The chemistry of xanthine oxidase. Reaction with iodoacetamide.
The chemistry of xanthine oxidase. Reaction with iodoacetamide.
复制标题
黄嘌呤氧化酶的化学性质。
作者：
R. Bray;D. Watts
1.用银-碘化银电极研究了牛奶黄嘌呤氧化酶与碘乙酰胺的反应。2.有黄嘌呤存在时，反应进行得比没有黄嘌呤时快得多。在厌氧条件下，当黄嘌呤过量时，反应以快速阶段发生，在该阶段酶被灭活，并且在该阶段中约有100%的酶被灭活。1巯基/mol。的酶反应，并作为一个缓慢的阶段，其中约12基团/mol。反应过来3.黄嘌呤氧化酶和碘乙酰胺的快速反应似乎是一级反应，且与黄嘌呤浓度无关，超过约3 mol。黄嘌呤/mol.酶。4.快速相的速度常数为0.26min。(-1)在25 ℃，pH7.0条件下，用1 mm-碘乙酰胺和17 mum-黄嘌呤氧化酶进行酶促反应。较慢阶段的速度常数约为该值的百分之一。5.在5.0-9.6的范围内，两个相的速度随着pH值的增加而增加。6.黄嘌呤可以用水杨醛代替，而不影响酶活性的损失率。用连二亚硫酸钠作还原剂，反应稍快。7.讨论了巯基在酶反应机理中的可能作用。
1. The reaction of milk xanthine oxidase with iodoacetamide has been studied with the silver-silver iodide electrode. 2. The reaction proceeds considerably faster in the presence of xanthine than in its absence. Anaerobically, with excess of xanthine, the reaction takes place as a rapid phase in which the enzyme is inactivated and in which approx. 1 thiol group/mol. of enzyme reacts and as a slower phase in which about 12 groups/mol. react. 3. The rapid reaction appears to be first-order with respect to xanthine oxidase and iodoacetamide and independent of the xanthine concentration with more than about 3mol. of xanthine/mol. of enzyme. 4. The velocity constant of the rapid phase is 0.26min.(-1) at 25 degrees and pH7.0, with 1mm-iodoacetamide and 17mum-xanthine oxidase. The velocity constant for the slower phase is about one-hundredth of this value. 5. The velocities of both phases increase with increasing pH in the range 5.0-9.6. 6. Xanthine may be replaced by salicylaldehyde without affecting the rate of loss of enzymic activity. With sodium dithionite as reducing agent, the reaction is slightly faster. 7. The possible function of thiol groups in the reaction mechanism of the enzyme is discussed.