvents as ether and benzene.

4. Melting points of fats are usually low, but higher than the solidification point,

Chemical Properties of fats and oilsChemical Properties of fats and oils:

1-Hydrolysis:1-Hydrolysis:

• They are hydrolyzed into their constituents (They are hydrolyzed into their constituents (fatty acids and fatty acids and glycerol)glycerol) by the action of super heated steam, acid, alkali or by the action of super heated steam, acid, alkali or enzyme (e.g., lipase of pancreas). enzyme (e.g., lipase of pancreas).

• - During their enzymatic and acid hydrolysis glycerol and - During their enzymatic and acid hydrolysis glycerol and free fatty acids are produced. free fatty acids are produced.

2-Saponification. Alkaline hydrolysis produces glycerol Alkaline hydrolysis produces glycerol and salts of fatty acids (and salts of fatty acids (soapssoaps).).

• Soaps cause emulsification of oily material this help easy Soaps cause emulsification of oily material this help easy washing of the fatty materialswashing of the fatty materials

CH 2 O

C HO

CH 2

C

C

O C

R1

R3

R2

O

O

OH2C OH

C HHO

H2C OH

ONaCR1

O

ONaCR3

O

+ ONaCR2

O

Triacylglycerol Glycerol Sodium salts of fatty acids (soap)

3 NaOH

3-Halogenation3-Halogenation

• Neutral fats containing unsaturated fatty acids have the Neutral fats containing unsaturated fatty acids have the ability of adding halogens (e.g., hydrogen or hydrogenation ability of adding halogens (e.g., hydrogen or hydrogenation and iodine or iodination) at the double bonds.and iodine or iodination) at the double bonds.

• - It is a very important property to determine the degree of - It is a very important property to determine the degree of unsaturation of the fat or oil that determines its biological unsaturation of the fat or oil that determines its biological value value

CH (CH 2)7 COO HCHCH2CH

Linoleic acidCH(CH 2)4CH3

2 I2

CH (CH 2)7 COO HCHCH2CH

Stearate-tetra-iodinate

CH(CH 2)4CH3

II I I

4-Hydrogenation or hardening of oils: • It is a type of addition reactions accepting hydrogen at the

double bonds of unsaturated fatty acids.• The hydrogenation is done under high pressure of

hydrogen and is catalyzed by finely divided nickel or copper and heat.

• It is the base of hardening of oils (margarine manufacturing), e.g., change of oleic acid of fats (liquid) into stearic acid (solid).

• It is advisable not to saturate all double bonds; otherwise It is advisable not to saturate all double bonds; otherwise margarine produced will be very hard, of very low margarine produced will be very hard, of very low biological value and difficult to digest.biological value and difficult to digest.

Oils(liquid)

(with unsaturated fatty acids, e.g., oleic)

Hard fat(margarine, solid)(with saturated

fatty acids, e.g., stearic)

Hydrogen, high pressure, nickel

Advantages for hydrogenatedAdvantages for hydrogenated oil or fat are as followsoil or fat are as follows:1.1. It is more pleasant as cooking fat.It is more pleasant as cooking fat.2.2. It is digestible and utilizable as normal animal fats and It is digestible and utilizable as normal animal fats and

oils.oils.3.3. It is less liable to cause gastric or intestinal irritation.It is less liable to cause gastric or intestinal irritation.4.4. It is easily stored and transported and less liable to It is easily stored and transported and less liable to

rancidity.rancidity. Disadvantages of hydrogenatedDisadvantages of hydrogenated • fats include lack of fat-soluble vitamins (A, D, E and K) fats include lack of fat-soluble vitamins (A, D, E and K)

and essential fatty acidsand essential fatty acids

5-Oxidation(Rancidty)5-Oxidation(Rancidty)

• This toxic reaction of triglycerides leads to unpleasant odour or taste of oils and fats developing after oxidation by oxygen of air, bacteria, or moisture.

• Also this is the base of the drying oils after exposure to atmospheric oxygen.

Example is linseed oil, which is used in paints and varnishes manufacturing

RancidityRancidityDefinition:Definition:• It is a physico-chemical change in the natural It is a physico-chemical change in the natural

properties of the fat leading to the development of properties of the fat leading to the development of unpleasant odor or taste or abnormal colorunpleasant odor or taste or abnormal color particularly on aging after exposure to particularly on aging after exposure to atmospheric oxygen, light, moisture, bacterial or atmospheric oxygen, light, moisture, bacterial or fungal contamination and/or heat. fungal contamination and/or heat.

• Saturated fats resist rancidity more than Saturated fats resist rancidity more than unsaturated fats that have unsaturated double unsaturated fats that have unsaturated double bonds.bonds.

Types and causes of RancidityTypes and causes of Rancidity:

1.1. Hydrolytic rancidityHydrolytic rancidity2.2. Oxidative rancidityOxidative rancidity3.3. Ketonic rancidityKetonic rancidity1-Hydrolytic rancidity1-Hydrolytic rancidity:• It results from slight hydrolysis of the fat by lipase It results from slight hydrolysis of the fat by lipase

from bacterial contamination leading to the liberation from bacterial contamination leading to the liberation of free fatty acids and glycerol at high temperature and of free fatty acids and glycerol at high temperature and moisture. moisture.

• Volatile short-chain fatty acids have unpleasant odor.Volatile short-chain fatty acids have unpleasant odor.

CH 2 O

C HO

CH 2

C

C

O C

R1

R3

R2

O

O

O

3 H2O

H2C OH

C HHO

H2C OH

OHCR1

O

OHCR3

O

+ OHCR2

OLipase

Triacylglycerol Glycerol Free fatty acids(volatile, bad odor)

2-Oxidative Rancidity2-Oxidative Rancidity: • It is oxidation of fat or oil catalyzed by exposure to It is oxidation of fat or oil catalyzed by exposure to

oxygen, light and/or heat producing peroxide oxygen, light and/or heat producing peroxide derivatives which on decomposition give derivatives which on decomposition give substances, e.g., substances, e.g., peroxides, aldehydes, ketones and peroxides, aldehydes, ketones and dicarboxylic acids that are toxic and have bad odor.dicarboxylic acids that are toxic and have bad odor.

• This occurs due to oxidative addition of oxygen at This occurs due to oxidative addition of oxygen at the unsaturated double bond of unsaturated fatty the unsaturated double bond of unsaturated fatty acid of oils.acid of oils.

Polyunsaturated fatty acid

Peroxyradical

Oxidant, O2

Hydroperoxide

Hydroxy fatty acid

Cyclic peroxide

Aldehydessuch as malondialdehyde

Other fragmentssuch as dicarboxylic acids

3-Ketonic Rancidity:

• It is due to the contamination with certain It is due to the contamination with certain fungi such as Asperigillus Niger on fats such fungi such as Asperigillus Niger on fats such as coconut oil.as coconut oil.

• Ketones, fatty aldehydes, short chain fatty Ketones, fatty aldehydes, short chain fatty acids and fatty alcohols are formed. acids and fatty alcohols are formed.

• Moisture accelerates ketonic rancidity.Moisture accelerates ketonic rancidity.

• Prevention of rancidity is achieved by:Prevention of rancidity is achieved by:1.1. Avoidance of the causes (Avoidance of the causes (exposure to light, oxygen, exposure to light, oxygen,

moisture, high temperature and bacteria or fungal moisture, high temperature and bacteria or fungal contaminationcontamination). By keeping fats or oils in well-). By keeping fats or oils in well-closed containers in cold, dark and dry place (i.e., closed containers in cold, dark and dry place (i.e., good storage conditions).good storage conditions).

2.2. Removal of catalysts such as lead and copper that Removal of catalysts such as lead and copper that catalyze rancidity.catalyze rancidity.

3.3. Addition of Addition of anti-oxidantsanti-oxidants to prevent peroxidation in to prevent peroxidation in fat (i.e., rancidity). They include phenols, naphthols, fat (i.e., rancidity). They include phenols, naphthols, tannins and hydroquinones. tannins and hydroquinones. The most common The most common natural antioxidant is vitamin E that is important natural antioxidant is vitamin E that is important in in vitrovitro and and in vivoin vivo..

Analysis and Identification of fats and oilsAnalysis and Identification of fats and oils

(Fat Constants(Fat Constants))

• Fat constants or numbers are tests used for:Fat constants or numbers are tests used for:

1.1. Checking the purity of fat for detection of adulteration.Checking the purity of fat for detection of adulteration.

2.2. To quantitatively estimate certain properties of fat.To quantitatively estimate certain properties of fat.

3.3. To identify the biological value and natural characteristics To identify the biological value and natural characteristics of fat.of fat.

4.4. Detection of fat rancidity and presence of toxic hydroxy Detection of fat rancidity and presence of toxic hydroxy fatty acidsfatty acids..

Hazards of Rancid Fats:

1.1. The products of rancidity are toxic, i.e., The products of rancidity are toxic, i.e., causes food poisoning and cancer.causes food poisoning and cancer.

2.2. Rancidity destroys the fat-soluble vitamins Rancidity destroys the fat-soluble vitamins (vitamins A, D, K and E).(vitamins A, D, K and E).

3.3. Rancidity destroys the polyunsaturated Rancidity destroys the polyunsaturated essential fatty acids.essential fatty acids.

4.4. Rancidity causes economical loss because Rancidity causes economical loss because rancid fat is inediblerancid fat is inedible..

1-Iodine number1-Iodine number (or value):• Definition: It is the number of grams of iodine

absorbed by 100 grams of fat or oil.• Uses: It is a measure for the degree of unsaturation of the

fat, as a natural property for it. • Unsaturated fatty acids absorb iodine at their double

bonds, therefore, as the degree of unsaturation increases iodine number and hence biological value of the fat increase.

• It is used for identification of the type of fat, detection of adulteration and determining the biological value of fat.

2-Saponification number (or value):• DefinitionDefinition: It is the number of It is the number of milligrams ofmilligrams of KOHKOH

required to completely saponify required to completely saponify one gramone gram of fat of fat.• UsesUses: • Since each carboxyl group of a fatty acid reacts with Since each carboxyl group of a fatty acid reacts with

one mole of KOH during saponification, therefore, one mole of KOH during saponification, therefore, the amount of alkali needed to saponify certain the amount of alkali needed to saponify certain weight of fat depends upon the number of fatty acids weight of fat depends upon the number of fatty acids present per weight.present per weight.

• Thus, fats containing short-chain acids will have Thus, fats containing short-chain acids will have more carboxyl groups per gram than long chain fatty more carboxyl groups per gram than long chain fatty acids and consume more alkali, i.e., will have higher acids and consume more alkali, i.e., will have higher saponification number.saponification number.

3-Acids Number3-Acids Number (or value):

• DefinitionDefinition: :

• It is the number of It is the number of milligrams of KOHmilligrams of KOH required to neutralize the free fatty acids required to neutralize the free fatty acids present in present in one gramone gram of fat. of fat.

• Uses:Uses:

• It is used for detection of hydrolytic rancidity It is used for detection of hydrolytic rancidity because it measures the amount of free fatty because it measures the amount of free fatty acids present.acids present.

4-Reichert- Meissl Number4-Reichert- Meissl Number (or value):

• DefinitionDefinition: : It is the number of It is the number of milliliters ofmilliliters of 0.1 N0.1 N KOHKOH required to neutralize the water-soluble fatty required to neutralize the water-soluble fatty acids distilled from acids distilled from 5 grams5 grams of fat. Short-chain fatty of fat. Short-chain fatty acid (less than 10 carbons) is distillated by steam.acid (less than 10 carbons) is distillated by steam.

• UsesUses: This studies the natural composition of the fat : This studies the natural composition of the fat and is used for detection of fat adulteration.and is used for detection of fat adulteration.

• Butter that has high percentage of short-chain fatty Butter that has high percentage of short-chain fatty acids has highest Reichert-Meissl number compared acids has highest Reichert-Meissl number compared to margarine. to margarine.

5-Acetyl Number5-Acetyl Number (or value):• DefinitionDefinition: It is number of It is number of milligrams ofmilligrams of KOH KOH

needed to neutralize the acetic acid liberated from needed to neutralize the acetic acid liberated from hydrolysis of hydrolysis of 1 gram of acetylated fat1 gram of acetylated fat (hydroxy fat (hydroxy fat reacted with acetic anhydridereacted with acetic anhydride).

• Uses:Uses: The natural or rancid fat that contains fatty The natural or rancid fat that contains fatty acids with free hydroxyl groups are converted into acids with free hydroxyl groups are converted into acetylated fat by reaction with acetic anhydride. acetylated fat by reaction with acetic anhydride.

• Thus, acetyl number is a measure of number of Thus, acetyl number is a measure of number of hydroxyl groups present. hydroxyl groups present.

• It is used for studying the natural properties of the fat It is used for studying the natural properties of the fat and to detect and to detect adulteration adulteration and rancidity.and rancidity.

B-Waxes• DefinitionDefinition: Waxes are solid simple lipids containing a Waxes are solid simple lipids containing a

monohydric alcohol (with a higher molecular weight than monohydric alcohol (with a higher molecular weight than glycerol) esterified to long-chain fatty acids. Examples of glycerol) esterified to long-chain fatty acids. Examples of these alcohols are these alcohols are palmitoyl alcohol, cholesterol, vitamin A or D.

• Properties of waxesProperties of waxes: Waxes are insoluble in water, but : Waxes are insoluble in water, but soluble in fat solvents and are negative for acrolein test. soluble in fat solvents and are negative for acrolein test.

• Waxes are not easily hydrolyzed as the fats and are Waxes are not easily hydrolyzed as the fats and are indigestible by lipases and are very resistant to rancidity. indigestible by lipases and are very resistant to rancidity.

• Thus they are of no nutritional value.Thus they are of no nutritional value.

Type of WaxesType of Waxes:

• - Waxes are widely distributed in nature such as the secretion of certain insects as bees-wax, protective coatings of the skins and furs of animals and leaves and fruits of plants. They are classified into true-waxes and wax-like compounds as follows:

A-True waxesA-True waxes: include:

• Bees-waxBees-wax is secreted by the honeybees that use it to form the combs. It is a mixture of waxes with the chief constituent is mericyl palmitate.

C15H31 C OHO

+C30H61OH C15H31 C OO

C30H61

H2OPalmiticacid

Mericylalcohol

Mericylpalmitate

B-Wax-like compounds: • Cholesterol estersCholesterol esters: Lanolin (or wool fat) is prepared Lanolin (or wool fat) is prepared

from the wool-associated skin glands and is secreted from the wool-associated skin glands and is secreted by sebaceous glands of the skin. by sebaceous glands of the skin.

• It is very complex mixture, contains both free and It is very complex mixture, contains both free and esterified cholesterol, e.g., cholesterol-palmitate and esterified cholesterol, e.g., cholesterol-palmitate and other sterolsother sterols.

Differences between neutral lipids and waxes:

Waxes Neutral lipids

1.Digestibility: Indigestible (not hydrolyzed by lipase).

Digestible (hydrolyzed by lipase).

2-Type of alcohol:

Long-chain monohydric Long-chain monohydric alcohol + one fatty acid.alcohol + one fatty acid.

Glycerol (trihydric) + 3 fatty acidsGlycerol (trihydric) + 3 fatty acids

3-Type of fatty acids:

Fatty acid mainly palmitic Fatty acid mainly palmitic or stearic acid.or stearic acid.

Long and short chain fatty acids.Long and short chain fatty acids.

4-Acrolein test: Negative.Negative. Positive.Positive.

5-Rancidability: Never get rancid.Never get rancid. Rancidible.Rancidible.

6-Nature at room temperature.

Hard solid.Hard solid. Soft solid or liquid.Soft solid or liquid.

7-Saponification Nonsaponifiable.Nonsaponifiable. Saponifiable.Saponifiable.

8-Nutritive value:

No nutritive value.No nutritive value. Nutritive.Nutritive.

9-Example: Bee & carnuba waxes.Bee & carnuba waxes. Butter and vegetable oils.Butter and vegetable oils.

2-Compound LipidsDefinition:• They are lipids that contain additional

substances, e.g., sulfur, phosphorus, amino group, carbohydrate, or proteins beside fatty acid and alcohol.

• Compound or conjugated lipids are classified into the following types according to the nature of the additional group:

1. Phospholipids2. Glycolipids.3. Lipoproteins4. Sulfolipids and amino lipids.

A-PhospholipidsA-PhospholipidsDefinition:: Phospholipids or phosphatides are compound Phospholipids or phosphatides are compound

lipids, which contain phosphoric acid group in their lipids, which contain phosphoric acid group in their structure. structure.

Importance: 1.1. They are present in large amounts in the liver and brain as They are present in large amounts in the liver and brain as

well as blood. Every animal and plant cell contains well as blood. Every animal and plant cell contains phospholipids. phospholipids.

2.2. The membranes bounding cells and subcellular organelles The membranes bounding cells and subcellular organelles are composed mainly of phospholipids. Thus, the transfer of are composed mainly of phospholipids. Thus, the transfer of substances through these membranes is controlled by substances through these membranes is controlled by properties of phospholipids.properties of phospholipids.

3.3. They are important components of the lipoprotein coat They are important components of the lipoprotein coat essential for secretion and transport of plasma lipoprotein essential for secretion and transport of plasma lipoprotein complexes. Thus, they are lipotropic agents that complexes. Thus, they are lipotropic agents that prevent fatty liver.

4.4. Myelin sheath of nerves is rich with phospholipids.Myelin sheath of nerves is rich with phospholipids.

5-Important in digestion and absorption of neutral lipids 5-Important in digestion and absorption of neutral lipids and excretion of cholesterol in the bile.and excretion of cholesterol in the bile.

6-Important function in blood clotting and platelet 6-Important function in blood clotting and platelet aggregation.aggregation.

7-They provide lung alveoli with 7-They provide lung alveoli with surfactants that surfactants that prevent its irreversible collapseprevent its irreversible collapse..

8-Important role in signal transduction across the cell 8-Important role in signal transduction across the cell membrane.membrane.

9-Phospholipase A2 in snake venom hydrolyses 9-Phospholipase A2 in snake venom hydrolyses membrane phospholipids into hemolytic membrane phospholipids into hemolytic lysolecithin or lysocephalin.lysolecithin or lysocephalin.

10-They are source of polyunsaturated fatty acids for 10-They are source of polyunsaturated fatty acids for synthesis of synthesis of eicosanoids.eicosanoids.

Sources: They are found in all cells (plant and They are found in all cells (plant and animal), milk and egg-yolk in the form of animal), milk and egg-yolk in the form of lecithins.lecithins.

Structure: phospholipids are composed of:phospholipids are composed of:

1.1. Fatty acidsFatty acids (a saturated and an unsaturated (a saturated and an unsaturated fatty acid).fatty acid).

2.2. Nitrogenous baseNitrogenous base (choline, serine, threonine, (choline, serine, threonine, or ethanolamine).or ethanolamine).

3.3. Phosphoric acid.Phosphoric acid.4.4. Fatty alcoholsFatty alcohols (glycerol, inositol or (glycerol, inositol or

sphingosine).sphingosine).

• Classification of Phospholipids are classified into 2 are classified into 2 groups according to the type of the groups according to the type of the alcohol present into present into two types:two types:

A-Glycerophospholipids: They are regarded as derivatives of phosphatidic acids that are the simplest type of phospholipids and include:

1. Phosphatidic acids.2. Lecithins3. Cephalins.4. Plasmalogens.5. Inositides.6. Cardiolipin.B-Sphingophospholipids: They contain sphingosine as an

alcohol and are named Sphingomyelins.

A-GlycerophospholipidsA-Glycerophospholipids 1-Phosphatidic acids:1-Phosphatidic acids:They are metabolic intermediates in synthesis They are metabolic intermediates in synthesis

of triglycerides and glycerophospholipids in the body and may of triglycerides and glycerophospholipids in the body and may have function as a have function as a second messengersecond messenger. They exist in two forms . They exist in two forms according to the position of the phosphateaccording to the position of the phosphate

CH2 O

C HO

CH2

C

C

O P

R1

R2

O

O

-Phosphatidic acid

OH

OH

O

Saturatedfatty acidPolyunsaturated

fatty acid

Phosphate

CH2 O

C H

CH2

C

O

R1

O

-Phosphatidic acid

Saturatedfatty acid

Polyunsaturatedfatty acid

Phosphate PHO

OH

O

C R2

O

O

2-Lecithins:2-Lecithins:• DefinitionDefinition: Lecithins are glycerophospholipids that contain Lecithins are glycerophospholipids that contain

choline as a base beside phosphatidic acid. They exist in 2 choline as a base beside phosphatidic acid. They exist in 2 forms forms - and - and -lecithins. Lecithins are a common cell -lecithins. Lecithins are a common cell constituent obtained from brain (constituent obtained from brain (-type), egg yolk (-type), egg yolk (-type), -type), or liver (both types). Lecithins are important in the or liver (both types). Lecithins are important in the metabolism of fat by the liver.metabolism of fat by the liver.

• Structure:Structure: Glycerol is connected at C2 or C3 with a Glycerol is connected at C2 or C3 with a polyunsaturated fatty acid, at C1 with a saturated fatty acid, polyunsaturated fatty acid, at C1 with a saturated fatty acid, at C3 or C2 by phosphate to which the choline base is at C3 or C2 by phosphate to which the choline base is connected. The common fatty acids in lecithins are stearic, connected. The common fatty acids in lecithins are stearic, palmitic, oleic, linoleic, linolenic, clupandonic or palmitic, oleic, linoleic, linolenic, clupandonic or arachidonic acids.arachidonic acids.

LysolecithinLysolecithin causes hemolysis of RBCs. This partially causes hemolysis of RBCs. This partially explains toxic the effect of snake venom,. The venom explains toxic the effect of snake venom,. The venom contains contains lecithinaselecithinase, which hydrolyzes the , which hydrolyzes the polyunsaturated fatty converting lecithin into polyunsaturated fatty converting lecithin into lysolecithin. Lysolecithins are intermediates in lysolecithin. Lysolecithins are intermediates in metabolism of phospholipidsmetabolism of phospholipids.

CH2 O

C HO

CH2

C

C

O P

R1

R2

O

O

-Lecithin

O

OH

O

CH2 O

C H

CH2

C

O

R1

O

-Lecithin

P

OH

O

C R2

O

CH2 CH2 N

CH3

CH3

CH3

+

OCH2CH2N

CH3

CH3

CH3

+

Choline

CholineO

• Lung surfactant • Is a complex of dipalmitoyl-lecithin, sphingomyelin and a

group of apoproteins called apoprotein A, B, C, and D. • It is produced by type II alveolar cells and is anchored to the

alveolar surface of type II and I cells. • It lowers alveolar surface tension and improves gas

exchange besides activating macrophages to kill pathogens. • In premature babies, this surfactant is deficient and they

suffer from respiratory distress syndrome. • Glucocorticoids increase the synthesis of the surfactant

complex and promote differentiation of lung cells.

3-Cephalins (or Kephalins):

• Definition: They are phosphatidyl-ethanolamine or serine. Cephalins occur in association with lecithins in tissues and are isolated from the brain (Kephale = head).

• Structure: Cephalins resemble lecithins in structure except that choline is replaced by ethanolamine, serine or threonine amino acids.

• Certain cephalins are constituents of the complex mixture of Certain cephalins are constituents of the complex mixture of phospholipids, cholesterol and fat that constitute the lipid phospholipids, cholesterol and fat that constitute the lipid component of the lipoprotein “component of the lipoprotein “thromboplastinthromboplastin” which ” which accelerates the clotting of blood by activation of prothrombin to accelerates the clotting of blood by activation of prothrombin to thrombin in presence of calcium ionsthrombin in presence of calcium ions.

CH 2 O

C HO

CH 2

C

C

O P

R1

R2

O

O

-Cephalin

O

OH

O

CH 2 CH 2 NH 2 Ethanolamine

HO CH 2 CH COO H Serine

NH 2

HO CH CH COO H Threonine

NH 2CH 3

4-Plasmalogens:• Definition: Plasmalogens are found in the cell

Plasmalogens resemble lecithins and cephalins in structure but differ in the presence of ,-unsaturated fatty alcohol rather than a fatty acid at C1 of the glycerol connected by ether bond.

• At C2 there is an unsaturated long-chain fatty acid, however, it may be a very short-chain fatty acid

• membrane phospholipids fraction of brain and muscle (10% of it is plasmalogens), liver, semen and eggs.

• Structure:

• Properties: Similar to lecithinsSimilar to lecithins.

CH 2

C HO

CH 2

C

O P

R2

O

-Plasmalogen

O

OH

O

CH 2 CH 2 N

CH 3

CH 3

CH 3

+

-Unsaturatedfatty alcoholCH CH R1O

5-Inositides5-Inositides:• DefinitionDefinition::

• - They are phosphatidyl inositol.They are phosphatidyl inositol.

• StructureStructure: They are similar to lecithins or cephalins but They are similar to lecithins or cephalins but they have the cyclic sugar alcohol, they have the cyclic sugar alcohol, inositol inositol as the base. as the base. They are formed of glycerol, one saturated fatty acid, one They are formed of glycerol, one saturated fatty acid, one unsaturated fatty acid, phosphoric acid and inositolunsaturated fatty acid, phosphoric acid and inositol

CH2

C HO

CH2

C

O P

R2

O

-Phosphatidylinositol

O

OH

O

C R1OO

H

H

OHOH

HOH

H

OHOH

H H1

2 3

4

56

• Source: Brain tissues.• Function: • Phosphatidyl inositol is a major component of cell

membrane phospholipids particularly at the inner leaflet of it.

• They play a major role as second messengers during signal transduction for certain hormone..

• On hydrolysis by phospholipase C, phosphatidyl-On hydrolysis by phospholipase C, phosphatidyl-inositol-4,5-diphosphate produces diacyl-glycerol and inositol-4,5-diphosphate produces diacyl-glycerol and inositol-triphosphate both act to liberate calcium from inositol-triphosphate both act to liberate calcium from its intracellular stores to mediate the hormone effects.its intracellular stores to mediate the hormone effects.

6-Cardiolipins:6-Cardiolipins:

• DefinitionDefinition: They are diphosphatidyl-glycerol. They are They are diphosphatidyl-glycerol. They are found in the inner membrane of mitochondria initially found in the inner membrane of mitochondria initially isolated from heart muscle (cardio). It is formed of 3 isolated from heart muscle (cardio). It is formed of 3 molecules of glycerol, 4 fatty acids and 2 phosphate molecules of glycerol, 4 fatty acids and 2 phosphate

groupsgroups..• FunctionFunction: : Used in serological diagnosis of autoimmunity Used in serological diagnosis of autoimmunity

diseases.diseases.

CH2

C HO

CH2

C

O P

R2

O

Cardiolipin

O

OH

O

C R1OO

CH2

CH OH

CH2 CH2

CH O

CH2

C

OP

R3

O

O

OH

O

CR4 O

O

B-SphingophospholipidsB-Sphingophospholipids1-Sphingomyelins1-Sphingomyelins• Definition:Definition: Sphingomyelins are found in large amounts in Sphingomyelins are found in large amounts in

brain and nerves and in smaller amounts in lung, spleen, brain and nerves and in smaller amounts in lung, spleen, kidney, liver and bloodkidney, liver and blood.

• Structure:Structure: Sphingomyelins differ from lecithins and Sphingomyelins differ from lecithins and cephalins in that they contain sphingosine as the alcohol cephalins in that they contain sphingosine as the alcohol instead of glycerol, they contain two nitrogenous bases: instead of glycerol, they contain two nitrogenous bases: sphingosine itself and choline. sphingosine itself and choline.

• Thus, sphingomyelins contain sphingosine base, one long-Thus, sphingomyelins contain sphingosine base, one long-chain fatty acid, choline and phosphoric acid. chain fatty acid, choline and phosphoric acid.

• To the amino group of sphingosine the fatty acid is To the amino group of sphingosine the fatty acid is attached by an amide linkage.attached by an amide linkage.

• Ceramide This part of sphingomyelin in which the This part of sphingomyelin in which the amino group of sphingosine is attached to the fatty acid by amino group of sphingosine is attached to the fatty acid by an amide linkage. an amide linkage.

• Ceramides have been found in the free state in the spleen, Ceramides have been found in the free state in the spleen, liver and red cells. liver and red cells.

CH CH NH

CH2

CHCH(CH2)12CH3

OH

Sphingosine

C R1

O

O

P O

OH

O CH2 CH2 N

CH3

CH3

CH3

+

Choline

Fatty acid

Phosphate

Ceramide

Sphingomyelin

B-GlycolipidsB-Glycolipids• DefinitionDefinition: They are lipids that contain carbohydrate They are lipids that contain carbohydrate

residues with sphingosine as the alcohol and a very long-residues with sphingosine as the alcohol and a very long-chain fatty acid (24 carbon series). chain fatty acid (24 carbon series).

• They are present in cerebral tissue, therefore are called They are present in cerebral tissue, therefore are called

cerebrosidescerebrosides • ClassificationClassification: According to the number and nature of According to the number and nature of

the carbohydrate residue(s) present in the glycolipids the the carbohydrate residue(s) present in the glycolipids the following arefollowing are

1. Cerebrosides. They have one galactose molecule They have one galactose molecule (galactosides).(galactosides).

2. Sulfatides. They are cerebrosides with sulfate on the They are cerebrosides with sulfate on the sugar (sulfated cerebrosides).sugar (sulfated cerebrosides).

3. Gangliosides. They have several sugar and sugaramine They have several sugar and sugaramine residues.residues.

1-Cerebrosides:1-Cerebrosides:• Occurrence: They occur in myelin sheath of nerves and white matter of

the brain tissues and cellular membranes. They are important for nerve conductance.

• Structure: They contain sugar, usually -galactose and may be glucose or

lactose, sphingosine and fatty acid, but no phosphoric acid.

CH CH NH

CH 2

CHCH(CH 2)12CH 3

OH

Sphingosine

C R1

O

O

Psychosin

Fatty acid

Ceramide

Cerebroside

OOH

H HH

OHH

OH

CH 2OH

HGalactose

• TypesTypes:: According to the type of fatty acid and carbohydrate present, there are 4 different types of cerebrosides isolated from the white matter of cerebrum and in myelin sheaths of nerves. Rabbit cerebrosides contain stearic acid.

1.1. KerasinKerasin contains lignoceric acid (24 carbons) and galactose.

2.2. Cerebron (Phrenosin)Cerebron (Phrenosin) contains cerebronic acid (2- contains cerebronic acid (2-hydroxylignoceric hydroxylignoceric acid) and galactose.

3.3. NervonNervon contains nervonic acid (unsaturated lignoceric acid at C15) and galactose.

4.4. OxynervonOxynervon contains oxynervonic acid (2-hydroxynervonic acid) and galactose.

2-2-SulfatidesSulfatides:• They are sulfate esters of kerasin or phrenosin in which the They are sulfate esters of kerasin or phrenosin in which the

sulfate group is usually attached to the –OH group of C3 or C6 sulfate group is usually attached to the –OH group of C3 or C6 of galactose. Sulfatides are usually present in the brain, liver, of galactose. Sulfatides are usually present in the brain, liver, muscles and testes.muscles and testes.

CH CH NH

CH 2

CHCH 2(CH 2)12CH 3

OH

C R1

O

O

Sulfatides (sulfated cerebroside)

OOH

H HH

OHH

OSO3H

CH 2OH

H

3-3-GangliosidesGangliosides:: • They are more complex glycolipids that occur in the gray They are more complex glycolipids that occur in the gray

matter of the brain, ganglion cells, and RBCs. They transfer matter of the brain, ganglion cells, and RBCs. They transfer biogenic amines across the cell membrane and act as a cell biogenic amines across the cell membrane and act as a cell membrane receptor. membrane receptor.

• GangliosidesGangliosides contain contain sialic acid (N-acetylneuraminicsialic acid (N-acetylneuraminic acid),acid), ceramide (sphingosine + fatty acid of 18-24 carbon atom ceramide (sphingosine + fatty acid of 18-24 carbon atom length), 3 molecules of hexoses (1 glucose + 2 galactose) and length), 3 molecules of hexoses (1 glucose + 2 galactose) and hexosamine. The most simple type of it the hexosamine. The most simple type of it the monosialoganglioside,. It works as a receptor for cholera monosialoganglioside,. It works as a receptor for cholera toxin in the human intestine.toxin in the human intestine.

Ceramide-Glucose-Galactose-N-acetylgalactosamine-Galactose

Monosialoganglioside

Sialic acid

• Plasma lipoproteins can be separated by two methodsPlasma lipoproteins can be separated by two methods::1.1. Ultra-centrifugationUltra-centrifugation: Using the rate of floatation in sodium : Using the rate of floatation in sodium

chloride solution leading to their sequential separation into chloride solution leading to their sequential separation into chylomicronschylomicrons, very low density lipoproteins (, very low density lipoproteins (VLDL or pre-VLDL or pre---lipoproteinslipoproteins), low density lipoproteins (), low density lipoproteins (LDL or LDL or -lipoproteins-lipoproteins), ), high density lipoproteins (high density lipoproteins (HDL or HDL or -lipoproteins-lipoproteins) and ) and albumin-albumin-free fattyfree fatty acids complex. acids complex.

2.2. ElectrophoresisElectrophoresis:: is the migration of charged particles in an electric is the migration of charged particles in an electric field either to the anode or to the cathode. It sequentially separates field either to the anode or to the cathode. It sequentially separates the lipoproteins into the lipoproteins into chylomicronschylomicrons, , pre-pre--, -, -, and -, and --lipoprotein lipoprotein and and albumin-free fattyalbumin-free fatty acids complex. acids complex.

Polar lipids(phospholipids)

Nonpolar lipids(cholesterol and its esters

and triacylglycerols)Structure of a plasma lipoprotein complex

Polar apolipoproteins

C-LipoproteinsC-Lipoproteins• DefinitionDefinition: Lipoproteins are lipids combined with proteins in Lipoproteins are lipids combined with proteins in

the tissues. The lipid component is phospholipid, cholesterol the tissues. The lipid component is phospholipid, cholesterol or triglycerides. The holding bonds are secondary bonds.or triglycerides. The holding bonds are secondary bonds.

• They include:They include:1.1. Structural lipoproteinsStructural lipoproteins: These are widely distributed in : These are widely distributed in

tissues being present in cellular and subcellular membranes. In tissues being present in cellular and subcellular membranes. In lung tissues acting as a surfactant in a complex of a protein lung tissues acting as a surfactant in a complex of a protein and lecithin. In the eye, rhodopsin of rods is a lipoprotein and lecithin. In the eye, rhodopsin of rods is a lipoprotein complex.complex.

• Transport lipoproteinsTransport lipoproteins: : • These are the forms present in blood plasma. They are These are the forms present in blood plasma. They are

composed of a protein called composed of a protein called apolipoprotein apolipoprotein and different and different types of lipids. (Cholesterol, cholesterol esters, phospholipids types of lipids. (Cholesterol, cholesterol esters, phospholipids and triglycerides). As the lipid content increases, the density and triglycerides). As the lipid content increases, the density of plasma lipoproteins decreasesof plasma lipoproteins decreases

a) Chylomicronsa) Chylomicrons: They have the largest diameter and the They have the largest diameter and the least density. They contain least density. They contain 1-2% protein1-2% protein only and only and 98-99% 98-99% fatfat. The main lipid fraction is triglycerides absorbed from . The main lipid fraction is triglycerides absorbed from the intestine and they contain the intestine and they contain small amountssmall amounts of the of the absorbed cholesterol and phospholipids.absorbed cholesterol and phospholipids.

b) Very low-density lipoproteins (VLDL) or pre-b) Very low-density lipoproteins (VLDL) or pre---lipoproteinslipoproteins: Their diameter is smaller than Their diameter is smaller than chylomicrons. They contain about chylomicrons. They contain about 7-10% protein7-10% protein and and 90-90-93% lipid93% lipid. The lipid content is mainly triglycerides formed . The lipid content is mainly triglycerides formed in the liver. They contain phospholipid and cholesterol in the liver. They contain phospholipid and cholesterol more thanmore than chylomicrons. chylomicrons.

c) Low-density lipoproteins (LDL) or ) Low-density lipoproteins (LDL) or -lipoproteins-lipoproteins: They contain They contain 10-20% proteins10-20% proteins in the form of in the form of apolipoprotein B. Their apolipoprotein B. Their lipid content varies from 80-90%.lipid content varies from 80-90%. They contain about 60% of total blood cholesterol and They contain about 60% of total blood cholesterol and 40% of total blood phospholipids. As their percentage 40% of total blood phospholipids. As their percentage increases, the liability to atherosclerosis increases.increases, the liability to atherosclerosis increases.

d) High-density lipoproteins (HDL) or d) High-density lipoproteins (HDL) or --LipoproteinsLipoproteins: They contain They contain 35-55% proteins35-55% proteins in the in the form of apolipoprotein A. They contain form of apolipoprotein A. They contain 45-65% 45-65% lipidslipids formed of cholesterol ( formed of cholesterol (40% of40% of total blood total blood contentcontent) and phospholipids () and phospholipids (60%60% of total blood of total blood contentcontent). They act as cholesterol ). They act as cholesterol scavengersscavengers, as , as their percentage increases, the liability to their percentage increases, the liability to atherosclerosis decreases. They are higher in atherosclerosis decreases. They are higher in females than in males. Due to their high protein females than in males. Due to their high protein content they possess the highest density.content they possess the highest density.

e) Albumin-free fatty acids complex:e) Albumin-free fatty acids complex: It is a It is a proteolipid complex with proteolipid complex with 99% protein99% protein content content associated with long-chain free fatty acids for associated with long-chain free fatty acids for transporting them.transporting them.

Cholesterol:Cholesterol:• Importance:Importance: - - • acetyl-CoA (1gm/day, cholesterol does not exist in plants) and acetyl-CoA (1gm/day, cholesterol does not exist in plants) and

is also taken in the diet (is also taken in the diet (0.3 gm/day as in, butter, milk, egg0.3 gm/day as in, butter, milk, eggIt is It is the most important sterol in animal tissues as the most important sterol in animal tissues as free alcoholfree alcohol or in or in an esterified form (an esterified form (with linoleicwith linoleic, , oleic, palmitic acids or other oleic, palmitic acids or other fatty acidsfatty acids).).

• Steroid hormones, bile salts and vitamin D are derivatives Steroid hormones, bile salts and vitamin D are derivatives from it. from it.

• Tissues contain different amounts of it that serve a structural Tissues contain different amounts of it that serve a structural and metabolic role, e.g., and metabolic role, e.g., adrenal cortexadrenal cortex content is 10%,content is 10%, whereas, whereas, brain is 2%,brain is 2%, others 0.2-0.3%. others 0.2-0.3%.

• SourceSource:: - It is synthesized in the body from It is synthesized in the body from yolk, brain, meat yolk, brain, meat and animal fat).and animal fat).

Physical propetiesPhysical propeties::It has a hydroxyl group on C3, a double bond between C5 and C6, 8 asymmetric carbon atoms and a side chain of 8 carbon atoms.

• It is found in all animal cells, corpus luteum and adrenal cortex, human brain (17% of the solids).

• In the blood (the total cholesterol amounts about 200 mg/dL of which 2/3 is esterified, chiefly to unsaturated fatty acids while the remainder occurs as the free cholesterol.

CH 3

CH 3

HO

CH 3

CH 3

CH 3

Cholesterol

• Chemical propertiesChemical properties Intestinal bacteria reduce Intestinal bacteria reduce cholesterol into cholesterol into coprosterol and dihydrocholesterolcoprosterol and dihydrocholesterol. .

• - It is also oxidized into- It is also oxidized into 7-Dehydrocholesterol 7-Dehydrocholesterol and further and further unsaturated cholesterol with a second double bond between unsaturated cholesterol with a second double bond between C7 and C8.C7 and C8. When the skin is irradiated with ultraviolet light When the skin is irradiated with ultraviolet light 7-dehydrocholesterol is converted to vitamin 7-dehydrocholesterol is converted to vitamin D3.D3. This This explains the value of sun light in preventing explains the value of sun light in preventing ricketsrickets..

CH 3

CH 3

HO

CH 3

CH 3

CH 3

Coprosterol,in feces

H

CH 3

CH 3

HO

CH 3

CH 3

CH 3

Dihydrocholesterol,in blood and other tissues

H

• ErgosterolErgosterol differs from 7-dehydrocholesterol in the differs from 7-dehydrocholesterol in the side chain. Ergosterol is converted to vitamin D2 by side chain. Ergosterol is converted to vitamin D2 by irradiation with UV Ergosterol and 7- dehydrocholesterol irradiation with UV Ergosterol and 7- dehydrocholesterol are called Pro-vitamins D or precursors of vitamin D. are called Pro-vitamins D or precursors of vitamin D.

• - It was first isolated from ergot, a fungus then from yeast. - It was first isolated from ergot, a fungus then from yeast. Ergosterol is less stable than cholesterol (Ergosterol is less stable than cholesterol (because of having because of having 3 double bonds3 double bonds).).

CH 3

CH 3

HO

CH 3

CH 3

CH 3

7-dehydrocholesterol

CH 3

CH 3

HO

CH 3CH 3

CH 3

Ergosterol

CH 3

SteroidsSteroids• Steroids constitute an important class of biological Steroids constitute an important class of biological

compounds.compounds.• Steroids are usually found in association with fat. Steroids are usually found in association with fat.

They can be separated from fats after They can be separated from fats after saponification since they occur in the saponification since they occur in the unsaponifiable residue.unsaponifiable residue.

• They are They are derivatives of cholesterolderivatives of cholesterol that is formed that is formed of steroid ring or nucleus.of steroid ring or nucleus.

• Biologically important groups of substances, which contain Biologically important groups of substances, which contain this ring, are:this ring, are:

1.1. Sterols.Sterols.2.2. Adrenal cortical hormones.Adrenal cortical hormones.3.3. Male and female sex hormones.Male and female sex hormones.4.4. Vitamin D group.Vitamin D group.5.5. Bile acids.Bile acids.6.6. Cardiac glycosides.Cardiac glycosides.

• General consideration about naturally occurring steroids: A typical member of this group is cholesterol. Certain facts have to be

considered when drawing steroid formula:1) There is always oxygen in the form of hydroxyl or ketone on C3.2) Rings C and D are saturated (stable).3) Methyl groups at C18 C19. In case of vitamin D, the CH3 group at

C19 becomes a methylene group (=CH2) and the ring B is opened, whereas, this methyl group is absent in female sex hormones (estrogens).

4) In estrogens (female sex hormones) ring A is aromatic and there is no methyl group on C10.

CH 3

CH 3

HO

Steroid ring

12

3 45

6 7

89

10

11

12

13

14 15

1617

18

19

A B

C D

• Bile acids:

• They are produced from oxidation of cholesterol in the liver producing cholic and chenodeoxycholic acids that are conjugated with glycine or taurine to produce glycocholic, glycochenodeoxycholic, taurocholic and taurochenodeoxycholic acids. They react with sodium or potassium to produce sodium or potassium bile salts.

• Their function is as follows:1. Emulsification of lipids during digestion.2. Help in digestion of the other foodstuffs.3. Activation of pancreatic lipase.4. Help digestion and absorption of fat-soluble vitamins.5. Solubilizing cholesterol in bile and prevent gall stone

formation.6. Choleretic action (stimulate their own secretion).7. Intestinal antiseptic that prevent putrefaction

CH 3

CH 3

HO

CH 3

C

Sodium-tauro orglyco-cholate

CH 3

CH 3

HO

CH 3

C

Sodium-tauro orglyco-chenodeoxycholate

OH

OH OH

O O

R1 or R2 R1 or R2

(CH 2)2 SO3-Na+H2NCH 2 COO -Na+H2N

Sodium taurateSodium glycateR1 R2