CELLULAR-DISTRIBUTION OF BRANCHED-CHAIN AMINO-ACID AMINOTRANSFERASE ISOENZYMES AMONG RAT-BRAIN GLIAL-CELLS IN CULTURE

The first step in the catabolism of branched-chain amino acids (BCAA), reversible transamination, is catalyzed by one of the two isoforms of branched-chain amino acid aminotransferase (BCAT). The mitochondrial isoenzyme (BCATm) is widely distributed among tissues, whereas the cyt osolic isoenzyme (BCATc) is restricted to only a few organs. Remarkabl y, BCATc is the prominent isoenzyme found in brain. The physiological significance of the subcellular compartmentation of BCAT is still not understood. To contribute to the elucidation of the cellular distribut ion of the two isoenzymes in brain, we used cultured rat glial cells i n an immunocytochemical study to determine the pattern of BCAT isoenzy me expression by glial cells. Antiserum against BCATm generated a punc tate staining pattern of astroglial cells, confirming the mitochondria l location of this isoenzyme. In contrast, the cytosol of galactocereb roside-expressing oligodendroglial cells and O2A progenitor cells disp layed intense staining only for BCATc. In addition, subpopulations of astroglial cells exhibited BCATc immunoreactivity. The presence of BCA Tm in astrocytes is consistent with the known ability of these cells t o oxidize BCAA. Furthermore, our results on BCATc provide support for the hypothesis that BCATs are also involved in nitrogen transfer from astrocytes to neurons.