Mertvetsov N P, Chesnokov V N, Sakhno L V, Salganik R I
Biokhimiia. 1976 Aug;41(8):1352-66.
Rat liver contains two groups of tyrosine aminotransferase (TAT) isoenzymes; during electrophoresis in agar gel one of the groups moves to the anode and the other--to the catode. Cortisol is shown to induce only the anode isoenzymes of TAT, which were isolated, purified and thoroughly analyzed. The inducible anode isoenzyme of TAT spearated from other proteins is more sensitive to the effect of proteases (trypsin and chymotrypsin) than the catode isoenzyme. Some kinetic parameters of the purified TAT isoenzymes were studied. Both isoenzymes have pH optimum around 7.5; their apparent Km values for tyrosine are also similar. However, the catode isoenzyme of TAT possesses a higher affinity for alpha-ketoglutarate than does the anode isoenzyme. Unlike the latter, the former isoenzyme may use oxaloacetate as an amino group acceptor. Pyridoxal phosphate is firmly bound to the catode isoenzyme and can be readily spearated from the anode isoenzyme during dyalisis. An increased sensitivity of the inducible isoenzyme to proteases is due not only to the possibility of coenzyme dissociation, but also to some specific properties of the apoenzyme. The results obtained support the assumption that a high sensitivity of the inducible isoenzymes to proteases provides for a removal of excessive amounts of the enzymes from the cells under cessation of hormonal induction, thus maintaining enzymatic homostasis in the cell.
大鼠肝脏含有两组酪氨酸转氨酶（TAT）同工酶；在琼脂凝胶中进行电泳时，其中一组向阳极移动，另一组向阴极移动。已证明皮质醇仅诱导TAT的阳极同工酶，对其进行了分离、纯化并全面分析。从其他蛋白质中分离出的可诱导TAT阳极同工酶比阴极同工酶对蛋白酶（胰蛋白酶和糜蛋白酶）的作用更敏感。研究了纯化的TAT同工酶的一些动力学参数。两种同工酶的最适pH均在7.5左右；它们对酪氨酸的表观Km值也相似。然而，TAT的阴极同工酶对α-酮戊二酸的亲和力高于阳极同工酶。与后者不同，前者同工酶可以使用草酰乙酸作为氨基受体。磷酸吡哆醛与阴极同工酶紧密结合，在透析过程中很容易与阳极同工酶分离。可诱导同工酶对蛋白酶敏感性的增加不仅归因于辅酶解离的可能性，还归因于脱辅基酶的一些特殊性质。所得结果支持以下假设：可诱导同工酶对蛋白酶的高敏感性使得在激素诱导停止时从细胞中去除过量的酶，从而维持细胞内的酶稳态。
