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Intestinal absorptive cells contain high levels of expression of two homologous fatty acid-binding proteins (FABP), liver FABP (L-FABP), and intestinal FABP (I-FABP). Both bind long chain fatty acids with relatively high affinity. The functional distinction, if any, between these two proteins remains unknown. It is often hypothesized that FABP are important in intracellular transport of fatty acids. To assess whether fatty acid transport properties might differ between the two enterocyte FABPs, we examined the rate and mechanism of transfer of fluorescent anthroyloxy fatty acids (AOFA) from these proteins to model membranes using a resonance energy transfer assay. The results show that the absolute rate of AOFA transfer from I-FABP is faster than from L-FABP. Moreover, the apparent mechanism of fatty acid transfer is different between the two proteins. The rate of AOFA transfer from I-FABP is independent of ionic strength, directly dependent on the concentration of acceptor membrane vesicles, and dramatically regulated by the lipid composition of the membranes. These data strongly suggest that fatty acid transfer from I-FABP to membranes occurs by direct collisional interaction of the protein with the phospholipid bilayer. In contrast, the characteristics of fatty acid transfer from L-FABP are consistent with an aqueous diffusion-mediated process. Thus the two enterocyte FABPs may perform different functions within the intestinal absorptive cell in the regulation of fatty acid transport and utilization. It is hypothesized that L-FABP may act as a cytosolic buffer for fatty acids, maintaining the unbound fatty acid concentration, whereas I-FABP may be involved in the uptake and/or specific targeting of fatty acid to subcellular membrane sites.
Hsu et al. (Sat,) studied this question.