Ertl R P, Stegeman J J, Winston G W
Department of Biochemistry, Louisiana State University, Baton Rouge 70803, USA.
Biochem Pharmacol. 1998 May 1;55(9):1513-21. doi: 10.1016/s0006-2952(98)00003-3.
Alligator mississippiensis has at least two classes of inducible hepatic microsomal cytochromes P450 (CYP): (1) those induced by 3-methylcholanthrene (3MC), and (2) those induced by phenobarbital (PB). The rates of induction by these xenobiotic compounds are significantly slower than those reported for mammals. Carbon monoxide binding, western blots, and enzymatic activity measurements indicated that at least 48-72 hr are required to reach full induction. A methoxy-, ethoxy-, pentoxy, and benzyloxyphenoxazone (resorufin) O-dealkylation (MROD, EROD, PROD, and BROD) profile was indicative of substrate selectivity typical of 3MC- and PB-induced P450s. MROD and BROD showed the greatest ability to discriminate between alligator hepatic microsomes induced by 3MC and PB, respectively. This is in contrast to mammals, in which EROD is a biomarker of polycyclic aromatic hydrocarbon exposure because of its ability to discriminate the induction of CYP 1A. In a similar manner, PROD is a highly preferred activity of CYP 2B in mammals; thus, it is used to indicate CYP 2B induction. The induction of P450 by PB is a general phenomenon in mammals and birds. To the best of our knowledge, this is the first report demonstrating PB induction of P450 activities typical of the mammalian CYP 2 family isoforms in alligator or any reptilian liver. The importance of this finding to the evolution of CYP 2 family regulation by PB is heightened by the fact that induction by this xenobiotic is not common to fish and other lower vertebrates (Ertl RP and Winston GW, Comp Biochem Physiol, in press). Although indicating the presence of CYP 1A- and CYP 2B-like isoforms in alligator, it remains to be established how closely related these alligator P450s are to mammalian isoforms.
密西西比鳄至少有两类可诱导的肝微粒体细胞色素P450（CYP）：（1）由3-甲基胆蒽（3MC）诱导的那些，以及（2）由苯巴比妥（PB）诱导的那些。这些外源性化合物的诱导速率明显慢于哺乳动物的报道速率。一氧化碳结合、蛋白质免疫印迹和酶活性测量表明，至少需要48 - 72小时才能达到完全诱导。甲氧基、乙氧基、戊氧基和苄氧基苯并恶唑酮（试卤灵）O-脱烷基化（MROD、EROD、PROD和BROD）谱表明了3MC和PB诱导的P450典型的底物选择性。MROD和BROD分别显示出区分3MC和PB诱导的密西西比鳄肝微粒体的最大能力。这与哺乳动物不同，在哺乳动物中，EROD是多环芳烃暴露的生物标志物，因为它能够区分CYP 1A的诱导。以类似的方式，PROD是哺乳动物中CYP 2B的高度优选活性；因此，它用于指示CYP 2B的诱导。PB诱导P450是哺乳动物和鸟类中的普遍现象。据我们所知，这是第一份证明PB诱导密西西比鳄或任何爬行动物肝脏中典型的哺乳动物CYP 2家族同工型P450活性的报告。这一发现对于PB对CYP 2家族调控进化的重要性因以下事实而增强：这种外源性化合物的诱导在鱼类和其他低等脊椎动物中并不常见（Ertl RP和Winston GW，《比较生物化学与生理学》，即将出版）。虽然表明密西西比鳄中存在CYP 1A和CYP 2B样同工型，但这些密西西比鳄P450与哺乳动物同工型的亲缘关系有多近仍有待确定。
