Essentially, lipoproteins are grouped into three major categories:
1. Chylomicron (CM) and very low density lipoprotein (VLDL). These are relatively low in protein, phospholipid, and cholesterol, but high (55 to 95 percent) in triglyceride. In more general terms, these particles are referred to as triglyceride-rich lipoproteins.
2. Intermediate density lipoproteins (IDL) and low density lipoproteins (LDL). These are characterized by high levels of cholesterol, mainly in the form of cholesteryl esters. The latter form of cholesterol is highly insoluble. Since up to 50 percent of the LDL mass is cholesterol, it is not surprising that LDL has a significant role in the development of atherosclerotic disease.
3. High density lipoprotein (HDL). The hallmarks of these particles are their high protein content (50 percent) and relatively high phospholipid content (30 percent). HDL are generally divided into two subclasses HDL2 and HDL3; of the two, HDL2 are large and less dense, and HDL3 are smaller and more dense.
PLASMA LIPOPROTEIN CONCENTRATIONS
The most commonly used clinical indicator for measuring potential risk of premature cardiovascular disease is the level of plasma lipids. Fasting levels of triglyceride, cholesterol, and HDL-cholesterol can often be used to identify possible abnormalities. Females characteristically have lower triglyceride concentrations (80 mg/dl) than males (120 mg/dl) and have higher HDL cholesterol (55 mg/dl versus 43 mg/dl for males). For comparison, the lipid levels from cordblood of normal, full-term newborns are also provided. The newborn infant has triglyceride and total cholesterol levels one-half to one-third those of the adult. The HDL cholesterol levels are relatively high (35 mg/dl) in the newborn, where the ratio of total cholesterol to HDL cholesterol is 2 compared with the adult values of 3.5 for females and 4.6 for males. HDL are considered beneficial; that is, they are protective against atherosclerosis, whereas LDL are positive risk factors. Therefore, the lipid levels in infants are perhaps the most "ideal," for at birth, plasma total cholesterol is low while HDL cholesterol is relatively high. Except for genetic abnormalities (such as homozygous familial hypercholesterolemia, to be discussed later), the vascular walls of neonates are free of fatty streaks. Fat accumulation appears in the first years of life, indicating that dietary input and environmental factors probably influence the initiation and progression of atherosclerosis. At birth, no distinction can be seen between male and female infants since sex hormone concentrations are low in levels and apparently have little metabolic influence at this stage of development.