Butler A M, Murray M
Department of Medicine, University of Sydney, Westmead Hospital, Australia.
J Pharmacol Exp Ther. 1997 Feb;280(2):966-73.
Studies in rat liver have shown that cytochrome P450 (CYP) enzymes mediate the oxidative biotransformation of the phosphorothioate pesticide parathion to paraoxon and 4-nitrophenol. Transfer of the phosphorothioate thionosulfur atom to the CYP apoprotein results in amino acid modification and enzyme inactivation. Our study investigated the role of human hepatic CYP in parathion oxidation and their relative susceptibilities to inhibition and inactivation. Rates of parathion oxidation varied about 10-fold in microsomes from 23 individual livers (1.72-18.33 nmol total metabolites/mg protein/min). Linear regression of rates of parathion oxidation with those of other microsomal CYP reactions implicated CYP3A4 in the reaction. Thus, parathion oxidation was correlated strongly with testosterone 6beta-hydroxylation (r2 = 0.95, n = 11), but not with activities mediated by CYP 1A2, 2C9 or 2E1. CYP 3A4 expressed in lymphoblastoid cell lines was an efficient catalyst of parathion oxidation, although CYP 1A2 and 2B6 also catalyzed the activity. The CYP3A4 inhibitors ketoconazole and triacetyloleandomycin decreased the observed rate of microsomal parathion oxidation, but chemicals known to interact preferentially with other human CYP were essentially noninhibitory. P450 was lost during parathion biotransformation in human hepatic microsomes. Thus, incubation (10 min) of parathion (25 microM) with NADPH-supplemented microsomes led to an apparent 19 +/- 4% decrease in holo-P450 content. Several CYP-specific oxidation reactions were inhibited and inactivated by parathion. Testosterone 6beta-hydroxylation (mediated by CYP3A4), 7-ethylresorufin O-deethylation (CYP1A2) and tolbutamide methyl hydroxylation (CYP2C9/10), but not aniline 4-hydroxylation (CYP2E1), were inhibited effectively by parathion. Preincubation of microsomes with parathion and NADPH intensified the extent of inhibition (i.e., elicited inactivation) of reactions mediated by 3A4 and 1A2 and, to a lesser extent, 2C9. In summary, these findings strongly implicate CYP 3A4 as the principal catalyst of parathion oxidation in human liver, although other CYP may play a lesser role. During parathion oxidation CYP3A4 undergoes significant inactivation. In view of the role of this enzyme in the oxidation of many therapeutic agents, exposure to phosphorothioate pesticides may adversely affect drug elimination in humans.
对大鼠肝脏的研究表明，细胞色素P450（CYP）酶介导了硫代磷酸酯农药对硫磷氧化生物转化为对氧磷和4-硝基苯酚的过程。硫代磷酸酯的硫原子转移至CYP脱辅基蛋白会导致氨基酸修饰及酶失活。我们的研究调查了人肝脏CYP在对硫磷氧化中的作用及其对抑制和失活的相对敏感性。来自23个个体肝脏的微粒体中，对硫磷氧化速率变化约10倍（1.72 - 18.33 nmol总代谢产物/毫克蛋白/分钟）。对硫磷氧化速率与其他微粒体CYP反应速率的线性回归表明该反应涉及CYP3A4。因此，对硫磷氧化与睾酮6β-羟基化密切相关（r2 = 0.95，n = 11），但与CYP 1A2、2C9或2E1介导的活性无关。淋巴母细胞系中表达的CYP 3A4是对硫磷氧化的有效催化剂，尽管CYP 1A2和2B6也催化该活性。CYP3A4抑制剂酮康唑和三乙酰夹竹桃霉素降低了观察到的微粒体对硫磷氧化速率，但已知优先与其他人CYP相互作用的化学物质基本无抑制作用。在人肝脏微粒体对硫磷生物转化过程中P450会损失。因此，对硫磷（25 microM）与补充了NADPH的微粒体孵育（10分钟）导致全酶P450含量明显下降19±4%。几种CYP特异性氧化反应会被对硫磷抑制和失活。睾酮6β-羟基化（由CYP3A4介导）、7-乙基试卤灵O-脱乙基化（CYP1A2）和甲苯磺丁脲甲基羟基化（CYP2C9/10），但苯胺4-羟基化（CYP2E1）不会被对硫磷有效抑制。微粒体与对硫磷和NADPH预孵育会增强由3A4和1A2介导反应的抑制程度（即引发失活），对2C9介导反应的抑制程度较小。总之，这些发现有力地表明CYP 3A4是人肝脏中对硫磷氧化的主要催化剂，尽管其他CYP可能起较小作用。在对硫磷氧化过程中CYP3A4会发生显著失活。鉴于该酶在许多治疗药物氧化中的作用，接触硫代磷酸酯农药可能会对人体药物消除产生不利影响。
