CHOLESTEROL
Amy M. Sharma
PHM 142 REM
University of Toronto
Sept. 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
O
Cholesterol Synthesis:
Step 1:
Mitochondria
C
thiolase
Acetoacetyl CoA
+
Acetyl CoA
+
synthase
H2O
S
CoA
CH2
HO
C
CH3
CH2
COO
3-Hydroxy-3-methylglutaryl CoA
(HMG CoA)
Committed and ratelimiting step:
Reduction of HMG-CoA
to Mevalonate
NADPH
Lyase
NADP
NADH
NAD+
*-OH butyrate
Acetyl CoA
+
*Acetoacetate
*acetone
(breath)
+
HMG CoA
reductase
H2C
OH
CH2
ratelimiting
step
CHOLESTEROL or
STATIN: feedback
inhibition by
cholesterol
HO
C
CH3
CH2
COO
Mevalonate
COO-
ATP
CH2
Cholesterol Synthesis:
Step 2:
HO
C
COO-
ADP
CH3
CH2
HO
C
CH3
phosphotransferase
CH2
CH2
O
CH2 OH
CH2 O
P
O-
O-
Synthesis of isopentenyl
pyrophosphate from
mevalonate occurs in the
PEROXISOMES
Mevalonate
5-phosphomevalonate
ATP
kinase
COO-
ADP
CH2
HO
C
CH3
CH2
O
CH2 O
P
O
O
O-
P
O-
O-
(pyrophosphate)
5-pyrophosphomevalonate
ATP
decarboxylase
ADP + Pi + CO2
CH2
C
CH3
CH2
O
CH2 O
P
O-
O
O
P
O-
Isopentenyl
pyrophosphate
O-
CH3
H 3C
Cholesterol Synthesis:
C
O
C
C
H2
O
P
O
O
O-
H
O-
P
O-
isomerase
Dimethylallyl pyrophosphate
Step 3:
CH2
peroxisome
Synthesis of squalene occurs in the
peroxisomes
then the endoplasmic reticulum
PPi
CH3
H 3C
C
O
C
H2
C
H2
isopentenyl
pyrophosphate
prenyltransferase (head-to-tail)
CH3
C
H
O
H 3C C C C O P O P OH2 H2
OO-
O
C
C
C
H2
O
O
P
O
O-
H
O-
P
O-
Geranyl pyrophosphate
CH2
peroxisome
H 3C C C C O
H2 H2
PPi
CH3
H 3C
C
C
H2
C
H2
C
C
H2
C
H2
C
P
O-
O-
O-
O
C
C
H2
O
P
O
O
O-
H
Farnesyl pyrophosphate
O-
P
O-
Farnesyl pyrophosphate + NADPH
e.r.
CoQ, heme A
Dolichol-PP
Prenylated
proteins
P O
CH3
C
H
isopentenyl
pyrophosphate
O
prenyltransferase
CH3
C
H
O
Squalene synthase
DIMERIZATION
NADP+ + 2PPi + H+
CH3
H 3C
C
CH3
C
H
C
H2
H 2C
C
CH3
CH3
C
H
CH2
H 2C
2
Squalene
C
H
C
C
H2
CH2
2
C
H
C
CH3
C H3
H3 C
Squalene
C H3
C H3
C
Cholesterol Synthesis:
C H3
C H3
C H3
er P450, O2, NADPH
CH3
H3C
Step 4:
CH3
CH3
Squalene epoxide
C
CH3
O
Synthesis of cholesterol occurs
in the ER
CH3
CH3
cyclase
Lanosterol
diet
CH3
H3C
CH3
H
C
3
CH3
C
H
3
CH3
sunlight
Dehydrocholesterol
C
H
3
H
C
2
HO
NADPH
P450 reductase
H3C
CH3
CH3
H
C
3
Vitamin
D3
H
H
O
Unsat. FA acyl CoA
CH3
CH3
+cholesterol acyltransferase
Cholesterol ester
(ACAT)
CHOLESTEROL
HO
Drug therapy to decrease plasma Clt
Statins (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
-
O
O
O
-
O
HO
OH
3-hydroxy-3-methylglutaryl CoA
HO
O
SCoA
Mevinolin
O
O
Mevinolin (Fungal), a competitive inhibitor of HMG CoA reductase, resembles
3-hydroxy-3-methyl-glutaryl CoA, the substrate.
Peroxisome
But Statins may also decrease plasma ubiquinone antioxidant
HMG-CoA
TYROSINE
4-OH-benzoate
ER, Golgi
Decaprenyl4-OH
benzoate
transf.
Decaprenyl-PP
Isopentenyl-PP
Dimethylallyl-PP
Geranyl-PP
Farnesyl-PP
Endogenous synthesis
of ubiquinone and
cholesterol. Formation
of mevalonate is the
rate limiting step in
synthesis.
Polyprenyl-PP
trans-prenyltransf.
Squalene
synthase
Decaprenyl-4-OH-benzoate
Squalene
COENZYME Q
Free Rad. Biol. Med. 29, 285-94 (2000)
Lancet 356, 391-5 (2000)
Dolichol
N-glycosylates secretory
proteins = Export glycoproteins
 cholesterol
 LDL receptors (induced)
Cholesterol  LDL uptake  LDL
 risk of atherosclerosis
Drug therapy to decrease plasma Clt
Resins (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 Clt
Hypolipidemic ie antihyperlipidemic fibrate drugs
• CLOFIBRATE: 2g/day
• (also: Gemfibrozil)
CH3O
Cl
O C C O C2H5
CH3
1. ↑ lipoprotein lipase activity
2. ↑ fatty acid oxidation by inducing
PEROXISOMES
 serum triglycerides
 serum triglyceride-rich lipoprotein
3.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
mitochondriacarbon dioxide and water.
PEROXISOME (numerous genetic diseases)
Peroxisomal fatty acid -oxidation forms H2O2 which is removed by catalase
located in the peroxisomes. Medium-chain fatty acids (C8-18) prefer
mitochondrial -oxidation that doesn’t form H2O2.
Long chain or 3 or branched fatty acids
Acyl
carnitine
MITOCHONDRIA
-oxidation
Synthase
Acetyl CoA
oxidase*
Fatty acyl CoA
O2
heat
H2
Shorter-chain fatty acid
NADH
+
Acetyl
CoA
O
2
catalase
H2O + O2
Cholesterol
CoQ10*
Bile acids
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)
Drug therapy to decrease plasma Clt
Niacin (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 NADPH
derivatives 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 Clt
Blocking intestinal cholesterol permease
Ezetimibe - drug that blocks cholesterol uptake by inhibiting
intestinal sterol permease (packaged with a statin).
Natural therapy to decrease plasma Clt
Plant 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)
Summary:
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:
Dietary mechanisms to decrease cholesterol are additive (e.g., use in
patients resistant or intolerant to statins).
1.
2.
3.
4.
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.
Competitive inhibition of cholesterol absorption from the
gut, e.g., plant sterols margarine, almonds, flaxseed.
Increase LDL receptor-mediated LDL cholesterol uptake and
degradation, e.g., soy proteins, soy milk.
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 cholestasis
Cholestasis 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 cholestasis
Cholestasis 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
G) BILE ACID SYNTHESIS BY LIVER
Endoplasmic reticulum (except CYP27)
OH
cholesterol 7ahydroxylase
CYP7A
Cholesterol
CYP27
mitoch.
COO-
er, NADPH
inhibited by bile acids HO
or fasting;
induced by cholesterol
or thyroxine
7
OH
H
er, CYP12
er
side chain
oxidation
CYP7B
er
side chain
oxidation
er
OH
COOblocked
bile duct
CHENODEOXYCHOLIC
ACID
3
HO
OH
H
er, UGT
UDPGA
er
thioesterase (ligase)
acetylCoA
CHOLIC ACID
(Bile acid, "detergent")
3
CHOLESTASIS
TOXICITY
Cholyl CoA
Hepatocyte death
Gluc O
LIVER
CIRRHOSIS
AND DEATH
Biochemistry. 31, 4737-49, (1992)
CYTOSOL
28
Ga) BILE ACID SYNTHESIS BY LIVER (cont)
Endoplasmic reticulum
CYTOSOL
Cholic Acid
Cholyl CoA
sulfotransferase
(SULT2A1)
+ PAPS
SULT2A1
N-acetyltransferase
+ taurine
N-acetyltransferase
+ glycine
O
taurine
OH
NH
Glycocholate
glycine
NH
3
O3SO
TAUROCHOLATE
sulfate
HO
OH
H
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.