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 found in high abundance in the cytosol of adipose cells. To better understand the role of this protein in intracellular free fatty acid (ffa) transport, 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 aqueous solubility, such as shorter chain length and unsaturation, did not increase the AOffa transfer rate. In addition, solution conditions that increase the aqueous solubility of ffa, such as decreasing ionic strength and increasing pH, had little effect on AOffa transfer from A-FABP to membranes. These results suggest that AOffa do not transfer through the aqueous phase. The small entropic contribution to the free energy of the transfer process provides further evidence that AOffa may not travel through the surrounding aqueous environment when transferred from A-FABP to phospholipid membranes. Finally, the rate of AOffa transfer from A-FABP was directly dependent on the concentration of the acceptor membranes. These studies suggest that AOffa transfer from A-FABP to phospholipid vesicles may occur via transient collisional interactions 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 possesses 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., & Storch, J. (1992) J. Biol. Chem. 267, 77–82]. These differences indicate that structural divergence among FABP may be translated into functional differences, as evidenced here by the mechanism of AOffa transfer to membranes.