Upload
elyssa-aduddell
View
227
Download
3
Tags:
Embed Size (px)
Citation preview
CHOLESTEROLAmy M. SharmaPHM 142 REMUniversity of TorontoSept. 27 2012
HDL• Stage 4 GOOD HDL : Reverse cholesterol transport and function
• 1. HDL is synthesized and secreted from the liver and the intestine. HDL contains 65% protein + free fatty acids, cholesterol, triacylglyceride and phospholipids.
• • 2 Function: • HDL picks up cholesterol released into the plasma from dying cells• and from membranes undergoing turnover and returns it to the liver
• 3. HDL contains cholesterol, cholesterol ester, phospholipid and Lecithin:Cholesterol Acyl Transferase (LCAT) - synthesised in the liver that catalyses :
• LECITHIN + CHOLESTEROL • LYSOLECITHIN + CHOLESTEROL ESTER
• LCAT is activated by apo-A1 and deficiency in LCAT means that HDL can’t take up cholesterol from tissue, therefore cholesterol and lecithin in tissue
Increasing HDL to decrease tissue Clt• Anacetrapib (Merck drug) NEW• Niacin best• Fibrate drugs• bile acid binding resins• Exercise , -3 fatty acids,red wine,orange juice,beans, soy,oat
bran• tr.fatty acids, high carbohydrate. decr. HDL
Cholesterol Synthesis • Essential molecule in many animals including humans• However not required from diet – all cells can synthesize it• Majority produced in the liver • Synthesis occurs in 4 stages:
• 1. Condensation of 3 acetate units to form 6-carbon intd, Mevalonate• 2. Mevalonate to activated isoprene units • 3. Polymerization of six 5-carbon isoprene units to form Squalene• 4. Squalene cyclization to form four rings of steroid nucleus followed by
more changes to form final cholesterol
Acetoacetyl CoA + Acetyl CoA + H2O
3-Hydroxy-3-methylglutaryl CoA (HMG CoA)
Mevalonate
synthase
NADPH
NADP+
rate-limitingstep
CHOLESTEROL or STATIN: feedbackinhibition bycholesterol
thiolase
Acetyl CoA+
*Acetoacetate
*-OH butyrate
NADH
*acetone (breath)
HMG CoA reductase
CH2 OH
CH2
C CH3OH
CH2
COO
Lyase
NAD+
COO
CH2
C CH3OH
CH2
C S CoA
O
Cholesterol Synthesis:
Step 1:Mitochondria
Committed and rate-limiting step:Reduction of HMG-CoA to Mevalonate
Cholesterol Synthesis:
Step 2:
Synthesis of isopentenyl pyrophosphate frommevalonate occurs in the PEROXISOMES
COO-
CH2
CHO CH3
CH2
CH2OH
COO-
CH2
CHO CH3
CH2
CH2O P O-
O-
O
COO-
CH2
CHO CH3
CH2
CH2O P O
O-
O
P
O-
O-
O
CH2
C CH3
CH2
CH2O P O
O-
O
P
O-
O-
O
ATP ADP
ATP
ADP + Pi + CO2
Mevalonate 5-phosphomevalonate
5-pyrophosphomevalonate
Isopentenylpyrophosphate
ATP
ADP
phosphotransferase
kinase
(pyrophosphate)
decarboxylase
Cholesterol Synthesis:
Step 3:
Synthesis of squalene occurs in the peroxisomesthen the endoplasmic reticulum
peroxisome
Squalene synthase DIMERIZATIONe.r.
peroxisome
H3C C
CH3
C
H
CH2
O P
O-
O
O
P
O
O-
O-
CH2
C
CH3
C
H
CH2
O P
O-
O
O
P
O
O-
O-CH2
CH
C
CH3
H3C
H3C C CH2
CH2
CH2
O P
O-
O
O
P
O
O-
O-
PPi
H3C C CH2
CH2
CH2
O P
O-
O
O
P
O
O-
O-
PPi
CH2
C
CH3
C
H
CH2
O P
O-
O
O
P
O
O-
O-CH2
CH
C
CH3
CH2
CH2
CH
C
CH3
H3C
Dimethylallyl pyrophosphate
Geranyl pyrophosphate
Farnesyl pyrophosphate
H3C C CH
CH3
CH2
H2C C
CH3
CH
CH2 CH2C
CH3
CH
H2C CH2
CH
C
CH3
CH3
2 2
Squalene
Farnesyl pyrophosphate + NADPH
NADP+ + 2PPi + H+
isopentenylpyrophosphate
isopentenylpyrophosphate
isomerase
prenyltransferase (head-to-tail)
prenyltransferase
CoQ, heme ADolichol-PPPrenylated proteins
Cholesterol Synthesis:
Step 4:
Synthesis of cholesterol occurs in the ER
CH3
CH3H3C
CH3
CH3
CH3
CH3
C
CH3
CH3H3C
CH3
CH3
CH3
OCH3
C
CH3
CH3H3C
CH3
CH3
H3C CH3
HO
CH3
Dehydrocholesterol
Squalene
er P450, O2, NADPH
cyclase
Squalene epoxide
Lanosterol
CHOLESTEROL
CH3
CH3H3C
HO
CH3
CH3
NADPHP450 reductase
sunlight
diet
Unsat. FA acyl CoAOC
O
R
Cholesterol ester
H2C
HO
CH3
H3C
H
CH3
H3C
Vitamin D3
+cholesterol acyltransferase(ACAT)
CH3
CH3H3C
HO
CH3
CH3
Drug therapy to decrease plasma CltStatins (HMG-CoA reductase inhibitors)• Inhibit cholesterol biosynthesis in liver to decrease plasma LDL
cholesterol and cut the risk of heart attacks and strokes by at least 33%
• HMG CoA reductase inhibitors can induce rhabdomyolysis (test for muscle/kidney damage)
• Lipitor, Zocor, Crestor lower risk of heart attack, death and stroke
OO-
OH
O
SCoA
OO-
OH
OH
O
O
Mevinolin (Fungal), a competitive inhibitor of HMG CoA reductase, resembles 3-hydroxy-3-methyl-glutaryl CoA, the substrate.
Mevinolin3-hydroxy-3-methyl-glutaryl CoA
cholesterol LDL receptors (induced) LDL uptake LDL risk of atherosclerosis
Endogenous synthesisof ubiquinone and cholesterol. Formationof mevalonate is therate limiting step insynthesis.
Isopentenyl-PP
Dimethylallyl-PP
Geranyl-PP
Farnesyl-PP
Squalene synthase
Squalene
Cholesterol
Decaprenyl-PP
TYROSINE
4-OH-benzoate
Decaprenyl-4-OHbenzoatetransf.
Decaprenyl-4-OH-benzoate
COENZYME Q
trans-prenyltransf.
But Statins may also decrease plasma ubiquinone antioxidant
Free Rad. Biol. Med. 29, 285-94 (2000)Lancet 356, 391-5 (2000)
HMG-CoA
Polyprenyl-PP
Dolichol N-glycosylates secretory
proteins = Export glycoproteins
ER, GolgiPe
roxis
om
e
Drug therapy to decrease plasma CltResins (hydrocarbon secretion)Bind bile acids to lower cholesterol• Cholesterol is reabsorbed from intestine by forming complexes with bile acids. Liver then replaces
bile acids by oxidizing cholesterol (catalysed by CYP7A).
Prescription therapeutic resins bind bile acids and prevent cholesterol reabsorption: problem of constipation, ↓absorption of fat sol. vitamin A,D,E,K• e.g. colestipol • cholestyramine• colesevelam
Nonprescription bulk forming laxatives (soluble fibres)• Psyllium husks (metamucil)• Ispaghula husks• Oat bran (-glucan binds bile acids )
• Action of bile acid binders• cholesterol excretion• hepatic cholesterol 7a hydroxylase (CYP7A) activity which oxidizes cholesterol to bile acids.
(feedback inhibitor is normally bile acids)
Drug therapy to decrease plasma CltHypolipidemic ie antihyperlipidemic fibrate drugs
• CLOFIBRATE: 2g/day• (also: Gemfibrozil)
Cl O C
CH3
CH3
C
O
O C2H5
1. ↑ lipoprotein lipase activity2. ↑ fatty acid oxidation by inducing PEROXISOMES serum triglycerides serum triglyceride-rich lipoprotein3.Antioxidant action prevent LDL oxidation
Perioxisomes• Peroxisomes (microbodies) • found in virtually all eukaryotic cells.• Involved in the catabolism of very long chain fatty acids,
branched chain fatty acids, D-amino acids, polyamines, and biosynthesis of plasmalogens, i.e. ether phospholipids critical for the normal function of mammalian brains and lungs.
• Major function: breakdown of very long chain fatty acids through beta-oxidation.
• converted to medium chain fatty acids to mitochondriacarbon dioxide and water.
PEROXISOME (numerous genetic diseases)Peroxisomal fatty acid b-oxidation forms H2O2 which is removed by catalase located in the peroxisomes. Medium-chain fatty acids (C8-18) prefer mitochondrial b-oxidation that doesn’t form H2O2.
Acylcarnitine
Fatty acyl CoA
SynthaseAcetyl CoA
O2 H2O2
oxidase*
heat
Shorter-chain fatty acid
MITOCHONDRIAb-oxidation
NADH+
Acetyl CoA
CholesterolCoQ10*
Bile acidsH2O + O2
catalase
H2O2 also formed by peroxisomal glycolate/glyoxylate oxidases,xanthine oxidase, uricase
* Peroxisomes induced by peroxisome proliferators via a cytosolic receptor (PPAR)e.g., hypolipidemic drugs, e.g., clofibrate; plasticizers, e.g., phthalate (DEHP); endogenous steroids formed by the adrenal glands e.g., dehydroepiandrosterone.
Ann Rev Biochem. 61, 157-97 (1992)Ann Rev Nutr. 14, 343-70 (1994)
Long chain or 3 or branched fatty acids
Drug therapy to decrease plasma CltNiacin (Vitamin B3) • Deficiency causes pellagra (rough photo-sensitive skin, dementia, etc) Flour now fortified with niacin; B 1 thiamine; B2 riboflavin
tryptophan niacin nicotinamide NAD NADP NADPHderivatives NAD, NADH, NAD+, and NADP play essential roles in energy
metabolism and DNA repair.
• Niacin 1.5-3g/day ↓ plasma LDL cholesterol & triglycerides; best for ↑ HDL But early hot flashes so use slow delivery formula
Rare hepatotoxicity or hyperglycemia
Drug therapy to decrease plasma CltBlocking intestinal cholesterol permease
Ezetimibe - drug that blocks cholesterol uptake by inhibiting intestinal sterol permease (packaged with a statin).
Natural therapy to decrease plasma CltPlant sterols/stanols
Ezetimibe - drug that blocks cholesterol uptake by inhibiting intestinal sterol permease (packaged with a statin).
• Sitosterol , Sitostanol• Clt lowering action of plant sterols on the diet
Plant sterol not absorbed by gut (2g/day) so inhibits gut absorption of cholesterol from diet. “functional margarine”)e.g. Becel pro-activ.in Loblaws
Natural therapy to decrease plasma Clt• Chitosan (shellfish exoskeleton) (LIBRACOL is polychitosamine:
amine groups bind cholesterol)• Policosanol (sugar cane wax or rice wax alcohol ie.
Octacosanol) CH3(CH2)26CH2OH)
DIETARY WAYS OF DECREASING THE ATHEROSCLEROSIS RISK:
1. cholesterol and saturated fatty acids plant stanols (2g/margarine day)
2. polyunsaturated fatty acids which cholesterol oxidation to bile acids
LDL catabolism cholesterol excretion into intestine
3. smoking, obesity, lack of exercise, low Ca2+
4. high HDL in premenopausal women protects but not after menopause.
Summary:
Summary:
Dietary mechanisms to decrease cholesterol are additive (e.g., use in patients resistant or intolerant to statins).
1. Decrease intestinal bile acids by binding them to viscous fibres, e.g., oats (b-glucan), barley, psyllium (metamucil), egg plant,ochra.
Glucan is also a soluble fibre & an antioxidant which prev. oxidn of PUFA & cholesterol. Amer.J.Clin.Nutrition 75(2002)834-9.
2. Competitive inhibition of cholesterol absorption from the gut, e.g., plant sterols margarine, almonds, flaxseed.
3. Increase LDL receptor-mediated LDL cholesterol uptake and degradation, e.g., soy proteins, soy milk.
4. Decrease oxidized LDL using antioxidants, e.g., almonds (Vit E), soy proteins (isoflavones).
Bile: function & importance Bile (gall) - fluid produced by the liver of most vertebrates • Composed of water, bile salts, mucus and pigments, fats,
inorganic salts and cholesterol• Main functions:
• Essential in the process of digestion of lipids from the small intestine • For protection of small intestines from oxidative damage• For excretion of endogenous xenobiotic compounds
Also:• Aids in absorption of fat-soluble vitamins A, D, E, K• Bile salts are also bactericidal (protect from microbial products
in foodstuff)
Bile: dysfunction & implications • Drugs that slow/block bile flow from the liver to the gallbladder
& gut may cause liver failure. e.g. chlorpromazine,prochlorperazine, penicillin
ampicillin,estradiol, nitrofurantoin, sulindac.
• Symptoms include dark urine, pale stool, jaundice, fever/rash persistent itching.
MUST discontinue the drug.
Bile: potential for toxicity • Bile acids are cytotoxic to hepatocytes (can cause liver failure
via free radical formation).• Mechanism:• Reduces Fe3+ which reduces H2O2 to form hydroxyl radicals and
reactive oxygen species (ROS)• Radicals oxidize nucleic acids, proteins, and unsaturated lipids
to form other radicals.• BUT…• Antioxidants & blockers of mitochondrial permeability
transition prevent apoptosis
Cholestasis defined
Cholestasis - any condition in which the flow of bile from the liver is blocked.
i.e. where bile cannot flow from the liver to duodenum
• Obstructive cholestasis - mechanical blockage in the duct system such i.e. as a results of gallstone or malignancy
• Metabolic cholestasis - disturbance in bile formation due to genetic defects or as an adverse effect of a drug
Extra-hepatic cholestasisCholestasis occurring outside the liver
• Caused by blockage of bile duct or ducts• Possible causes:- bile duct tumors - Pancreatic tumor or pseudocyst- Cysts- Narrowing of the bile duct- Stones- Pancreatitis- Pressure on an organ due to tumor or nearby mass
Intra-hepatic cholestasisCholestasis occurring within the liver
• Caused by significant blockage of small ducts or by disorders, such as hepatitis, that impair the body's ability to eliminate bile
• Possible causes:- Alcoholic liver disease- Amyloidosis- Bacterial abscess- Lymphoma
• Drugs- Primary biliary cirrhosis- Tuberculosis- Sepsis,- Viral hepatitis
Biochemistry. 31, 4737-49, (1992)
G) BILE ACID SYNTHESIS BY LIVEREndoplasmic reticulum (except CYP27)
COO-OH
HOHOH
COO-OH
HOHOH
Cholesterol
cholesterol 7 hydroxylaseCYP7A
er, CYP12
erside chainoxidation
erthioesterase (ligase)acetylCoA
CHOLIC ACID(Bile acid, "detergent")
LIVER CIRRHOSIS AND DEATH
CYP27mitoch.
CYP7Ber
side chain oxidation
CHENODEOXYCHOLIC ACID
er, NADPHinhibited by bile acids or fasting; induced by cholesterol or thyroxine
er
Cholyl CoA
3
er, UGT
UDPGA
OGluc3
7
CHOLESTASISTOXICITY
CYTOSOL
blocked bile duct
Hepatocyte death
28
Then efflux into bile and stored in gall bladder.Then released by bile duct into upper-small intestine (ileum).Then metabolised (deconjugation (CO2), dehydroxylation) by anaerobic bacteria of colon to deoxycholate, lithocholate, urodeoxycholate.Then actively reabsorbed and recirculates via liver 8 times / day.
Ga) BILE ACID SYNTHESIS BY LIVER (cont)
OHH
OH
OH NH
O
taurine
Cholyl CoA
N-acetyltransferase+ taurine
CYTOSOL
Glycocholate
SULT2A1
O3SO
sulfotransferase(SULT2A1)+ PAPS
3 TAUROCHOLATE
N-acetyltransferase+ glycine
sulfate
Cholic Acid
NHglycine
Endoplasmic reticulum