Lipids and Lipoproteins

Lipids• A group of water-insoluble substances that

are extractable by nonpolar (fat) solvents, such as alcohol and ether.

• Include: fatty acids, neutral fats, waxes andsteroids.

• Compound lipids: glycolipids, lipoproteins, and phospholipids.

• Main groups: cholesterol and esters, glycerolesters (TG), fatty acids, phospholipids.

Cholesterol

• A sterol that is turned into bile acids and steroid hormones and is a key constituent of cell membranes.

• Largely endogenous and synthesized in liver.• Diet influences blood levels by 10 to 20%. • 30 to 60% of cholesterol in diet is absorbed

mixed with conjugated bile acids, phospho-lipids, fatty acids, and monoacylglycerides.

Triglycerides

• Most abundant dietary fat and compose 95% of all fat stored in adipose tissue.

• Prime function: furnish energy for the cell.• In the intestines, in the presence of lipases and

bile acids are hydrolyzed into fatty acids, glycerol and monoglycerides.

• After absorption, are reconstituted into chylo-microns.

• Unlike cholesterol, diet greatly affects levels.

Lipoproteins• Lipid-protein complexes in which lipids (which

are hydrophobic) are transported in the blood.

• Lipoprotein particles consist of a spherical hydrophobic core of TG or cholesterol esters surrounded by an amphophilic mono- layer of phospholipids, cholesterol, and apolipoproteins.

Lipoprotein Metabolism• Exogenous metabolism: from dietary fat to

chylomicrons to glycerol, free fatty acids, and monoglycerides.

• Endogenous metabolism: from chylomicron remnant to the liver to synthesis of VLDL, IDL, LDL, HDL.

Liver

IDL

Glycerol,free fatty acids,monoglycerides

Glycerol,free fatty acids,monoglycerides

Chylo-micron

RemnantRemnant

Lpl

Glycerol,free fatty acids,monoglycerides

Glycerol,free fatty acids,monoglycerides

Triglycerides (stored)Triglycerides (stored)

Energy (cells)Energy (cells)

VLDL

Lpl

Nascent HDL

HDL

Stored CholesterolStored CholesterolIntestinesIntestines

LCAT

Lpl

Lpl

LDLPeripheral

Tissues and Liver

Peripheral Tissues

and Liver

Lpl=lipoprotein lipase

EXOGENOUS LIPOPROTEIN METABOLISM

ENDOGENOUS LIPOPROTEIN METABOLISM

Chylomicrons

• Large particles produced by the intestines that are very rich in triglycerides (90%) of dietary origin, poor in cholesterol and phospholipids, and low in protein (1%).

• Less dense than water due to high lipid to protein ratio and floats.

• Cause of “milky” plasma.• Due to action of lipoprotein lipase, becomes

triglyceride-poor: REMNANT.

VLDL• Very-low-density lipoproteins.• Like chylomicrons, are triglyceride-rich (50%),

can float and make plasma turbid.• Unlike chylomicrons, are endogenous (liver).• Contains cholesterol and phospholipids (40%),

and protein (10%).• Action of Lpl gives rise to IDL.

LDL• Low-density lipoproteins• Make up 50% of total lipoproteins.• Even when in high concentration, does not

cause turbidity of plasma.• Esterified cholesterol makes up 50% of mass.• Subfraction: small particles with lower

cholesterol/apoB ratio that are seen in dyslipoproteinemia

associated with CAD.

HDL

• High-density lipoproteins• Contain 50% protein, mostly apoA-I and II.• Subclasses: HDL2 and HDL3.

• Low levels of apoA-I related to Coronary Artery Disease.

Apolipoproteins The hydrophilic components of lipoproteins (Lipids must

be in water soluble micellar structures for transport in plasma.)

Grouped by function: * ApoA: major protein of HDL; ApoA-I

activates LCAT, which esterifies cholesterol in plasma. * ApoB: major protein (95%) of LDL.

* ApoC: major protein of VLDL. ApoC-IIactivates lipoprotein lipase.

* ApoD and ApoE

Patient Preparation• Cholesterol:

* Nonfasting acceptable for screening* 12-14 hr fast for diagnosis

• HDL / LDL Cholesterol: 12 hr or more fasting• Triglycerides: 12 hr or more fasting• Apolipoproteins: 12 hr or more fasting

General Lab Precautions

Interference in laboratory testing will occur in specimens that are:* lipemic* turbid* hemolyzed* icteric

Do not use lubricated test tube stoppers for Triglyceride test as free glycerol can increase value (correct with sample blank).

Physiologic Variation*

Component Coefficient of Variation

• Total Cholesterol 5.0%• Triglycerides 17.8%• LDL-cholesterol 7.8%• HDL-cholesterol 7.1%• ApoA-1 7.1%• ApoB 6.4%

* Kafonek, 1992CV= SD/mean x 100

LIPID PROFILETotal Cholesterol, triglyceride, VLDL, HDL,

total cholesterol:HDL ratio,coronary heart disease risk

LIPID PROFILETotal Cholesterol, triglyceride, VLDL, HDL,

total cholesterol:HDL ratio,coronary heart disease risk

CholesterolCholesterol

>200 mg/dl

2 fastingsamples

TriglycerideTriglycerideNonfasting or fasting

< 200 mg/dl

Males > 160 mg/dlFemales > 135

Assessment of Lipids

2 fastingsamplesNormal

STOP

No 2o disorder 2o disorder presentPrimaryhyper-

lipidemia

Monitor lipid

profile

Screen for 2o disorders

HyperlipoproteinemiaTYPE LIPOPROTEIN PATTERN

I Extremely elevated TG due to chylomicrons

IIa Elevated LDLIIb Elevated LDL and VLDLIII Elevated cholesterol; presence of

-VLDL; VLDL-C/plasma TG ratio >0.3

IV Elevated VLDLV Elevated VLDL with chylomicrons

Fredrickson Classification*

Type Refrigerator Test Electrophoresis• I +, clear plasma Normal• IIa -, clear plasma High band• IIb -, cloudy plasma High & pre-• III +, cloudy plasma Broad band• IV -, cloudy plasma High -2band• V +, cloudy plasma High -2 band

*original method of classifying lipid-related diseases that enabled correlation of clinical disease syndromes to laboratory assessment.

PLA

SM

A A

T 4

o C

ClearFaintlyTurbid

1to2+Turbid

1to2+Turbid

Creamylayer

1to2+TurbidClear

Creamylayer

I II-A II-B III IV V

Standing Plasma Test

1. 2 ml of plasma in a test tube is allowed to stand inside a refrigerator

at 4o C undisturbed overnight.2. Chylomicrons accumulate as a floating “cream” layer.3. A turbid plasma contains excessive VLDL.

Lipid Disease Patterns• High cholesterol with High LDL-C• High Triglycerides with Normal Cholesterol• High Cholesterol and High Triglycerides with or

without Low HDL-C• Low Total Cholesterol with Low or Normal HDL• Isolated Low HDL• Isolated High HDL• Lp (a) Lipoprotein Excess

Diagnosing Hypertriglyceridemia

200-500 mg/dl >500 mg/dl< 200 mg/dl

Diet andretest inone year

Evaluate for risk factors:

AlcoholismDiabetes MellitusGlycogen Storage

DiseaseHypertensionHyperuricemiaHypothyroidism

MedicationsOral contraceptives

PancreatitisPregnancy

Renal disorder

No riskfactors+ Risk factor or+ Family history

Diet anddrugsDiet

Male > 160 mg/dl Female > 135 mg/dl

Lipid Interpretation for Coronary Heart Disease

Lipids normalT Chol <200 mg/dlLDL <130 mg/dlHDL >45 mg/dl

Lipids normalT Chol <200 mg/dlLDL <130 mg/dlHDL >45 mg/dl

Repeat after 5 yrsRepeat after 5 yrs

Lipids abnormalT Chol 200-239 mg/dlLDL 130-159 mg/dlHDL 35-45 mg/dl

Lipids abnormalT Chol 200-239 mg/dlLDL 130-159 mg/dlHDL 35-45 mg/dl

Lipids abnormalT Chol >240 mg/dlLDL >160 mg/dlHDL <35 mg/dl

Lipids abnormalT Chol >240 mg/dlLDL >160 mg/dlHDL <35 mg/dl

RISK FACTORSCerebrovascular diseaseCigarettes >10/dayDiabetes mellitusFH of CHD/vascular diseaseHypertensionMaleOcclusive peripheral vascular diseaseOverweight >30%

RISK FACTORSCerebrovascular diseaseCigarettes >10/dayDiabetes mellitusFH of CHD/vascular diseaseHypertensionMaleOcclusive peripheral vascular diseaseOverweight >30%

Diet and/orDrugs

Diet and/orDrugs

DietRetest in 1 yr

0-1 risk factorNo coronary HD0-1 risk factorNo coronary HD

2 or morerisk factors2 or morerisk factors

Risk Factors for CHD• POSITIVE RISK FACTORS:

- Age: Male >45 yr; Female >55 yr or premature menopause.

– Family history of premature CHD(<55 yr for father and <65 yr for mother)

– Current cigarette smoking– Hypertension or on anti-HPN medication

– Low HDL-cholesterol– Diabetes mellitus

• NEGATIVE RISK FACTOR:– High LDL-cholesterol: >60 mg/dl

from NCEP Guidelines

Adults: Cholesterol and HDL*• Total Cholesterol:

Desirable level <200 mg/dlBorderline High 200-239 mg/dl

High >240 mg/dl

• HDL-CholesterolLow <35 mg/dl

* National Cholesterol Education Program

Children of Hypercholesterolemics

T Chol LDL-C Category (mg/dl) (mg/dl)

Acceptable <170 <110Borderline 170-199 110-129High >200 >130

from NCEP Expert Panel on Children and Adolescents

GOOD DAY!