Apolipoproteins in lipid transport, an impressionist view.

The major function of the lipoproteins is to provide the organism with a transport system for triacylglycerol and cholesterol. This system appears to be both highly effective and dynamic in normal subjects. While about 150 g of fat per day are fully absorbed from the gut, the plasma triglyceride pool in the blood is approximately 3 g in the fasting state and does not exceed 12 g in the postabsorbtive state. About 1 g/day of cholesterol is produced in the liver and 0.2-0.5 g is absorbed from the gut, but the plasma pool is kept at a steady level under 6 g. It is therefore understandable that minor abnormalities in the proteins that regulate lipid transport lead to grossly abnormal plasma lipid levels. These abnormal lipid levels in turn are associated with lipid accumulations at abnormal sites: the artery wall, macrophages, tendons, skin and various other tissues. This causes the clinical picture doctors observe, such as atherosclerosis, xanthomas, corneal opacifications and lipaemia retinalis. Clinicians are used to the description of abnormal lipid transport in terms of the lipids proper, for example hypercholesterolaemia, hypertriglyceridaemia and combined hyperlipidaemia. These parameters can be measured easily and relatively accurately. Cholesterol and triglyceride are, however, innocent bystanders in lipoprotein disorders. We will therefore describe the transport disturbances in terms of the protein abnormalities of apolipoproteins, receptors, transfer proteins and enzymes. This enhances the pathophysiological understanding on the one hand and on the other provides arguments for measuring apolipoproteins in the assessment of atherosclerosis risk. This article is by no means a complete review of the literature; recently two excellent books have appeared with reviews of the different lipoprotein subjects.'2 Rather, it is an impressionist picture of a rapidly developing scene.