Carl G F, Benesh F C, Hudson J L
Biol Psychiatry. 1978 Dec;13(6):661-9.
Much greater increases in S-adenosylmethionine concentrations are observed in the liver in response to methionine-loading than in the brain due to differences in the methionine adenosyltransferase activities in these tissues. Liver methione adenosyltransferase exhibits a bimodal saturation curve with a nonlinear Line-weaver-Burk plot, indicating that high methionine concentrations are required for saturation. In the brain the methionine adenosyltransferase is saturated in vitro at a methionine concentration less than the normal physiological concentration. The increased S-adenosylmethionine concentrations in the livers of methionine-treated rats also account for the observed inhibition of N5,N10-methylenetetrahydrofolate reductase activity in this tissue. No inhibition of this enzyme is observed in the brain of methionine treated animals. Nor are S-adenosylmethionine concentrations increased significantly in brain. Serine hydroxymethyltransferase activity responds to methionine-loading by decreasing in brain and increasing in liver.
由于这些组织中蛋氨酸腺苷转移酶活性存在差异，与大脑相比，肝脏中因蛋氨酸负荷而观察到的S-腺苷甲硫氨酸浓度升高幅度要大得多。肝脏蛋氨酸腺苷转移酶呈现出双峰饱和曲线，Line-weaver-Burk图呈非线性，这表明饱和需要高浓度的蛋氨酸。在大脑中，蛋氨酸腺苷转移酶在体外蛋氨酸浓度低于正常生理浓度时就会饱和。蛋氨酸处理的大鼠肝脏中S-腺苷甲硫氨酸浓度升高也解释了在该组织中观察到的N5,N10-亚甲基四氢叶酸还原酶活性受到抑制的现象。在蛋氨酸处理动物的大脑中未观察到该酶受到抑制。大脑中的S-腺苷甲硫氨酸浓度也没有显著升高。丝氨酸羟甲基转移酶活性对蛋氨酸负荷的反应是在大脑中降低而在肝脏中升高。
