SEX-DIFFERENCES IN INTRACELLULAR FATTY-ACID TRANSPORT - ROLE OF CYTOPLASMIC-BINDING PROTEINS

Female liver clears long-chain fatty acids from plasma more rapidly th an male liver, and yet the basis for this sex difference is poorly und erstood. We tested the hypothesis that cytosolic fatty acid binding pr otein (FABP), which is more concentrated in female liver, may enhance fatty acid utilization by increasing the rate of transport through the cytoplasm. We modified the technique of fluorescence recovery after l aser photobleaching to measure the cytoplasmic diffusion rate of the f luorescent long-chain fatty acid methyl-(7-nitrobenz-2-oxa-1,3-diazol) aminostearate (NBD-stearate) in cultured hepatocytes from female and m ale rats. NBD-stearate was used because its hepatic handling is simila r to natural fatty acids. After uptake, NBD-stearate distributed unifo rmly in the cytoplasm but was excluded from the nucleus. Intracellular transport occurred by diffusion with no detectable convective flux. T he cytoplasmic diffusion rate at 37-degrees-C was 65% greater in femal e cells than in male cells (mean +/- SE, 5.03 +/- 0.37 vs. 3.05 +/- 0. 21 X 10(-9) cm2/s respectively; P < 0.001) and was two to three orders of magnitude slower than for either unbound NBD-stearate or FABP in w ater. A correspondingly greater fraction of cellular NBD-stearate was found in the aqueous cytosol in females (35.1 +/- 7.0 vs. 18.2 +/- 2.7 %), suggesting that FABP reduces binding of NBD-stearate to immobile c ytoplasmic membranes. These data indicate that intracellular transport of NBD-stearate, a typical amphipathic molecule, is slowed by binding to cytoplasmic membranes. The primary function of soluble binding pro teins such as FABP may be to enhance the diffusive fluxes of their lig ands by reducing membrane binding. If cytoplasmic transport of rapidly metabolized fatty acids such as palmitate is similarly slow, substant ial concentration gradients could develop within the cytoplasm of hepa tocytes at steady state. By catalyzing these diffusive fluxes, FABP ma y regulate fatty acid metabolism.