Liver fatty acid-binding protein binds monoacylglycerol in vitro and in mouse liver cytosol.

Abstract

Liver fatty acid-binding protein (LFABP; FABP1) is expressed both in liver and intestinal mucosa. Mice null for LFABP were recently shown to have altered metabolism of not only fatty acids but also monoacylglycerol, the two major products of dietary triacylglycerol hydrolysis (Lagakos et al., Am J Physiol. 2011). Nevertheless, the binding and transport of MG by LFABP is uncertain, with conflicting reports in the literature as to whether this single chain amphiphile is in fact bound by LFABP. In the present studies, gel filtration chromatography of liver cytosol from LFABP-/- mice shows the absence of the low molecular weight peak of radiolabeled monoolein present in the fractions that contain LFABP in cytosol from wild type mice, indicating that LFABP binds sn-2 MG in vivo. Further, solution state NMR spectroscopy demonstrates two molecules of sn-2-monoolein bound in the LFABP binding pocket, in positions similar to those found for oleate binding. Equilibrium binding affinities are approximately two-fold lower for MG compared to FA. Finally, kinetic studies examining the transfer of a fluorescent MG analogue show that the rate of transfer of MG is 7-fold faster from LFABP to phospholipid membranes than from membranes to membranes, and occurs by an aqueous diffusion mechanism. These results provide strong support for monoacylglycerol as a physiological ligand for LFABP, and further suggest that LFABP functions in the efficient intracellular transport of MG.