Fahien L A, Hsu S L, Kmiotek E
J Biol Chem. 1977 Feb 25;252(4):1250-6.
In previous studies it was found that: (a) aspartate aminotransferase increases the aspartate dehydrogenase activity of glutamate dehydrogenase; (b) the pyridoxamine-P form of this aminotransferase can form an enzyme-enzyme complex with glutamate dehydrogenase; and (c) the pyridoxamine-P form can be dehydrogenated to the pyridoxal-P form by glutamate dehydrogenase. It was therefore concluded (Fahien, L.A., and Smith, S.E. (1974) J. Biol. Chem 249, 2696-2703) that in the aspartate dehydrogenase reaction, aspartate converts the aminotransferase into the pyridoxamine-P form which is then dehydrogenated by glutamate dehydrogenase. The present results support this mechanism and essentially exclude the possibility that aspartate actually reacts with glutamate dehydrogenase and the aminotransferase is an allosteric activator. Indeed, it was found that aspartate is actually an activator of the reaction between glutamate dehydrogenase and the pyridoxamine-P form of the aminotransferase. Aspartate also markedly activated the alanine dehydrogenase reaction catalyzed by glutamate dehydrogenase plus alanine aminotransferase and the ornithine dehydrogenase reaction catalyzed by ornithine aminotransferase plus glutamate dehydrogenase. In these latter two reactions, there is no significant conversion of aspartate to oxalecetate and other compounds tested (including oxalacetate) would not substitute for aspartate. Thus aspartate is apparently bound to glutamate dehydrogenase and this increases the reactivity of this enzyme with the pyridoxamine-P form of aminotransferases. This could be of physiological importance because aspartate enables the aspartate and ornithine dehydrogenase reactions to be catalyzed almost as rapidly by complexes between glutamate dehydrogenase and the appropriate mitochondrial aminotransferase in the absence of alpha-ketoglutarate as they are in the presence of this substrate. Furthermore, in the presence of aspartate, alpha-ketoglutarate can have little or no affect on these reactions. Consequently, in the mitochondria of some organs these reactions could be catalyzed exclusively by enzyme-enzyme complexes even in the presence of alpha-ketoglutarate. Rat liver glutamate dehydrogenase is essentially as active as thebovine liver enzyme with aminotransferases. Since the rat liver enzyme does not polymerize, this unambiguously demonstrates that monomeric forms of glutamate dehydrogenase can react with aminotransferases.
（a）天冬氨酸转氨酶可提高谷氨酸脱氢酶的天冬氨酸脱氢酶活性；（b）该转氨酶的磷酸吡哆胺形式可与谷氨酸脱氢酶形成酶-酶复合物；（c）磷酸吡哆胺形式可被谷氨酸脱氢酶脱氢为磷酸吡哆醛形式。因此得出结论（法欣，L.A.，和史密斯，S.E.（1974年）《生物化学杂志》249，2696 - 2703），在天冬氨酸脱氢酶反应中，天冬氨酸将转氨酶转化为磷酸吡哆胺形式，然后被谷氨酸脱氢酶脱氢。目前的结果支持这一机制，并基本排除了天冬氨酸实际与谷氨酸脱氢酶反应且转氨酶是变构激活剂的可能性。事实上，发现天冬氨酸实际上是谷氨酸脱氢酶与转氨酶的磷酸吡哆胺形式之间反应的激活剂。天冬氨酸还显著激活了由谷氨酸脱氢酶加丙氨酸转氨酶催化的丙氨酸脱氢酶反应以及由鸟氨酸转氨酶加谷氨酸脱氢酶催化的鸟氨酸脱氢酶反应。在这后两个反应中，天冬氨酸没有明显转化为草酰乙酸，并且所测试的其他化合物（包括草酰乙酸）不能替代天冬氨酸。因此，天冬氨酸显然与谷氨酸脱氢酶结合，这增加了该酶与转氨酶的磷酸吡哆胺形式的反应活性。这可能具有生理重要性，因为在没有α-酮戊二酸的情况下，天冬氨酸能使谷氨酸脱氢酶与适当的线粒体转氨酶之间的复合物几乎与在有该底物存在时一样快速地催化天冬氨酸和鸟氨酸脱氢酶反应。此外，在有天冬氨酸存在的情况下，α-酮戊二酸对这些反应几乎没有或没有影响。因此，在某些器官的线粒体中，即使在有α-酮戊二酸存在的情况下，这些反应也可能仅由酶-酶复合物催化。大鼠肝脏谷氨酸脱氢酶与转氨酶反应时的活性基本上与牛肝脏酶相同。由于大鼠肝脏酶不会聚合，这明确表明谷氨酸脱氢酶的单体形式可以与转氨酶反应。
