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大鼠体内存在三种分枝氨基酸转氨酶(EC2.6.1.6)同工酶。同工酶Ⅰ和Ⅲ分布广泛,同工酶Ⅱ仅存在于肝脏,K_m值较高,底物特异性严格,只能催化亮氨酸与α-酮戊二酸之间的转氨反应,并且受体内糖皮质激素和高蛋白膳食的诱导。同工酶Ⅲ是分枝氨基酸转氨酶同工酶的胚胎分子形式存在于肾、脑、卵巢和胎盘组织中。文献报导,各种Yoshida腹水型肝癌、Morris肝癌、各种化学致癌剂和致癌病毒转化的肝细胞以及自发转化的细胞中都出现胚胎分子同工酶Ⅲ。近十余年来,大量的研究证明,癌瘤组织中基因表达发生紊乱,产生一些相应的正常分化组织所没有的、或含量极微的基因表达产物,同时正常分化组织所特有的一些功能蛋白则降低或消失。
There are three branched amino acid transaminase (EC2.6.1.6) isozymes in rats. Isoenzyme I and III are widely distributed. Isoenzyme II exists only in the liver with higher K_m and substrate specificity. It can only catalyze the transamination reaction between leucine and α-ketoglutarate, and Induction of glucocorticoid and high protein diet in the body. Isoenzyme III is a branched amino acid aminotransferase isoenzyme in the form of embryonic molecules in the kidney, brain, ovary and placenta. Reported in the literature, a variety of Yoshida ascites hepatocellular carcinoma, Morris liver cancer, a variety of chemical carcinogens and oncovirus transformation of liver cells and spontaneous transformation of cells are embryonic molecule isozyme Ⅲ. In the recent ten years, a large number of studies have shown that the gene expression in cancerous tissues is disordered, resulting in some gene expression products that are not found in the corresponding normal differentiated tissues or are extremely low in expression level, while some functional proteins specific to normal differentiated tissues Decrease or disappear.