week 9: Lipids and lipoproteins

Roles of lipids (fats)

• rich source of energy and efficient way for body to store calories

• integral part of cell membranes and structure 

Fatty acids

linear chains of carbon-hydrogen bonds terminating in carboxyl group

triglycerides

3 fatty acid molecules attached to 1 molecule of glycerol by ester bonds

Phospholipids

• similar to triglycerides, except with only 2 esterified fatty acids

• third position on glycerol backbone contains phospholipid head group

• types of head groups: choline, inositol, serine, ethanolamine, all of which are hydrophilic in nature

Cholesterol

• unsaturated steroid alcohol containing 4 rings and single side chain tail

• synthesized almost exclusively by animals; not readily catabolized by most cells, not a source of fuel

All lipoproteins

• typically spherical; diameters of 10-1,200 nm

• composed of lipids and proteins; deliver fuel to peripheral cells

Chylomicrons

• largest and least dense; diameters as large as 1,200nm

• produced by intestine; deliver dietary lipids to hepatic and peripheral cells

• transport exogenous triglycerides

• normally not in fasting serum

• plasma appearance

Very-low density lipoproteins (VLDL)

• produced by liver

• major carriers of endogenous triglycerides

• Apo B

Low-density lipoproteins (LDL)

• form as a result of lipolysis of VLDL

• readily taken up by cells via LDL receptors in liver and peripheral cells

• significantly smaller than VLDLs; can infiltrate extracellular space

Lipoprotein a (lpa)

• LDL-like particles; heterogeneous in size and density

• plasma levels of Lp(a) vary widely among individuals in population but remain relatively constant within a person 

High-density lipoproteins

• smallest and most dense; synthesized by liver and intestine

• can exist either as disk-shaped or spherical particles

• capable of removing excess cholesterol from peripheral cells

• highly heterogeneous; can be separated into 13 or 14 subfractions

lipids and lipoproteins

insoluble in water

primary lipids

• cholesterol

• triglycerides

• lipoproteins (Chylomicrons, LDL, HDL)

• phospholipids

• fatty acids

Exogenous lipid metabolism (digestion)

Endogenous triglycerides

• liver biosynthesis from glycerol and fatty acids

• transported as VLDL 

• hydrolyzed by lipoprotein lipase (LPL)

  • VLDL → LDL (cholesterol rich)

Endogenous cholesterol

• free cholesterol synthesized from acetyl CoA

  • bile acid (breaks down cholesterol)

  • steroid hormone precursor

  • synthesis = 90%, dietary = 10% of T. cholesterol

  • insoluble

Free cholesterol esterification (fatty acid addition)

• LCAT (Lecithin: cholesterol acyltransferase)

  • blood

• ACAT (Acyl-cholesterol acyltransferase)

  • intracellular 

• 1/3 free, 2/3 esterified 

Synthesis modulation by [free cholesterol]

• ↑ free cholesterol inhibits HMG-CoA reductase

  • rate limiting step

  • cholesterol-lowering drug (blocks chol syn by inhibiting HMG-CoA reductase) 

Transport by lipoproteins

• LDL transports to the cells

• HDl transports excess from cells to liver

  • excreted in bile, steroid hormone synthesis

Apolipoproteins

• proteins that binds lipids

• forms lipoproteins to help transport lipids

  • 5 types A→E

• protein on a lipid-protein molecule

  • ex. Apolipoprotein a: major protein on HDL

LDL “bad” cholesterol

• Apo B

• transports cholesterol to peripheral cells

• binds to LDL receptors

  • ↑↑ LDL → ↑ Atherosclerosis 

HDL “good” cholesterol (helpful)

• Apo A

• transports excess cholesterol from peripheral cells to liver

  • ↑↑HDL → ↓ Risk of atherosclerosis 

Lipoprotein (a) significance

• LDL-like molecules containing an extra apolipoprotein - apo (a) [usually on HDL]

• elevations thought to be independent factor for increased CHD risk

• genetic based, not dietary

• most LDL lowering drugs don’t lower Lp(a) 

Dyslipidemias (abnormal serum lipids)

• family history, physical exam, nutritional/social habits

• assess risk, diagnose, monitor: atherosclerosis and coronary artery disease (CAD)

• genetic or acquired

  • altered lipoprotein synthesis, transport catabolism

  • hyperlipoproteinemias/hypolipoproteinemias 

Hyperlipoproteinemias

• hypertriglyceridemia

• hypercholesterolemia

• combined hyperlipidemia

hyperlipoproteinemia diagnosis

• intital ↑↑ triglyceride and/or cholesterol

• repeat with 12hr fasting specimen

• Xanthomas (fat nodules underneath skin)

• primary or secondary disorder

1° Hypertriglyceridemia

• inherited chylomicron with VLDL clearance defect

• ↓ lipoprotein lipase activity

2° Hypertriglyceridemia

• Dietary

• hypothyroidism

• DM

• nephrotic syndrome

• obstructive liver disease

Hypertriglyceridemias 

• ↑↑ chylomicrons

• ↑↑ VLDL

• ↑↑ chylomicrons and VLDL

LDL-cholesterol

• familial hypercholesterolemia

  • ↓ LDL receptor #/activity

    • ↑atherosclerosis and CAD

    • marked ↑↑T. chol., LDL

• many 2° conditions

  • ↑↑LDL, TG, VLDL

↑HDL-Cholesterol

• familial hyper-alphalipoproteinemia

  • ↑↑HDL , sl ↑ T. chol

  • ↓CAD risk

Combined hyperlidpidemia

• Primary ↑↑ TG and T. Chol

  • dysbetalipoproteinemia 

• impaired VLDL → LDL

  • ↓ LDL

  • asymptomatic, possible xanthomas 

Combined hyperlipidemia conditions

• familial high cholesterol

• coronary heart disease

• diabetes

• alcohol

• hypothyroidism adds to problem 

Primary hypolipoproteinemias

• ↓T. Chol and HDL

• Tangier disease

  • ↓↓↓ Apo A synthesis

  • ↑ Cholesterol esters → orange-yellow tonsils

  • ↑↑CAD risk

Hypobetalipoproteinemia

• ↓ T. Chol and LDL

• ↓ Apo B synthesis

Secondary hypolipoproteinemias

• hyperthyroidism

• liver parenchymal disease-destruction and regeneration of liver cells leading to cirrhosis

Total cholesterol methods

HDL-cholesterol methods

• selective lipoprotein precipitation

  • reagent

    • divalent cations in buffer, heparin or dextran 

  • Apo B-containing lipoproteins precipitate, leaving HDL in supernatant

  • centrifuge

  • measure HDL in supernatant

• ↑ TG interferes → dilute and repeat PPT step

Homogenous (direct) HDL

• no manual separation step

• selective inhibition of non-HDL lipoproteins with T. Chol reagent

• ↑TG does not interfere with most methods

Friedewald calculation

• LDL-C = (total cholesterol) - (HDL-C) - (TG/5)

  • Valid only when TG < 400mg/dL

• ↑↑LDL/HDL ratio = ↑↑CAD risk

LDL-cholesterol methods

• analytical methods

  • selective lipoprotein preciptation

  • immunoseparation

4°C overnight EDTA plasma appearance

• creamy top layer = ↑ chylomicrons

• turbidity = ↑↑VLDL

  • yellow-orange color = ↑↑LDL??

Lipoprotein separation

• TG>400 mg/dL OR suspect abnormal lipoproteins: B-VLDL (floating B-lipoprotein)

• electrophoresis - lipid stain

• ultracentrifugation 

  • separates by relative density 

Triglyceride method 

Desirable lipid ranges

• Total cholesterol = <200mg/dL

• HDL-cholesterol = >37 mg/dL (male) >40 mg/dL (female)

• LDL-cholesterol = <130 mg/dL

• triglyceride = <250 mg/dL