The Plasma Lipoproteins: Current Concepts of Metabolism and Structure with Clinical Implications

Abstract

The work I will summarize on the plasma lipoproteins comes from many laboratories and has been recently reviewed in considerable detail (1–5). I will emphasize information which is pertinent to structure and metabolism with clinical medicine, beginning with definitions and classifications. All of the plasma lipoproteins appear spherical in negatively stained electron micrographs (Fig. 1). The chylomicrons, which carry dietary triglyceride, are the largest of the particles and are elevated in Type I and V hyperlipoproteinemia by the Fredrickson, Levy and Lees typing system. The very low density lipoproteins (VLDL) are the second largest in size. They carry the triglyceride made endogenously in the body and are increased in concentration in types IIb, III, IV and V hyperlipoproteinemia. The low density lipoproteins (LDL) carry one-half to two-thirds of the cholesterol and are high in types IIa and IIb. The high density or alpha-lipoproteins do not enter the typing classification but may be very important in atherosclerosis. There is also a family called intermediate density lipoproteins (IDL), which have a density between VLDL and IDL. Clinical interest in lipoproteins in recent years is due largely to the known risk of hyperlipoproteinemia in the development of premature atherosclerosis.