COMPARISON OF PLASMA-MEMBRANE FABP AND MITOCHONDRIAL ISOFORM OF ASPARTATE-AMINOTRANSFERASE FROM RAT-LIVER

A relationship between plasma membrane fatty acid binding protein (FAB P(pm)), a putative membrane transporter for long-chain fatty acids, an d the mitochondrial isoform of aspartate aminotransferase (m-AspAT) ha s been reported. Accordingly, we have compared the chemical and immuno logical properties of rat liver m-AspAT with those of rat liver FABP(p m) isolated by two procedures: 1) detergent solubilization of the memb ranes followed by purification via fatty acid affinity chromatography (FABP-1) or 2) salt extraction of the membranes and subsequent purific ation by high-performance liquid chromatography (HPLC; FABP-2). Compar ison of the three protein preparations revealed no differences with re spect to NH2-terminal amino acid sequence, amino acid composition, pep tides from tryptic digests, AspAT enzymatic activity, isoelectric poin t, mobility on sodium dodecyl sulfate-polyacrylamide gel electrophores is (SDS-PAGE), retention on five different HPLC columns, and immunopre cipitation and immunoblotting of SDS-PAGE separated proteins with poly clonal antisera. Examination of the proteins by nondenaturing PAGE sho wed a consistent second band in FABP-1 and FABP-2 not always present i n m-AspAT. However, whenever present, this band was immunoreactive wit h antibodies to both m-AspAT and FABP-1. Hence, FABP-1 and FABP-2 are indistinguishable from one another. They are also at least closely rel ated, if not identical, to m-AspAT.