Lipoprotein apoprotein metabolism.

The study of the plasma lipoproteins in recent years has significantly broadened our understanding of lipid metabolism and lipid transport. Incisive studies into the structure and function of the lipoprotein species have contributed to significant advances in our understanding and management of lipid disorders. All of the lipoproteins are related and interact dynamically within the vascular system. Many of the processes involved in lipoprotein metabolism are regulated by enzyme activity that can influence plasma lipoprotein levels. An understanding of these interactions and interrelationships has been made possible by numerous studies over the past decade. Until about 10 years ago lipoproteins were classically viewed as isolated families or species as defined either by ultracentrifugation or by electrophoresis. The operational separations by density or electrophoretic mobility allowed for the naming and quantification of various abnormalities, but provided little insight as to the cause of these abnormalities or of the role of the lipoproteins in lipid transport. A more functional view of the lipoproteins awaited the isolation, characterization and metabolic study of the lipoprotein apoproteins. Numerous reviews have appeared over the last several years summarizing our current state of knowledge of the plasma lipoproteins, their composition, structure, and function as well as the structure, distribution, and function of the lipoprotein apoproteins (1-6). It is not our intent herein to provide another comprehensive review of the plasma lipoproteins. Rather we will focus our attention on the functional interrelationships of the lipoprotein apoproteins and the information these relationships reveal about the mechanisms and determinants involved in the lipoprotein metabolism within the vascular bed. Though there are still some disagreements about nomenclature, in this review we will employ the nomenclature first introduced by Alaupovic et al. (7). It is now widely agreed that there are at least seven different lipoprotein apoproteins. These apoproteins differ in their primary, secondary, and tertiary structure as well as in their functions and distribution throughout the lipoprotein spectrum (Tables 1, 2). Five of these lipoprotein apoproteins have already been sequenced (8-16) and numerous reports have appeared detailing the unique characteristics that allow these proteins to bind lipid (1-6). Some, like apoprotein B and apoprotein A-I1 appear to have a primary role in the transport of lipid, while others like apoproteins A-I and C-I1 serve as activators (coenzymes) for enzyme systems involved in lipoprotein lipolysis and interconversion (1 -6). We will review what is known about the metabolism of the individual lipoprotein apoproteins and attempt to relate these apoprotein systems to one another as well as to normal and abnormal lipid transport.