The transport of long chain fatty acids across the cell plasma membrane and through the cell cytoplasm as well as their subsequent utilization for energy production and glycerolipid synthesis may depend, in part, upon their interaction with fatty acid binding proteins (FABP) present in both the cell plasma membrane and cytoplasm. Although only one molecular form of membrane-associated FABP has been characterized in liver (LPM- FABP; 40 dDa), three distinct 14-15 kDa cytoplamic FABP have been purified, respectively, from liver (L-FABP), intestine (I- FABP), and heart muscle (M-FABP). The cytoplasmic FABP may perform specialized functions in fatty acid transport and utilization in these different tissues. The nature of the precise functions of the FABP, however, remains undefined. L-FABP, in addition, is not uniformly expressed in liver cells, but predominates in lobular zone I (periportal) hepatocytes. However, the significance of the differential hepatocellular expression of L- FABP in relation to specific pathways of fatty acid metabolism and in relation to the ability of hepatocytes to withstand potentially toxic effects of long chain fatty acids is unknown. The aims of this proposal have as their broad goal, the elucidation of the function of the cellular FABP and are to: (1) determine the relationship between L-FABP abundance and long chain fatty acid utilization and toxicity in (a) hepatocytes isolated from zones abundant (periportal hepatocytes) and sparse (perivenous hepatocytes) in their expression of L-FABP, as well as in (b) hepatocyte monolayer cultures following alteration of the L- FABP pool size by liposome-mediated injection of anti-L-FABP IgG or pure L-FABPl (2) determine the comparative mechanisms of L-FABP and M-FABP effects on mitochondrial and microsomal long chain acyl-CoA synthesis and to determine whether radiolabeled L-FABP and M-FABP bind in a specific manner to microsomal and mitochondrial membranes; (3) define the putative function of LPM-FABP in membrane fatty acid translocation by identifying and characterizing liver plasma membrane proteins labeled with photoreactive long chain fatty acid probes and by developing improved methods for purifying LPM-FABP in order to determined its fatty acid transport function reconstituted in liposomes. Collectively, the proposed studies will provide important new information regarding the poorly understood function of cellular FABP and their role in the cellular transport and regulation of fatty acid metabolism.