FtEGULATION OF LIPID METABOLISM*

The past 15 years have witnessed an explosive increase in our under-standing of lipid metabolism and its central role in caloric homeostasis. This period of growth has seen the ascendency of adipose tissue from an ignored, inert repository for excess dietary fat, to the pinnacle of popu-larity for study of its biochemical virtuosity and its endocrinological promiscuity. In the next 30 minutes, I’d like to consider some aspects of this field, concentrating particularly on the various levels at which control of fat mobilization is exerted. To put these remarks in context, it will be useful to examine the overall features of lipid metabolism in mammals. A schematic repre-sentation is given in figure 1. The fatty acids which are stored in adipose tissue as triglycerides may arise from three sources: dietary fat, de novo synthesis in the liver, or synthesis within the adipose tissue itself from glucose or amino acid precursors. The relative importance of liver and adipose tissue for endogenous synthesis of fatty acids apparently varies widely from species to species, with the liver playing the dominant role in such diverse species as pigeons and man, and the adipose tissue playing the more important role in rats and mice. Regardless of their origin, fatty acids en route to adipose tissue travel in blood esterified with glycerol and in lipoprotein complexes. Some chylomicra formed in the gut reach the adipose tissue directly, but most apparently are re-packaged in the liver as smaller low density lipoprotein molecules. The hepatic production of the protein portion of lipoproteins and the repackaging process will be the subject of several papers presented later this afternoon. Fig.1. Schematic representation of fat metabolism. *Taken from the introductory remarks given at the session on Lipi