Cholesterol Structure and Properties

Lipids

• Lipids are constituents of living cells composed mostly of C-H bonds.
• They are molecules that combine water-insoluble dietary lipids and water-soluble proteins (apolipoproteins) for transportation throughout the body.

Classification of Lipids

• Fatty Acids:
  • Short (4-6), Medium (8-12), Long (12-18), and Very long (16-24) chains
  • Unesterified - bound to albumin
  • Esterified - constituent of triglycerides or phospholipids
  • Saturated - no double bonds
  • Monounsaturated - one double bond
  • Polyunsaturated - contain 2 or more double bonds
• Triglycerides:
  • Contain 3 fatty acids attached to 1 molecule of glycerol by ester bonds
  • No charged groups, water-insoluble, neutral lipid
• Phospholipids:
  • Contain 2 fatty acids attached to 1 molecule of glycerol
  • Amphipathic - hydrophobic head and hydrophilic tails
  • Phospholipid head groups (hydrophilic): choline, inositol, inositol phosphates, glycerol, serine, and ethanolamine

Lipoproteins

• Classification of Lipoproteins:
  • Chylomicrons:
    • Largest lipoproteins with the lowest density
    • Formed in the intestines, transport triglycerides after a meal
    • Composed of 86% triglyceride, 5% cholesterol, 7% phospholipid, and 2% apolipoprotein
  • VLDL (Very Low-Density Lipoprotein):
    • Carries endogenous triglycerides synthesized in the liver
    • Composed of 55% triglycerides, 19% cholesterol, 18% phospholipid, and 8% apolipoprotein
  • IDL (Intermediate Density Lipoprotein):
    • Transitional form between VLDL and LDL
    • Carries endogenous triglycerides and cholesterol esters
    • Composed of 23% triglycerides, 38% cholesterol, 19% phospholipid, and 19% apolipoprotein
  • LDL (Low-Density Lipoprotein):
    • Primary carrier of cholesterol
    • Also known as "bad cholesterol"
    • Composed of 50% cholesterol, 22% protein, and has apoprotein B-100 on its surface
  • HDL (High-Density Lipoprotein):
    • Also known as "good cholesterol"
    • Removes excess cholesterol in peripheral tissues and transports it to the liver
    • Composed of 50% protein, 28% phospholipids, 19% cholesterol, and has apoproteins A-I and A-II on its surface
  • Lp(a):
    • Composed primarily of cholesterol esters, phospholipids, and apolipoprotein (a) and B-100
    • Elevated levels associated with increased risk for coronary heart disease, myocardial infarction, and cerebrovascular disease

Lipoprotein Physiology and Metabolism

• Lipid absorption: lipids are absorbed in the intestine and packed into chylomicrons
• Endogenous pathway: TAG in the liver is packed into VLDL through lipolysis
• Exogenous pathway: lipoprotein lipase hydrolyzes TAG in chylomicrons
• Reverse cholesterol pathway: HDL removes excess cholesterol and delivers it to the liver### Lipid Profile
• Lipid profile includes total cholesterol, HDL-C, LDL-C, VLDL-C, and triglycerides.
• Reference method: 3-step procedure of ultracentrifugation, heparin manganese precipitation, and Avell-Kendall assay.
• LDL-C estimation: indirect method uses Friedewald equation, direct method uses precipitation or homogeneous method.

Triglyceride Measurement

• Enzymatic method involves glycerol kinase.
• Correction methods for endogenous free glycerol: blanking, lipase-free reagent, and designated calibration blanking.

Lipoprotein Electrophoresis

• Uses agarose gel or PAGE.
• Not desirable for quantitation but useful for qualitative analysis.

Ultracentrifugation

• Preparative ultracentrifugation: uses sequential density adjustments to fractionate major and minor classes of lipoproteins.
• Density-gradient methods: permits fractionation of several or all classes of lipoproteins in a single run.

Hyperlipoproteinemia

• Classification: type I, type IIa, type III, type IV, and type V.
• Characteristics: elevated chylomicrons, LDL, IDL, VLDL, and triglycerides.

Lipid Abnormalities

• Familial combined hyperlipidemia (FCHL): increased plasma levels of total and LDL cholesterol or triglyceride.
• Familial hypertriglyceridemia: moderate elevation of triglyceride with excess VLDL production.
• Familial hypercholesterolemia: increased LDL cholesterol with normal or moderately elevated triglyceride.
• Hyperapobetalipoproteinemia: associated with VLDL and apo B-100 overproduction in the liver.

Apo A-1, Apo B, and Lp(a) Clinical Significance

• Apo A-1: major protein found in HDL, activates lecithin cholesterol acyltransferase, and removes free cholesterol from extrahepatic tissues.
• Apo B-100: major protein found in LDL, associated with increased risk of coronary artery disease.
• Lp(a): independent risk factor associated with impaired plasminogen activation and decreased fibrinolysis.

Liver Chemistries

• Liver functions: synthetic function, detoxification and excretion, storage, and transformation.
• Liver disease: hepatomegaly, jaundice, and liver chemistry tests (serum bilirubin, ALT, AST, ALP, and GCT).

Bilirubin Test

• Elevation in the test suggests cholestasis or liver injury.
• Conjugated bilirubin: water-soluble, produced by liver cells, and released in the common bile duct.
• Unconjugated bilirubin: produced by excessive destruction of erythrocytes.

Methods of Bilirubin Determination

• Jendrassik & Grof method: reacts with diazo reagent to produce purple azobilirubin.
• Evelyn & Malloy method: based on Van Den Bergh reaction, susceptible to hemoglobin interference.### Liver Chemistries
• Liver performs various functions and releases metabolites into the bloodstream, which can indicate liver disorders
• Liver tests can diagnose or monitor liver disease or damage
• Classification of liver chemistries:
  • Bilirubin tests (serum bilirubin and urobilinogen tests)
  • Liver protein tests (albumin, globulin, and prothrombin time)
  • Liver enzyme tests (ALT, AST, GGT, and ALP)

Liver Enzyme Tests

• ALT (Alanine Aminotransferase):
  • Found mainly in the liver, but also in other tissues
  • Increases in serum when hepatic cells are damaged or destroyed
  • Normal range: 7-45 U/L
• AST (Aspartate Aminotransferase):
  • Found in the liver and other tissues
  • Increases in serum when hepatic cells are damaged or destroyed
  • Normal range: 5-35 U/L
• AST to ALT ratio: useful in distinguishing acute from chronic liver diseases
• Elevated levels of AST and ALT can indicate:
  • Hepatocellular damage
  • Acute viral hepatitis
  • Drug/toxin-related hepatic necrosis
  • Autoimmune hepatitis

Alkaline Phosphatase (ALP)

• Found in the bile canaliculi and bones
• Increases in serum during:
  • Biliary obstruction or cholestasis
  • Bone growth and development
  • Inherited hyperphosphatasia
• Decreases in serum during:
  • Inherited hypophosphatasia
• Normal range: varies with age and sex

Gamma-Glutamyl Transferase (GGT)

• Found in the cell membranes of the liver and gallbladder
• Increases in serum during:
  • Cholestasis
  • Chronic alcoholism
  • Warfarin and phenytoin use
• Normal range: 6-55 U/L (men), 5-38 U/L (women)

Lipid Determinations

• Lipids are transported in plasma via lipoproteins
• Lipoproteins:
  • Chylomicrons
  • High-density lipoprotein (HDL)
  • Low-density lipoprotein (LDL)
  • Very low-density lipoprotein (VLDL)
  • Intermediate-density lipoprotein (IDL)
• Lipid profile:
  • Total cholesterol
  • HDL cholesterol
  • LDL cholesterol
  • Triglycerides
• Importance of lipid profile:
  • Risk assessment for cardiovascular disease
  • Monitoring lipid-lowering therapy

Cholesterol Determination

• Total cholesterol level:
  • Important factor in cardiovascular disease process
  • High levels increase risk of atherosclerosis
• HDL cholesterol:
  • Responsible for cholesterol uptake from cells
  • High levels associated with reduced cardiovascular risk
• LDL cholesterol:
  • Involved in cholesterol transport to peripheral cells
  • High levels associated with increased cardiovascular risk

Triglycerides

• Triglycerides are esters of glycerol with three fatty acids
• Transported in plasma bound to apolipoproteins forming VLDL and chylomicrons
• Measurement used in:
  • Screening for lipid status
  • Monitoring lipid-lowering measures
• High triglyceride levels associated with:
  • Atherosclerotic risks
  • Liver, kidney, and pancreatic diseases