Bixel M G, Hutson S M, Hamprecht B
Physiologisch-chemisches Institut der Universität, Tübingen, Germany.
J Histochem Cytochem. 1997 May;45(5):685-94. doi: 10.1177/002215549704500506.
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 cytosolic isoenzyme (BCATc) is restricted to only a few organs. Remarkably, 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 distribution of the two isoenzymes in brain, we used cultured rat glial cells in an immunocytochemical study to determine the pattern of BCAT isoenzyme expression by glial cells. Antiserum against BCATm generated a punctate staining pattern of astroglial cells, confirming the mitochondrial location of this isoenzyme. In contrast, the cytosol of galactocerebroside-expressing oligodendroglial cells and O2A progenitor cells displayed intense staining only for BCATc. In addition, subpopulations of astroglial cells exhibited BCATc immunoreactivity. The presence of BCATm in astrocytes is consistent with the known ability of these cells to oxidize BCAA. Furthermore, our results on BCATc provide support for the hypothesis that BCATs are also involved in nitrogen transfer from astrocytes to neurons.
支链氨基酸（BCAA）分解代谢的第一步是可逆转氨作用，由支链氨基酸转氨酶（BCAT）的两种同工型之一催化。线粒体同工酶（BCATm）广泛分布于各种组织中，而胞质同工酶（BCATc）仅局限于少数器官。值得注意的是，BCATc是在脑中发现的主要同工酶。BCAT亚细胞区室化的生理意义仍不清楚。为了有助于阐明脑中这两种同工酶的细胞分布，我们在一项免疫细胞化学研究中使用培养的大鼠神经胶质细胞来确定神经胶质细胞中BCAT同工酶的表达模式。针对BCATm的抗血清产生了星形胶质细胞的点状染色模式，证实了该同工酶的线粒体定位。相比之下，表达半乳糖脑苷脂的少突胶质细胞和少突胶质前体细胞的胞质仅对BCATc显示强烈染色。此外，星形胶质细胞亚群表现出BCATc免疫反应性。星形胶质细胞中存在BCATm与这些细胞氧化BCAA的已知能力一致。此外，我们关于BCATc的结果支持了BCAT也参与从星形胶质细胞到神经元的氮转移这一假说。
