Regulation of fluorescent fatty acid transfer from adipocyte and heart fatty acid binding proteins by acceptor membrane lipid composition and structure.

Adipocyte and heart fatty acid binding proteins (A-FABP and H-FABP) are closely related members of the FABP family. Unlike the more distantly related liver FABP, these FABP have been proposed to transfer free fatty acids to model membranes by a collisional mechanism (Wootan, M. G., Bernlohr, D. A., and Storch, J. (1993) Biochemistry 32, 8622-8627; Kim, H. K., and Storch, J. (1992) J. Biol. Chem. 267, 20051-20056). Collisional transfer requires that the acceptor membranes interact with FABP during the transfer process. We, therefore, examined whether the acceptor membrane structure and lipid composition regulate the rate of anthroyloxy-labeled palmitate (2AP) transfer from A- and H-FABP, using a fluorescence resonance energy transfer assay. The results showed that 2AP transfer from A- and H-FABP was more rapid to acceptor vesicles containing acidic phospholipids and was slower to positively charged membranes. In addition, the rate of 2AP transfer from A- and H-FABP was enhanced by unsaturation of the phosphatidylcholine acyl chains and was slowed by the presence of cholesterol or sphingomyelin in the acceptor membranes. These latter changes were small but of a similar magnitude and together suggest that fatty acid transfer from A- and H-FABP was slower to membranes of greater lipid order. Since transfer by an aqueous diffusion mechanism would be unaffected by acceptor membrane properties, these studies strengthen the hypothesis that free fatty acid transfer from A- and H-FABP to membranes occurs via a collisional mechanism.