Mitochondrial Aspartate Aminotransferase Expressed on the Surface of 3T3-L1 Adipocytes Mediates Saturable Fatty Acid Uptake

Abstract

Physicochemical studies have suggested that the 43-kDa plasma membrane fatty acid binding protein (FABP_pm) is closely related to the mitochondrial isoform of aspartate aminotransferase (mAspAT). In the present studies, mAspAT was not detected immunohistochemically or by immunoblotting in plasma membranes of proliferating 3T3-L1 fibroblasts. During controlled differentiation to an adipocyte pheno-type, mAspAT became detectable by the second day of confluent growth, prior to accumulation of visible lipid droplets, and was strongly expressed in 8-day differentiated 3T3-L1 adipocytes. The pattern of expression paralleled the previously reported expression both of FABP_pm and of the V_max for saturable uptake of long chain free fatty acids. As with anti-FABP_pm, antibodies to mAspAT selectively inhibited the uptake of [³H]-oleate in 3T3-L1 adipocytes but not in fibroblasts, while having no effect on uptake of either 2-deoxyglucose or the medium chain fatty acid octanoate. Preabsorption of anti-FABP_pm with mAspAT, or of anti-mAspAT with FABP_pm, abolished immunopositivity in immunohistochemical and immunoblotting studies, as well as the ability of either antibody to inhibit [³H]-oleate uptake. These studies provide strong biologic evidence for the identity of FABP_pm and mAspAT, and for the hypothesis that FABP_pm/mAspAT mediates the uptake of long chain free fatty acids.