Yang C S, Tu Y Y, Koop D R, Coon M J
Cancer Res. 1985 Mar;45(3):1140-5.
The metabolism of nitrosamines by microsomal cytochrome P-450 (P-450) isozymes was studied in a reconstituted monooxygenase system. P-450 LM2, LM3a, LM3b and LM3c, LM4, and LM6 were purified, respectively, from the livers of phenobarbital-treated, ethanol-treated, untreated, isosafrole-treated, and imidazole-treated rabbits. Of these isozymes, LM3a had the highest N-nitrosodimethylamine demethylase (NDMAd) activity with a Km of 2.9 mM and Vmax of 9.3 nmol/min/nmol. LM2, LM4, and LM6 exhibited NDMAd activity only at high N-nitrosodimethylamine concentrations, and isozymes LM3b and LM3c had poor activity even at the highest substrate concentrations examined. LM2, however, was more active than LM3a in the metabolism of N-nitrosomethylaniline. With each isozyme (LM3a or LM4), only one Km for NDMAd was observed, whereas with rabbit liver microsomes, multiple Km of 0.07, 0.27, and 36.8 mM were obtained. P-450 isozymes also catalyzed the denitrosation of nitrosamines at rates comparable to or lower than the demethylation, and the ratio of these two reactions was different with different nitrosamines. 2-Phenylethylamine and 3-amino-1,2,4-triazole, which were believed previously to affect NDMAd by mechanisms independent of P-450, were shown to be potent inhibitors of P-450-dependent NDMAd. These results further establish the role of P-450 isozymes in the metabolism of nitrosamines and indicate that LM3a is apparently responsible for the increased N-nitrosodimethylamine metabolism associated with ethanol treatment.
在重组单加氧酶系统中研究了微粒体细胞色素P-450（P-450）同工酶对亚硝胺的代谢作用。分别从经苯巴比妥处理、乙醇处理、未处理、异黄樟素处理和咪唑处理的兔子肝脏中纯化出P-450 LM2、LM3a、LM3b、LM3c、LM4和LM6。在这些同工酶中，LM3a具有最高的N-亚硝基二甲胺脱甲基酶（NDMAd）活性，其Km为2.9 mM，Vmax为9.3 nmol/分钟/纳摩尔。LM2、LM4和LM6仅在高浓度N-亚硝基二甲胺时才表现出NDMAd活性，而同工酶LM3b和LM3c即使在检测的最高底物浓度下活性也很差。然而，LM2在N-亚硝基甲基苯胺的代谢中比LM3a更具活性。对于每种同工酶（LM3a或LM4），仅观察到一个NDMAd的Km，而用兔肝微粒体时，得到了0.07、0.27和36.8 mM的多个Km。P-450同工酶还以与脱甲基作用相当或更低的速率催化亚硝胺的脱亚硝基作用，并且这两种反应的比例因不同的亚硝胺而异。以前认为通过独立于P-450的机制影响NDMAd的2-苯乙胺和3-氨基-1,2,4-三唑被证明是P-450依赖性NDMAd的有效抑制剂。这些结果进一步证实了P-450同工酶在亚硝胺代谢中的作用，并表明LM3a显然与乙醇处理相关的N-亚硝基二甲胺代谢增加有关。
