Frasca V, Riazzi B S, Matthews R G
J Biol Chem. 1986 Dec 5;261(34):15823-6.
Nitrous oxide (N2O) is commonly used as an anesthetic agent. Prolonged exposure to N2O leads to megaloblastic anemia in humans and to loss of methionine synthase activity in vertebrates. We now report that purified preparations of cobalamin-dependent methionine synthase (5-methyltetrahydrofolate-homocysteine methyltransferase, EC 2.1.1.13) from both Escherichia coli and pig liver are irreversibly inactivated during turnover in buffers saturated with N2O. Inactivation by N2O occurs only in the presence of all components required for turnover: homocysteine, methyltetrahydrofolate, adenosylmethionine, and a reducing system. Reisolation of the inactivated E. coli enzyme after turnover in the presence of N2O resulted in significant losses of bound cobalamin and of protein as compared to controls where the enzyme was subjected to turnover in N2-equilibrated buffers before reisolation. However, N2O inactivation was not associated with major changes in the visible absorbance spectrum of the remaining enzyme-bound cobalamin. We postulate that N2O acts by one-electron oxidation of the cob(I)alamin form of the enzyme which is generated transiently during turnover with the formation of cob(II)alamin, N2, and hydroxyl radical. Generation of hydroxyl radical at the active site of the enzyme could explain the observed irreversible loss of enzyme activity.
一氧化二氮（N₂O）通常用作麻醉剂。长期接触N₂O会导致人类巨幼细胞贫血，并导致脊椎动物甲硫氨酸合酶活性丧失。我们现在报告，来自大肠杆菌和猪肝的纯化的钴胺素依赖性甲硫氨酸合酶（5-甲基四氢叶酸-同型半胱氨酸甲基转移酶，EC 2.1.1.13）制剂在饱和N₂O的缓冲液中周转期间会不可逆地失活。N₂O引起的失活仅在周转所需的所有成分存在时发生：同型半胱氨酸、甲基四氢叶酸、腺苷甲硫氨酸和一个还原系统。与在重新分离前将酶在N₂平衡的缓冲液中进行周转的对照相比，在N₂O存在下周转后重新分离失活的大肠杆菌酶，导致结合的钴胺素和蛋白质大量损失。然而，N₂O失活与剩余酶结合钴胺素的可见吸收光谱的主要变化无关。我们推测，N₂O通过在周转过程中与钴胺素（II）、N₂和羟基自由基的形成一起瞬时产生的酶的钴胺素（I）形式的单电子氧化起作用。在酶的活性位点产生羟基自由基可以解释观察到的酶活性的不可逆丧失。
