Guengerich F P, Johnson W W, Shimada T, Ueng Y F, Yamazaki H, Langouët S
Department of Biochemistry and Center in Molecular Toxicology, Vanderbilt University School of Medicine, Nashville, TN 37232, USA.
Mutat Res. 1998 Jun 18;402(1-2):121-8. doi: 10.1016/s0027-5107(97)00289-3.
Aflatoxin B1 (AFB1) is a potent hepatocarcinogen in experimental animals and a hazard to human health in several parts of the world. Implementation of rational intervention plans requires understanding of aspects of the roles of individual chemical steps involved in its disposition. AFB1 is activated to AFB1 exo-8,9-epoxide primarily by cytochrome P450 (P450) enzymes, particularly P450 3A4. However, P450 3A4 and other P450s also oxidize AFB1 to less dangerous products. The exo-epoxide is unstable in H2O (t1/2 1 s at 25 degreesC, k=0.6 s-1) and the diol product undergoes base-catalyzed rearrangement to a dialdehyde that reacts with protein lysine residues. AFB1 exo-8, 9-epoxide reacts with DNA to give adducts in high yield (>98%). This interaction is characterized by a Kd of approximately 1.4 mM, intercalation between base pairs, and rapid reaction with the guanyl N7 atom (k approximately 40 s-1). A proton field on the periphery of DNA is postulated to catalyze hydrolysis and also conjugation. Rat and especially human epoxide hydrolase show very little rate acceleration of hydrolysis of AFB1 exo- or endo-8,9-epoxide. However, glutathione transferases (GSTs) can catalyze AFB1 exo-8,9-epoxide conjugation. Kinetic analysis indicates a range of ratios of kcat/Kd varying from 10 to 1700 s-1 M-1, with the polymorphic GST M1-1 having the highest activity of the human GSTs. Studies with human hepatocytes indicate a major role for GST M1-1 in AFB1 conjugation and that the model chemoprotective agent oltipraz can act by both inducing GSTs and inhibiting P450s.
黄曲霉毒素B1（AFB1）在实验动物中是一种强效的肝癌致癌物，在世界上多个地区对人类健康构成危害。实施合理的干预计划需要了解其代谢过程中各个化学步骤所起作用的相关方面。AFB1主要通过细胞色素P450（P450）酶，特别是P450 3A4，被激活为AFB1外-8,9-环氧化物。然而，P450 3A4和其他P450也会将AFB1氧化为危险性较低的产物。外-环氧化物在水中不稳定（25℃时半衰期为1秒，k = 0.6秒-1），二醇产物会发生碱催化重排生成二醛，二醛与蛋白质赖氨酸残基反应。AFB1外-8,9-环氧化物与DNA反应可高产率（>98%）生成加合物。这种相互作用的特征是解离常数（Kd）约为1.4 mM，在碱基对之间嵌入，并与鸟嘌呤N7原子快速反应（k约为40秒-1）。推测DNA外围的质子场催化水解和共轭反应。大鼠尤其是人类的环氧化物水解酶对AFB1外-或内-8,9-环氧化物水解的速率加速作用很小。然而，谷胱甘肽转移酶（GST）可催化AFB1外-8,9-环氧化物的共轭反应。动力学分析表明催化常数与解离常数的比值范围为10至1700秒-1 M-1，多态性GST M1-1在人类GST中活性最高。对人类肝细胞的研究表明GST M1-1在AFB1共轭反应中起主要作用，模型化学保护剂奥替普拉可通过诱导GST和抑制P450发挥作用。
