Lecture 27 & 28_Digestion of Lipids and Nutritional Importance of Lipids

8/2/2019 Lecture 27 & 28_Digestion of Lipids and Nutritional Importance of Lipids

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Digestion of lipids and nutritionalimportance of lipids I & II

Lecture 27 & 28

Dr. S. Annie JeyachristyLecturer in Biochemistry

Faculty of MedicineAIMST University


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Lecture 27to discuss digestive enzymes, their action on dietary

lipids, absorption of lipids and disorders of digestion

and absorption.

Lecture 28

to discuss nutritional role of lipids, naturally occurringfatty acids and the relative proportion of fatty acids in

lipids.

OBJECTIVES


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LEARNING OUTCOMESLecture 27

Describe the digestion, absorption, secretion and utilization of dietary

lipids.

Describe the role of enzymes in the digestion of lipids in mouth, stomach

and intestine.

Describe the causes, symptoms and treatment of steatorrhea.

Explain the general malabsorption disorders associated with lipids.

Lecture 28

Provide the sources of lipids and the relative proportion of fatty acids in

lipids.

Name the naturally-occurring fatty acids.

Give the values and reasons for the recommended intake of fats.


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LIPIDSHeterogeneous group of water-insoluble (hydrophobic)

organic molecules

Can be extracted from tissues by nonpolar solvents

Generally found compartmentalized because of its

insoluble nature. Eg. membrane associated lipids or

droplets of triacylglycerol in adipocytes, protein associated

lipids as lipoproteins for transport of lipids.


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CLASSIFICATION of LIPIDSLIPIDS

SIMPLE LIPIDS COMPOUND LIPIDS DERIVED LIPIDS

FATS & OIL

WAXES

PHOSPHOLIPIDS

GLYCOLIPIDS

LECITHIN

CEPHALIN

PLASMALOGEN

SPHINGOMYELIN

CEREBROSIDES

GANGLIOSIDES

ALCOHOL

FATTY ACIDS

STEROLS

ISOPRENOIDS

TERPENOIDS

CAROTENOIDS

SULFOLIPIDS

SIMPLE

MIXED


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CLASSIFICATION of FATTY ACIDSFatty acids

SATURATED

UNSATURATED

MONOUNSATURATED

POLYUNSATURATED

PROSTAGLANDINS

THROMBOXANES

STRAIGHT CHAIN CYCLIC

OMEGA 3

OMEGA 6

Cis-

Trans-


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Lipids/ Fat

In food In body

Energy EssentialNutrient

Flavor AdiposeTissue CellMembrane


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Functions

Major source of energy for the body

Also provides hydrophobic barrier that permits

partitioning of the aqueous contents of cells and

subcellular structures.

Fat-soluble vitamins have regulatory or coenzyme

functions.

Prostaglandins and steroid hormones plays role in

maintaining body homeostasis.


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Food Sources of Fat

Animal Fats

Meat Fat (bacon, sausage.)

Dairy Fats and products (cream, butter, cheese..)

Egg yolk

Plant Fats

Monounsaturated, polyunsaturated Fatty acidVegetable oil (safflower, corn, soybean, cottonseed,

olive oil)


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Characteristics Of Food Fat Sources

Visible fat

Butter, margarin, salad oils and dressing,shirteninig

fat meat

Invisible fat

Cheese, cream portion of homogenised milk, egg

yolk, nuts, seeds, olives..


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DIGESTION, ABSORPTION, SECRETION AND UTILIZATION

OF DIETARY LIPIDS

Ingestion of lipids by adults60-80g per day

More than 90% is normally triacylglycerolRemaining consists of cholesterol, cholesteryl esters,

phospholipids, unesterified (free) fatty acids.

Digestion of lipids begins in stomach.


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Digestion of lipids in stomach

Lingual lipase (originates from the glands at the back of the tongue)

Gastric lipase (secreted by gastric mucosa)

Both are acid stable lipases, with pH optimum of pH4 to pH8

Act on triacylglycerols with short and medium-length fatty acids (


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Digestion of lipids in small intestine

Emulsification of lipids

Emulsification increases the surface area of the hydrophobic lipid droplets sothat the digestive enzymes, can work at the interface of the droplet and the

surrounding aqueous solution can act effectively.

Bile saltsderivative of cholesterol, made in liver, stored in gall bladder,

association with glycine or taurine

Mechanical mixing due to peristalsis


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Bile Acids

1. The end products of cholesterol utilization are the bile acids, synthesized in theliver.

2. Primary bile acids in human bile are

1. chenodeoxycholic acid (45%) and

2. cholic acid (31%).

3. Secondary bile acids,

1. deoxycholate (from cholate) and

2. lithocholate (from chenodeoxycholate).

4. Conjugation with glycine or taurine before their secretion into the bile yield

1. Glycocholic acid

2. Taurocholic acid


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Micelles: tiny emulsified fat packets that can enter intestinal cells (enterocytes)


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Lipid (fats and oils) is insoluble in water (hydrophobic).

Lipids tend to coalesce into larger droplets which reduces the surface

area for digestion.

The hydrophobic lipid is only accessible to the water soluble lipases

at the interface between lipid and water.

To increase the access (increased surface area) and rate of lipid

digestion the lipid droplet must be broken up.

Bile salts secreted from the liver (via gallbladder) have moleculeswith a combination of hydrophobic and (lipophilic) hydrophilic

regions.

Bile salts break up the lipid droplet into many smaller droplets

thereby increasing the surface area of lipid-water access.


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Enzymatic degradation of lipids by pancreatic enzymes

Triacylglycerol degradation

Pancreatic lipase

Triacylglycerol molecules are too large

Cannot be taken up efficiently by the mucosal cells of the intestinal villi

Removes fatty acid at C1 and C3.

Products are 2 monacylglycerol and free fatty acids

Triglycerides 2 fatty acids + Mono glycerides

Colipase

Also secreted by pancreas and binds pancreatic lipase at a ratio of one to

one.

Anchors at lipid aqueous interface.

Procolipase

colipase


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Cholesteryl ester degradation

Mostly in free form ; 10-15% in esterified form

Cholesteryl esters are hydrolysed by pancreatic cholesterol ester

hydrolase (cholesterol esterase)

Products are cholesterol and free fatty acids

Cholesteryl esterase activity is greatly increased in the presence of bile

salts

Cholesteryl esters

fatty acids + cholesterol


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Phospholipid degradation

Phospholipids are hydrolysed by phospholipase A2.Pancreatic juice is rich in phospholipase A2; activated by trypsin;

requires bile salts for optimum activity

Products are lysophospholipid and free fatty acid

Lysophospholipase removes another fatty acid and the product is

glycerylphosphoryl base.

Glycerylphosphoryl base may be excreted in feces, further degraded or

absorbed.

Phosphatidyl choline fatty acid +lysophosphatidyl choline

Lysophosphatidylcholine

fatty acid +Glycerylphosphoryl choline


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Regulation of lipid digestion - Hormonally controlled

CholecystokininSecretin

Cholecystokinin (formerly called as pancreozymin)

Small peptide hormone

Produced by mucosal cells of jejunum and lower duodenum

Secreted in response to presence of lipids and partially digested proteins

entering upper small intestine

Mode of actionActs on gall bladder causing contraction and release of bile

Acts on exocrine cells of pancreas and aids release of digestive enzymes

Decreases gastric motility resulting in slower release of gastric contents

into the small intestine.


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SecretinSmall peptide hormone

Produced by intestinal cells

Secreted in response to low pH of the chyme entering the intestine.

Mode of action

Acts on pancreas and liver resulting in release of watery solution rich in

bicarbonate that helps in neutralization of pH of the intestinal contents.

Maintains the appropriate pH for enzymatic digestion by pancreaticenzymes.

Chymesemifluid mass of partially digested food that passes from the stomach to the duodenum


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Absorption of lipids by intestinal mucosal cells

Free fatty acids

Free cholesterol

2-monoacyl glycerol

Bile salts Mixed micelles

Brush border membrane

of intestinal mucosal cells

Together

with

Primary products

of lipid digestion

Disc shaped clusters of

amphipathic lipids(hydrophilic groups

outside and hydorphobic

groups inside)

This membrane is separated from the liquid contents of the intestinal lumen by an

unstirred water layer that is partially miscible with the bulky fluid.

Hydrophilic surface of micelles facilitates transport of hydrophobic lipids through

unstirred water layer for absorption


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Resynthesis of triacylglycerols and cholesteryl esters

Mixture of lipids

(absorbed)For biosynthesis of complex lipids

Migrates to

ER

In enterocytes

Fatty acids Fatty acyl CoA derivatives

Thiokinase (Fatty acylCoA synthetase)

Activation

Conversion

Monoacylglycerol TriacylglycerolMonoacylglyceol acyl transferase

Diacylglycerol acyl transferase

Lysophospholipids PhospholipidsAcyl transferases

Cholesterol Cholesteryl esterAcyl CoA cholesterol acyl

transferase


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Long- chain fatty acids

Resynthesis in the similar fashion

Small- and Medium- chain fatty acids

Need not form mixed micelles for absorption by the intestinal mucosa.

(dietary therapy for individuals with malabsorption of other lipids)

Not converted to CoA derivatives and not reesterified

They are released into portal circulation and are carried by serum albumin to

liver.

Cholesterol and other sterols are poorly absorbed. Overall, about 50% of dietarycholesterol is absorbed.

Dietary fat increases cholesterol absorption

Fiber (especially soluble fiber) and phytosterols decrease cholesterol absorption


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Secretion of lipids from enterocytes

Newly synthesized triacylglycerols and cholesteryl esters are hydrophobic and

aggregate in aqueous environment.

Hence, they are packaged as lipid droplets surrounded by thin layer of

phospholipids, unesterified cholesterol and protein molecule (apolipoprotein B48)

- chylomicrons

This thin layer stabilizes the lipid particle and increases solubility preventing

coalescing of multiple particles.Chylomicrons are released by exocytosis from enterocytes into lacteals (lymphatic

vessels villi of small intestine)

Enter thoracic duct and then to the left subciavian vein and finally enter into blood


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triglycerides


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Utilization of lipids by tissues

Triacylglycerol in chylomicrons are broken down in the capillaries of skeletal

muscle , adipose tissues, heart, lung, kidney and liver.

Triacylglycerol is degraded by to free fatty acids and glycerol by lipoprotein

lipase.

Lipoprotein lipase is synthesized primarily by adipocytes and muscle cells and is

secreted and associated with the luminal surface of the endothelial cells of the

capillary beds of the peripheral tissues

Deficiency of lipoprotein lipase or its coenzyme apo C-II leads to rare, autosomal

recessive disorderFamilial lipoprotein lipase deficiency (Type I

hyperlipoproteinemia) resulting in massive chylomicronemia


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Fate of free fatty acids

From the hydrolysis of triacylglycerols directly enters mucosal cells or

adipocytes.

Or transported in the blood with serum albumin until taken up by cells.

Most cells oxidize fatty acids for energy

Adipocytes reesterifies to produce triacylglycerol for storage purpose.

Fate of glycerol

Released from triacylglycerol is almost used by liver to produce glycerol-3-

phosphate which enters glycolysis or gluconeogenesis (oxidation of

dihydroxyacetone phosphate)

Fate of chylomicron components

Chylomicron remnants bind to receptors in liver and then endocytosed.

Defect in chylomicron remnant removal by liver resulting in accumulation of

chylomicron remnants in plasma leads to Familial dysbetalipoproteinemia (Type

III hyperlipoproteinemia)


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Source Destination Major lipids Functions

Chylomicrons Intestine Many organs Triglycerides, other

lipids

Deliver lipids of

dietary origin to

body cell.

VLDLs Liver Many organs Triglycerides,

Cholesterol

Deliver

endogenously

produced

triglycerides to

body cells.

LDLs Intraviscular

removal of

triglycerides from

VLDL

Blood vessels, Liver Cholesterol Deliver

endogenously

produced

cholesterol to

various organs.

HDLs Liver and intestine Liver and steroid-

hormone-producing

glands

Cholesterol Remove and

degrade

Cholesterol.

Lipids are transported in the plasma as lipoproteins

Transportation of lipids

Documents

Lecture 27 & 28_Digestion of Lipids and Nutritional Importance of Lipids