Intestinal fatty acid binding protein (IFABP) is an intracellular protein of 15.1 kDa that binds long-chain fatty acids (1). IFABP is abundantly expressed in small intestine enterocyte cytosol and together with liver fatty acid binding protein (LFABP) represents a high proportion of the total cellular protein. It is hypothesized that these fatty acid binding proteins (FABPs) are important in the intracellular transport of FAs, and accumulating indirect and direct evidence supports this idea (2). Several differences found between these two proteins, including tissue distribution, ligand specificity, and binding stoichiometry, have suggested that they are functionally different. Furthermore, we have demonstrated that they transfer FA to and from phospholipid membranes through markedly different mechanisms: whereas LFABP uses a diffusion-mediated mechanism, IFABP transfers fatty acids through direct collisional interactions (3, 4). We have shown that the ␣ -helical domain of IFABP is responsible for the collisional mechanism of transfer (5, 6) and have demonstrated that the ␣ -helical domain of LFABP is responsible for this protein's diffusion-mediated transfer mechanism (7).The collisional FA transfer mechanism described for IFABP has also been found for several other FABPs examined to date, including heart, adipocyte, keratinocyte, and brain (8-11). As most of the enzymes involved in FA metabolism are membrane-bound, it is possible that IFABP could modulate lipid metabolic enzyme activity via direct interactions with membrane lipids and/or proteins. To gain insight into IFABP function, it is important to determine those characteristics of the protein and the acceptor membrane that regulate the fatty acid transfer process.In the present study, we analyzed the influence of electrostatic interactions on the collisional mechanism of FA transfer from IFABP to phospholipid membranes. Using a fluorescence resonance transfer assay, the transfer of anthroyloxy-labeled fatty acids (AOFAs) from IFABP to model membranes with different phospholipid head group and acyl chain composition was analyzed. Furthermore, IFABP Abbreviations: 12AO, 12-(9-anthroyloxy)oleic acid; 12AS, 12-(9-anthroyloxy)stearic acid; AOFA, anthroyloxy-labeled fatty acid; CL, bovine heart cardiolipin; EPC, egg phosphatidylcholine; FA, fatty acid; FABP, fatty acid binding protein; IFABP, intestinal fatty acid binding protein; LFABP, liver fatty acid binding protein; NBD-PC, N -(7-nitro-2,1,3-benzoxadiazol-4-yl) egg phosphatidylcholine; PS, brain phosphatidylserine; SUV, small unilamellar vesicle.
