Sugiyama K, Kushima Y, Muramatsu K
Department of Agricultural Chemistry, Faculty of Agriculture, Shizuoka University, Japan.
J Nutr Sci Vitaminol (Tokyo). 1987 Jun;33(3):195-205. doi: 10.3177/jnsv.33.195.
The alleviation mechanism of methionine toxicity by dietary glycine was investigated in weanling rats fed a high-methionine diet. When rats were fed a 10% casein diet containing 2% methionine, the activities of methionine adenosyltransferase, cystathionine beta-synthase, and cystathionine gamma-lyase, which participate in the methionine metabolism in the transsulfuration pathway, were significantly enhanced. But the addition of 2% glycine to the high methionine diet did not cause further increase in these enzyme activities; the activities of methionine adenosyltransferase and cystathionine beta-synthase were rather decreased while cystathionine gamma-lyase activity was not altered. Methionine transaminase activity was essentially insensitive to the dietary addition of methionine and glycine. In rats fed a high methionine diet, the hepatic methionine level was significantly increased with a concomitant decrease in the levels of glycine, serine, and threonine. The addition of glycine to the high methionine diet effectively suppressed the enhancement of the hepatic methionine level and almost completely restored the glycine level, but it only partially restored the serine level and further decreased the threonine level. From these results, it is suggested that the alleviating effect of dietary glycine on methionine toxicity is primarily elicited by the restoration of the hepatic glycine level rather than by an increase in hepatic enzyme activity.
在喂食高蛋氨酸饮食的断奶大鼠中，研究了膳食甘氨酸对蛋氨酸毒性的缓解机制。当给大鼠喂食含2%蛋氨酸的10%酪蛋白饮食时，参与转硫途径中蛋氨酸代谢的蛋氨酸腺苷转移酶、胱硫醚β-合酶和胱硫醚γ-裂合酶的活性显著增强。但是在高蛋氨酸饮食中添加2%的甘氨酸并没有使这些酶的活性进一步增加；蛋氨酸腺苷转移酶和胱硫醚β-合酶的活性反而降低，而胱硫醚γ-裂合酶的活性没有改变。蛋氨酸转氨酶活性对膳食中添加蛋氨酸和甘氨酸基本不敏感。在喂食高蛋氨酸饮食的大鼠中，肝脏蛋氨酸水平显著升高，同时甘氨酸、丝氨酸和苏氨酸水平降低。在高蛋氨酸饮食中添加甘氨酸有效地抑制了肝脏蛋氨酸水平的升高，并几乎完全恢复了甘氨酸水平，但仅部分恢复了丝氨酸水平，并进一步降低了苏氨酸水平。从这些结果表明，膳食甘氨酸对蛋氨酸毒性的缓解作用主要是通过恢复肝脏甘氨酸水平而不是通过增加肝脏酶活性来实现的。
