LIPIDS(LIPID PROFILE)

Introduction

The major lipids present in the plasma are: fatty acids, Triglycerides, cholesterol and phospholipids.

Other lipid-soluble substances, present in much smaller amounts (e.g. steroid hormones ).

Elevated plasma concentrations of lipids, particularly cholesterol, are related to the pathogenesis of atherosclerosis.

Lipids transport Lipids are carried in the bloodstream

by complexes known as lipoproteins. This is because these lipids are not

soluble in the plasma water. Thus they travel in micelle-like

complexes composed of phospholipids, cholesterol and protein on the outside with cholesteryl esters, and triglycerides on the inside.

The four main types of lipoproteins are chylomicrons, VLDL, LDL, and HDL

Clinical Significance Cholesterol and triglycerides, like many

other essential components of the body, attract clinical attention when present in abnormal concentrations.

Increased or decreased levels usually occur because of abnormalities in the synthesis, degradation, and transport of their associated lipoprotein particles.

Increased or decreased plasma lipoproteins are named hyperlipoproteinemia & hypolipoproteinemia respectively.

Fatty streak

Thrombotic athero lesion, myocardial infarct

Early and late atherosclerotic lesions

Generic Lipoprotein

Metabolic Syndrome: Disease of the Modern Era

Constellation of several risk factors that increase chance of coronary artery disease, peripheral vascular disease, stroke and type 2 diabetes.

Combination of 3 or more of the following risks:

• Abdominal obesity

• Triglyceride levels above 150 mg/dL

• Low HDL cholesterol

• Elevated blood pressure (>130/85 mm Hg)

• Fasting blood glucose > 100 mg/dL

Aging a major contributor: prevalence in 20-29 yr olds = 6.7%; 60-69 yr olds = 43.5%

CM VLDL IDL LDL HDL

Lipoprotein Nomenclature and Composition

Major apoB apoB apoB apoB apoA-IProtein

Major TG TG CE CE CELipid

CM= chylomicron TG=triglycerideVLDL= very low density lipoprotein CE= cholesteryl esterIDL= intermediate density lipoproteinLDL= low density lipoproteinHDL= high density lipoproteinApo = apolipoprotein

Nascent-HDLapoA-I

Liver

VLDL

apoB-100

apoCs

apoE

IDL LDL

apoB-100apoE

apoB-100

apoB-48

CM

apoCs

Intestine

Nascent-HDLapoA-I

Site of Synthesis of Lipoproteins

Lipids as Biochemical Markers of Disease

Clinical chemistry laboratories offer many tests for lipid disorders.

One of the most common tests is the lipid profile. This panel of tests includes measurement of

triglycerides and cholesterol in the form of lipoprotein-cholesterol molecules,

low density lipoprotein cholesterol (LDL-C) and high-density lipoprotein cholesterol (HDL-C).

The results of testing for these lipids provide measures of risk for coronary artery disease.

It is therefore clear that lipid measurements should be made in all patients known to have vascular disease, and in those at increased risk.

Thus plasma lipids should be measured in the individuals with the following: CHD (and cerebrovascular and peripheral

vascular disease) a family history of premature coronary disease

(occurring at age <60 years) other major risk factors for CHD (e.g. diabetes

mellitus, hypertension) patients with clinical features of

hyperlipidaemia patients whose plasma is seen to be lipaemic.

Positive and Negative Risk Factors in Atherosclerosis

Positive Negative

Age: Males > 45 years Elevated HDL cholesterol

Females > 55 years Low LDL cholesterol

Family history of early CHD Good genes

Elevated LDL cholesterol (>130 mg/dL) Female gender (estrogen)

Elevated triglyceride (>150 mg/dL) Exercise

Diabetes mellitusHypertension

Obesity

Smoking

CHD, coronary heart disease

Triglycerides

13

Glycerol backbone with FA attached by ester bonds

Sources of Triglycerides:Exogenous source: DietaryEndogenous : Liver and tissue

storageH

CH OH

C

C

H OH

H

H OH

Glycerol

H

CH O

C

C

H O

H

H O

CO

CO

CO (CH2)n

(CH2)n

(CH2)n

CH3

CH3

CH3

Triglyceride

Triglycerides

Serum triglycerides measurements are done for the following clinical reasons:• Hypertriglyceridemia increases the risk

for pancreatitis.• Hypertriglyceridemia is associated with

the following clinical findings: eruptive xanthoma, lipemia retinalis, hepatomegaly, splenomegaly, depressed HDL-cholesterol.

• in characterizing risk of CVDs• For the estimation of LDL-cholesterol,

using the Friedewald equation

Enzymatic Method Glycerol, released from triglycerides after

hydrolysis with lipoprotein lipase, Tranformed by glycerolkinase into

glycerol-3-phosphate which is oxidized by glycerolphosphate

oxidase into dihydroxyacetone phosphate and hydrogen peroxide.

In the presence of peroxidase, the hydrogen peroxide oxidizes the chromogen 4-aminophenazone/ESPT to form purple quinoneimine whose intensity is proportional to the concentration of triglycerides in the sample.

Enzymatic Method

Triglycerides Glycerol + 3 fatty acids

Glycerol + ATP Glycerol-3 phosphate + ADP

Glycerol-3 phosphate dihydroxyacetone + H2O2 phosphate

H2O2 + 4-aminophenazone/ESPT Quinoneimine

Lipoprotein lipase

glycerolkinase

glycerolphosphate oxidase

peroxidase

Cholesterol

Cholesterol is a sterol compound that is found in all animal tissues

Serves many important physiological functions including: synthesis of bile acids, steroid hormones, and cell membranes.

Cholesterol also appears to be involved in atherosclerosis; thus cholesterol measurement is one of the most common laboratory tests used today.

Enzymatic Reaction Determination of cholesterol after enzymatic

hydrolysis and oxidation. The colorimetric indicator is quinoneimine which

is generated from 4-aminoantipyrine and hydroxybenzoate by hydrogen peroxide under the catalytic action of peroxidase

Cholesterol Esterase

Cholesterol oxidase

Peroxidase

Cholesterol-3-one

HDL HDL is a fraction of plasma lipoproteins It is composed of:

50% protein, 25% phospholipid, 20% cholesterol, and 5% triglycerides

Evidence suggests that high-density lipoprotein (HDL) cholesterol is a primary coronary heart disease (CHD) risk factor.

Analysis Methods Ultracentrifugation Polyacrylamide Gel Electrophoresis Precipitation

based on the ability of various agents to precipitate selectively the major lipoprotein fractions, except HDL

Immunological Antibodies against human lipoproteins are

used to form antigen-antibody complexes with LDL, VLDL and chylomicrons in a way that only HDL-cholesterol is selectively determined by an enzymatic cholesterol measurement

Precipitation Method

In the plasma, cholesterol is transported by three lipoproteins: high density lipoprotein, low density lipoprotein, and very low density lipoprotein

HDL lipoproteins are assayed, after precipitation of LDL and VLDL lipoproteins with polyethylene glycol (PEG) 6000.

HDL is left in the supernatant solution for cholesterol quantitation.

Low Density Lipoprotein

[LDL-chol] = [Total chol] - [HDL-chol] - ([TG]/2.2)

Where all concentrations are given in mmol/L (note that if calculated using all concentrations

in mg/dL then the equation is:

[LDL-chol] = [Total chol] - [HDL-chol] - ([TG]/5) The quotient ([TG]/5) is used as an estimate of

VLDL-cholesterol concentration. It assumes, first, that virtually all of the plasma

TG is carried on VLDL, and second, that the TG:cholesterol ratio of VLDL is constant at about 5:1

Limitations of the Friedewald equation

The Friedewald equation should not be used under the following circumstances: when chylomicrons are present when plasma triglyceride concentration

exceeds 400 mg/dL in patients with dysbetalipoproteinemia (type

III hyperlipoproteinemia