Metabolism of Very Low Density Lipoproteins

Abstract

The plasma lipoprotein system is traditionally separated into several lipoprotein families defined by physical, chemical, and immunochemical methods. Yet, during the last few years, it has been shown that all plasma lipoproteins are metabolically related. Studies carried out in humans (1 – 6) and rats (7 – 10.) have demonstrated unequivocally that apo C units, found predominantly in very low density lipoproteins (VLDL) and high density lipoproteins (HDL) represent one common pool of apoprotein, and that the apo B moiety of VLDL is the major precursor in plasma of the apoprotein moiety of low density lipoprotein (LDL). The transfer of apo C between VLDL and HDL was shown to occur via two different pathways: an exchange phenomenon (1, 2); and transfer of apo C molecules from VLDL (and chylomicrons) to HDL observed during degradation of the triglyceride-rich lipoproteins, and from HDL to VLDL (and chylomicrons) during synthesis and secretion of these lipoproteins (4, 5). The two pathways were first demonstrated in vivo and can be reproduced in vitro. In contrast, the conversion of apo B from VLDL to LDL was shown so far only in vivo, and presumably necessitates interactions present only in the intact animal. Whereas this general scheme is well substantiated, the fine details of the interconversion process are yet unknown. Thus, relatively little is known about the biochemical and physiochemical characteristics of partially degraded lipoproteins, the role of different lipoprotein-metabolizing enzymes in the interconversion process, and the biological significance of the interconversion process in regulating plasma lipoprotein levels. In the present report some of these problems are discussed in the light of our new studies.