[The role of apolipoproteins in lipid metabolism]

There is little doubt today that apolipoproteins play a key role in lipid metabolism and thus in atherogenesis. There are five major classes of apo Lp known: Apo AI, the main component of HDL not only mediates the action of LCAT, a key enzyme in cholesterol metabolism, but also through specific cell receptors is responsible for the reverse cholesterol transport, which is discussed as the main atherogenic process. Apo B is necessary for the secretion of neutral lipids out of the liver and the intestine. In addition, apo B containing lipoproteins are recognized by specific cell surface receptors leading to the fast removal of cholesterol rich fractions from circulation. Apo C proteins regulate the activity of lipoprotein lipase, the key enzyme of triglyceride metabolism. Apo E containing lipoproteins are recognized by the B/E-receptor with a 10 to 100 fold affinity. There exists, however, another specific receptor for Apo E, which is responsible for the fast removal of the atherogenic remnants from circulation. Apo E in addition serves to secrete deposited cholesterol out of macrophages and foam cells. Apolipoprotein(a) is a peculiar fraction of apo B containing lipoproteins whose biological function is completely unknown. Cloning of the cDNA revealed striking similarities of apo(a) with the structure of plasminogen. The cross connection of Lp(a) with hemostasis and thrombogenesis is currently focus of intensive research. The knowledge of the specific function of apolipoproteins in lipid metabolism arose to a great extent from the characterization of apo-Lp isoforms and their impact of atherogenesis. In addition, intensive research by molecular biology techniques helped to unravel the pathophysiology in a wide array.