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Lipids and LipoproteinsLipids and Lipoproteins

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Lipid Transport in PlasmaLipid Transport in Plasma

Free fatty acid (FFA) bound to albumin

As lipoprotein complexes

Surface layer:Polar lipids (free cholesterol,

phospholipids)

Proteins (Apolipoproteins)

Core:Non-polar lipids(triglycerides,

cholesterol esters)

Apolipoproteins: Lipid binding Cofactors for enzymes that metabolise lipoproteins

Interact with cell receptors to mediate uptake into cells

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LIPOPROTEIN STRUCTURELIPOPROTEIN STRUCTURE

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TEM of isolated lipoproteins

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MMAJORAJOR LIPOPROTEINSLIPOPROTEINS

1. Chylomicrons (extremely low density lipoproteins)

2. Very Low Density Lipoproteins (VLDL)

3. Intermediate Density Lipoproteins (IDL)

4. Low Density Lipoproteins (LDL)

5. High Density Lipoproteins (HDL)

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Note: 1. Density as particle size

2. Different major lipid constituents of different lipoproteins

3. Each class heterogeneous

Table 11.2 (Beckett et al); see Luxton Fig. 13.2

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TEM of isolated lipoproteins

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Triglyceride Metabolism

+ Fat

+ ApoB,C

TG

FFA + monoglycerides

TG

Into circulation via thoracicduct

Lipoprotein lipase Tissue Lipoprotein lipase

Loss of TG

Liver

AdiposeTissue

Lipoproteins

FFA

TG

VLDL

Cholesterol,apolipoproteins

lipase

HDL is formed in the liver andintestinal mucosa

Beckett et al, Fig. 11.2;see Luxton Fig. 13.2

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Cholesterol Metabolism

Dietary cholesterol

1-2 g cholesterol into intestine per day

400-700 mg dietary 30-60% absorbed

750-1250 mg as bile into chylomicrons

Endogenous cholesterol

Liver de novosynthesis (900 mg/day) (HMG-CoA Reductase control step)

(1) VLDL(2) bile acids

(3) free cholesterol in bile

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Cholesterol PathwaysCholesterol Pathways

1. VLDL LDL cholesterol supply to cells

2. Reverse cholesterol transport

Cholesterol in tissue cell membranes (free or unesterified) takenup by HDL

esterified by HDL LCAT (lecithin-cholesterol acyltransferase)

cholesterol ester

LCAT

Cholesterol + FA Cholesterol ester

transferred (by CETP) to LDL, VLDL, chylomicrons, or directly

to liver for excretion

[CETP cholesterol ester transfer protein]

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ApolipoproteinsApolipoproteins

1. Apo A-1, A-11

components of HDL

Apo A-I activates LCAT

2. Apo B: 2 major forms

apoB100 in LDL mainly

apoB48 only in chylomicrons

structural protein (for chylomicrons, VLDL, IDL, LDL)

binds tissue receptors

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3. Apo C-I, C-II, C-III

in HDL between meals

transfer to TG-rich lipoproteins (after meals)

Apo C-II activates lipoprotein lipase

4. Apo E-I to E-IV

in nascent HDL

transfers with CE from HDL to lipoprotein remnants

important in recognition of remnants

5. Apo (a)

component of Lp(a), along with apoB100

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AtheromaAtheroma andand ischaemicischaemic heart disease (IHD) /heart disease (IHD) /

coronary artery disease (CAD)coronary artery disease (CAD)(atheroma = atherosclerotic plaque in an artery)

Relationship between plasma lipoproteins andpathogenesis of atherosclerosis:

1. High plasma levels of LDL, IDL, but not VLDL alone areassociated with risk of premature atherosclerosis and CAD

2. Plasma HDL cholesterol is a negativenegativerisk factor

i.e. high HDL protection against CAD

low HDL risk of CAD

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CHD incidence vs cholesterol

Beckett et al

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Other Risk Factors

Hypertension

Glucose intolerance

Cigarette smoking Stress

Physical inactivity

Note: Lipoprotein (a)

similar to LDL; has similar lipid composition to LDL

may have a considerable role in atherogenesis

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Process of AtherosclerosisProcess of Atherosclerosis

Injured blood vessel (endothelial injury)

LDL attracted through chemotaxis

LDL oxidised (e.g. by oxygen free radicals)

Engulfed by macrophages (and smooth muscle cells)

Foam cells

Foam cells die to form core of atherosclerotic plaque

Non-contractile scar tissue

Cell rigidity, hypertension

Plaque rupture

Clot (thrombus)

(Marieb, Human Anatomy andPhysiology, 5th Ed, pp 722-3)

Platelets

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orkney-mcn.org

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LABORATORY ASSESSMENTLABORATORY ASSESSMENT

Plasma Cholesterol and Triglycerides

Blood sampling from subjects:

balanced diet 7 days fast 12-14 hr (esp. if measuring TGs)

no severe stress (e.g. AMI, burns)

( plasma TG and variable cholesterol)

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Reference ranges

1.1. TriglycerideTriglyceride < 1.6 mmol/L

2. Cholesterol2. Cholesterol

Difficult to defineDifficult to define

Age: LDL from birth to age 60-70, then Gender: HDL higher in women until menopause

Geography - plasma cholesterol is:

low in rural populations of developing countries

high in advanced societies due to diet

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Desirable levels of plasma cholesterol (wrt IHD)

Note: No clear cut-off

Previous guidelines

Either:Either:

adults 5.5 mmol/L ( > 6.5 mmol/L high risk)

Or:Or:

Desirable < 5.17 mmol/L

Borderline 5.17 - 6.18

High > 6.18 mmol/L

Need to take account of other risk factors

Children 4.5mmol/L

Note: levels are higher in elderly

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Primary classification ofPrimary classification of hyperlipidemiashyperlipidemiasNote: all fastingfastingmeasurements

1. Hypercholesterolemia

2. Hypertriglyceridemia

3. Hypercholesterolemia and hypertriglyceridemia(combinedcombinedhyperlipidemia)

adequate classification normally

otherwise need further tests

Note the need to differentiate cholesterol classes Total cholesterol vs LDL + HDL.

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Methods of AnalysisMethods of Analysis

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1. UltracentrifugationUltracentrifugation (Reference method)

separation on basis of density

NOTE: expensive instrumentation; long analyses not routine

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2. ElectrophoresisElectrophoresis

Fasting serum

Cellulose acetate/Agarose

Stain with lipophilic dye(e.g. Oil red O)

Chylomicrons:

absent from normal fasting plasma

Remain at origin

Does not provide a disease diagnosisonly a description of the phenotype

The hyperlipidemia may be due to avariety of diseases states (primary andsecondary)

+

LDL VLDL HDL

IDL

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Appearance ofAppearance of serumserum on standingon standing

Back-up data for electrophoresis

Turbidity related to size of lipoprotein (chylo>VLDL>LDL>HDL). Creamy

layer = chylomicrons; turbidity = VLDL See Luxton Fig. 13.3

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Appearance of serum on standingAppearance of serum on standing

NOTE the following definitions:

Lipemic = presence of (fine emulsion) of lipid in blood

due to TG

Lactescence = a milky appearancedue to TG

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3. HDL cholesterolHDL cholesterol

add heparin/Mn2+ to plasma

selective pptn of chylomicrons, VLDL, LDL

HDL left in solution

add polyethylene glycol (PEG-6000)

pptn of lipoproteins containing apo-B HDL left in solution

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4. Cholesterol4. Cholesterol

cholesterol esters free cholesterol + FAs[cholesterol esterase]

cholesterol cholest-4-ene-3-one + H2O2[cholesterol oxidase]

H2O2 + phenol + 4-aminophenazone

o-quinoneimine dye + 2H2O

[peroxidase]

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R-C-O-IIO

Cholesterol esterCholesterol ester

(Fatty acid fatty acyl ester )

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4. Cholesterol4. Cholesterol

cholesterol esters free cholesterol + FAs[cholesterol esterase]

cholesterol cholest-4-ene-3-one + H2O2[cholesterol oxidase]

H2O2 + phenol + 4-aminophenazone

o-quinoneimine dye + 2H2O

[peroxidase]

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5. Triglycerides5. Triglycerides

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5. Triglycerides5. Triglycerides

Triglycerides glycerol + FFAs[lipase]

Glycerol glycerol 3-P + ADP[glycerol kinase]

glycerol 3-P + O2 DHAP + H2O2[glycerophosphate oxidase]

H2O2

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HYPERLIPIDEMIASHYPERLIPIDEMIAS

Table 11.3 (Beckett et al)Table 11.3 (Beckett et al)

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PrimaryPrimary HyperlipidaemiaHyperlipidaemia

1.1. HypercholesterolaemiaHypercholesterolaemiaPrimary hypercholesterolaemia

LDL

frequency of 1/500 Most (~ 95%) patients have an unclassified disease that

has genetic/dietary influences

A small proportion of patients have familialfamilialhypercholesterolaemiahypercholesterolaemia

Type IIa

autosomal dominant

deficient/abnormal LDL receptors (i.e. apoB100receptors) on cells

plasma cholesterol: heterozygotes 8-15 mmol/L

homozygotes 15-30 mmol/L

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+

LDL VLDL HDL

IDL

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LDL uptake and catabolismLDL uptake and catabolism

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LDL uptake and catabolismLDL uptake and catabolism

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2.2. HypertriglyceridaemiaHypertriglyceridaemia

FamilialFamilial hypertriglyceridaemiahypertriglyceridaemia (Type IV)(Type IV)

VLDL

1/1000, autosomal dominant production or catabolism of VLDL

(i.e. A group of diseases)

Some patients with this disease also developchylomicronaemia: this appears to occur secondary toanother, underlying disease such as diabetes or alcoholism.

i.e. these patients have VLDL and chylomicrons= (Type V)

may see eruptive xanthomas

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+

LDL VLDL HDL

IDL

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Lipoprotein lipase deficiency (Type 1)Lipoprotein lipase deficiency (Type 1)

chylomicrons

rare disease; presents in infants

deficiency of lipoprotein lipase or its activator apo-C II

eruptive xanthomas

+

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+

LDL VLDL HDL

IDL

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3. Combined3. Combined hypercholesterolaemiahypercholesterolaemia andand

hypertriglyceridaemiahypertriglyceridaemia Familial combined hyperlipidaemia

heterogeneous group

LDL and/or VLDL ( LDL, VLDL Type IIb) risk of IHD

Remnant hyperlipoproteinaemia

IDL rich in cholesterol and TG see broad -band on electrophoresis (Type III) apoE defect plus additional unknown defects

defective hepatic uptake of chylomicronremnants and partially degraded VLDL accelerated atherosclerosis

rare

cutaneous xanthomas

+

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+

LDL VLDL HDL

IDL

C bi d3 C bi d h h l l ih h l l i dd

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3. Combined3. Combined hypercholesterolaemiahypercholesterolaemia andand

hypertriglyceridaemiahypertriglyceridaemia Familial combined hyperlipidaemia

heterogeneous group

LDL and/or VLDL ( LDL, VLDL Type IIb) risk of IHD

Remnant hyperlipoproteinaemia

IDL rich in cholesterol and TG see broad -band on electrophoresis (Type III) apoE defect plus additional unknown defects

defective hepatic uptake of chylomicronremnants and partially degraded VLDL accelerated atherosclerosis

rare

cutaneous xanthomas

+

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+

LDL VLDL HDL

IDL

S dS d h li id ih li id i

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SecondarySecondary hyperlipidaemiahyperlipidaemia

Less than 20% of hyperlipidaemias are secondary to otherdisease

Examples:

Hypothyroidism hypercholesterolaemia throughincreased LDL due to LDL catabolism

Diabetes mellitus hypertriglyceridaemia due tomobilisation of adipose tissue TG

increased VLDL due to VLDL synthesis

K tK t b d f ti db d f ti d k t id ik t id i

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KetoneKetone body formation andbody formation and ketoacidosisketoacidosis

I di i f l li idI di ti f l li id t

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Indications for plasma lipid assessmentIndications for plasma lipid assessment

1. Clinical

Xanthomas (RE macrophages of connective tissuefilled with lipid

TG (eruptive Xanthomas)

cholesterol (tendon Xanthomas)

Tendon

Eruptive

Indications for plasma lipid assessmentIndications for plasma lipid assessment

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Indications for plasma lipid assessmentIndications for plasma lipid assessment

Xanthelasma (lipid-filled cells in dermis yellow raisedplaques around eyelids) (due to cholesterol)

Indications for plasma lipid assessmentIndications for plasma lipid assessment

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Indications for plasma lipid assessmentIndications for plasma lipid assessment

Premature arcus - opaque ring at edge of cornea

(due to cholesterol)

I di ti f l li id tI di ti f l li id t

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Indications for plasma lipid assessmentIndications for plasma lipid assessment

ClinicalClinical

premature vascular disease

LaboratoryLaboratory

lipaemic plasma

Screen groups with:Screen groups with:

vascular disease of early onset (esp. premature IHD)

Diabetes mellitus, hypertension

family history of hyperlipidaemia or prematureatherosclerosis

clinical manifestations suggestive of hyperlipidaemia

Medical ManagementMedical Management

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ed ca a age e tg

Look for 2hyperlipidaemia and if found treat 1disorder.Otherwise:

1. Diet1. Diet

particularly wrt LDL-C

diet for 6 months

reduce saturated fats, cholesterol

(saturated fats and cholesterol suppress hepatic

LDL receptor plasma clearance of LDL)

replace saturated fats with mono- or poly-unsaturated fats

Note: obesity/caloric excess HDL, LDL

aerobic exercise HDL, LDL

2 Drugs to reduce cholesterol2 Drugs to reduce cholesterol

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2. Drugs to reduce cholesterol2. Drugs to reduce cholesterol

bile acid sequesterants (Cholestyramine, colestipol)

interrupt enterohepatic circulation of bile acids (bind bile

salts in intestine excretion of cholesterol)

Niacin (Nicotinic acid)

reduces VLDL synthesis

inhibitors of HMG-CoA reductase

(enzyme that catalyses HMG-CoA Mevalonate)

eg. statins:

Lovastatin, Simvastatin, Atorvastatin (Lipitor)