Godber B L, Doel J J, Goult T A, Eisenthal R, Harrison R
Department of Biology and Biochemistry, University of Bath, Claverton Down, Bath BA2 7AY, UK.
Biochem J. 2001 Sep 1;358(Pt 2):325-33. doi: 10.1042/0264-6021:3580325.
Xanthine oxidoreductase (XOR) is progressively inactivated while catalysing the reduction of inorganic nitrite to NO by xanthine. Inactivation results from conversion of the enzyme into its desulpho-form. The rate of inactivation increases with nitrite concentration. Similar behaviour was shown when NADH replaced xanthine as reducing substrate. A kinetic model is proposed incorporating a 'suicide' inactivation involving an enzyme-substrate (product) complex, rather than inactivation by free NO. The model provides a good fit to progress curves of the reaction of xanthine or NADH with nitrite in the presence of the oxidase or dehydrogenase forms of the enzyme. Inorganic nitrate, like nitrite, was shown to be reduced at the molybdenum site of XOR. With xanthine as reducing substrate, nitrite was produced in essentially a 1:1 stoichiometric ratio with respect to urate. Unlike the case of nitrite, the enzyme was not significantly inactivated, implying that inactivation during nitrite reduction depends on the presence of nascent NO in its enzyme complex.
黄嘌呤氧化还原酶（XOR）在催化黄嘌呤将无机亚硝酸盐还原为一氧化氮（NO）的过程中会逐渐失活。失活是由于该酶转化为其脱硫形式所致。失活速率随亚硝酸盐浓度的增加而加快。当用烟酰胺腺嘌呤二核苷酸（NADH）替代黄嘌呤作为还原底物时，也表现出类似的行为。提出了一个动力学模型，该模型包含一种涉及酶 - 底物（产物）复合物的“自杀式”失活，而非由游离NO导致的失活。该模型与在酶的氧化酶或脱氢酶形式存在下黄嘌呤或NADH与亚硝酸盐反应的进程曲线拟合良好。无机硝酸盐与亚硝酸盐一样，被证明在XOR的钼位点被还原。以黄嘌呤作为还原底物时，亚硝酸盐的生成与尿酸盐的化学计量比基本为1:1。与亚硝酸盐的情况不同，该酶没有明显失活，这意味着亚硝酸盐还原过程中的失活取决于其酶复合物中新生NO的存在。
