MECHANISM OF FLUORESCENT FATTY-ACID TRANSFER FROM ADIPOCYTE FATTY-ACID-BINDING PROTEIN TO MEMBRANES

Adipocyte fatty acid binding protein (A-FABP) is a 15-kDa protein foun d in high abundance in the cytosol of adipose cells. To better underst and the role of this protein in intracellular free fatty acid (ffa) tr ansport, the mechanism of ffa transfer from A-FABP to model membranes was examined by monitoring the transfer of fluorescent anthroyloxy ffa (AOffa) to small unilamellar phospholipid vesicles, using a resonance energy transfer assay. Structural features of ffa that increase aqueo us solubility, such as shorter chain length and unsaturation, did not increase the AOffa transfer rate. In addition, solution conditions tha t increase the aqueous solubility of ffa, such as decreasing ionic str ength and increasing pH, had little effect on AOffa transfer from A-FA BP to membranes. These results suggest that AOffa do not transfer thro ugh the aqueous phase. The small entropic contribution to the free ene rgy of the transfer process provides further evidence that AOffa may n ot travel through the surrounding aqueous environment when transferred from A-FABP to phospholipid membranes. Finally, the rate of AOffa tra nsfer from A-FABP was directly dependent on the concentration of the a cceptor membranes. These studies suggest that AOffa transfer from A-FA BP to phospholipid vesicles may occur via transient collisional intera ctions between the protein and membranes. Such a mechanism is similar to that found recently for AOffa transfer from heart FABP [Kim, H. K., & Storch, J. (1992) J. Biol. Chem. 267, 20051-20056], an FABP which p ossesses a high degree of sequence homology (62% identity) with A-FABP , but different from the aqueous diffusion mechanism described for the more distantly related (20% homology) liver FABP [Kim, H. K., & Storc h, J. (1992) J. Biol. Chem. 267, 77-82]. These differences indicate th at structural divergence among FABP may be translated into functional differences, as evidenced here by the mechanism of AOffa transfer to m embranes.