Plasma membrane fatty acid-binding protein and mitochondrial glutamic-oxaloacetic transaminase of rat liver are related

The hepatic plasma membrane fatty acid-binding protein (h-FABP_PM) and the mitochondrial isoenzyme of glutamic-oxaloacetic transaminase (mGOT) of rat liver have similar amino acid compositions and identical amino acid sequences for residues 3-24. Both proteins migrate with an apparent molecular mass of 43 kDa on SDS/polyacrylamide gel electrophoresis, have a similar pattern of basic charge isomers on isoelectric focusing, are eluted similarly from four different high-performance liquid chromatographic columns, have absorption maxima at 435 nm under acid conditions and 354 nm at pH 8.3, and bind oleate with a K_a ≈ 1.2-1.4 × 10⁷ M^-1. Sinusoidally enriched liver plasma membranes and purified h-FABP_PM have GOT enzymatic activity; the relative specific activities (units/nig) of the membranes and purified protein suggest that h-FABP_PM constitutes 1-2% of plasma membrane protein in the rat hepatocyte. Monospecific rabbit antiserum against h-FABP_PM reacts on Western blotting with mGOT, and vice versa. Antisera against both proteins produce plasma membrane immunofluorescence in rat hepatocytes and selectively inhibit the hepatocellular uptake of [³H]oleate but not that of [³⁵S]sulfobromophthalein or [¹⁴C]taurocholate. The inhibition of oleate uptake produced by anti-h-FABP,_PM can be eliminated by preincubation of the antiserum with mGOT; similarly, the plasma membrane immunofluorescence produced by either antiserum can be eliminated by preincubation with the other antigen. These data suggest that h-FABP_PM and mGOT are closely related.