Garcia J D, Jennette K W
J Inorg Biochem. 1981 Jul;14(4):281-95. doi: 10.1016/s0162-0134(00)80286-x.
The kinetics of chromate reduction by liver microsomes isolated from rats pretreated with phenobarbital or 3-methylcholanthrene with NADPH or NADH cofactor have been followed. Induction of cytochrome P-450 and NADPH-cytochrome P-450 reductase activity in microsomes by phenobarbital pretreatment caused a decrease in the apparent chromate-enzyme dissociation constant, Km, and an increase in the apparent second-order rate constant, kcat/Km, but did not affect the kcat of NADPH-mediated microsomal metabolism of chromate. Induction of cytochrome P-448 in microsomes by 3-methylcholanthrene pretreatment did not affect the kinetics of NADPH-mediated reduction of chromate by microsomes. The kinetics of NADH-mediated microsomal chromate reduction were unaffected by the drug treatments. The effects of specific enzyme inhibitors on the kinetics of microsomal chromate reduction have been determined. 2'-AMP and 3-pyridinealdehyde-NAD, inhibitors of NADPH-cytochrome P-450 reductase and NADH-cytochrome b5 reductase, inhibited the rate of microsomal reduction of chromate with NADPH and NADH. Metyrapone and carbon monoxide, specific inhibitors of cytochrome P-450, inhibited the rate of NADPH-mediated microsomal reduction of chromate, whereas high concentrations of dimethyl-sulfoxide (0.5 M) enhanced the rate. These results suggest that the electron-transport cytochrome P-450 system is involved in the reduction of chromate by microsomal systems. The NADPH and NADH cofactors supply reducing equivalents ultimately to cytochrome P-450 which functions as a reductase in chromate metabolism. The lower oxidation state(s) produced upon chromate reduction may represent the ultimate carcinogenic form(s) of chromium. These studies provide evidence for the role of cytochrome P-450 in the activation of inorganic carcinogens.
对从用苯巴比妥或3-甲基胆蒽预处理过的大鼠中分离出的肝微粒体，在有NADPH或NADH辅助因子的情况下铬酸盐还原的动力学进行了追踪。苯巴比妥预处理诱导微粒体中的细胞色素P-450和NADPH-细胞色素P-450还原酶活性，导致铬酸盐-酶表观解离常数Km降低，表观二级速率常数kcat/Km增加，但不影响NADPH介导的微粒体铬酸盐代谢的kcat。3-甲基胆蒽预处理诱导微粒体中的细胞色素P-448，不影响微粒体对NADPH介导的铬酸盐还原的动力学。药物处理不影响NADH介导的微粒体铬酸盐还原的动力学。已确定了特异性酶抑制剂对微粒体铬酸盐还原动力学的影响。NADPH-细胞色素P-450还原酶和NADH-细胞色素b5还原酶的抑制剂2'-AMP和3-吡啶醛-NAD，抑制了用NADPH和NADH时微粒体铬酸盐还原的速率。细胞色素P-450的特异性抑制剂甲吡酮和一氧化碳，抑制了NADPH介导的微粒体铬酸盐还原的速率，而高浓度的二甲基亚砜（0.5M）提高了该速率。这些结果表明，电子传递细胞色素P-450系统参与微粒体系统对铬酸盐的还原。NADPH和NADH辅助因子最终向细胞色素P-450提供还原当量，细胞色素P-450在铬酸盐代谢中起还原酶的作用。铬酸盐还原时产生的较低氧化态可能代表铬的最终致癌形式。这些研究为细胞色素P-450在无机致癌物活化中的作用提供了证据。
