Conversion of the 2,2,6,6-Tetramethylpiperidine Moiety to a 2,2-Dimethylpyrrolidine by Cytochrome P450: Evidence for a Mechanism Involving Nitroxide Radicals and Heme Iron

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• 作者：Wenji Yin ; Kaushik Mitra ; Ralph A. Stearns ; Thomas A. Baillie ; and Sanjeev Kumar
• 刊名：Biochemistry
• 出版年：2004
• 出版时间：May 11, 2004
• 年：2004
• 卷：43
• 期：18
• 页码：5455 - 5466
• 全文大小：215K
• 年卷期：v.43,no.18(May 11, 2004)
• ISSN：1520-4995

文摘

Earlier we described a novel cytochrome P450 (CYP) catalyzed metabolism of the 2,2,6,6-tetramethylpiperidine (2,2,6,6-TMPi) moiety in human liver microsomes to a ring-contracted 2,2-dimethylpyrrolidine (2,2-DMPy) [Yin, W., et al. (2003) Drug Metab. Dispos. 31, 215-223]. In the currentreport, evidence is provided for the involvement of 2,2,6,6-TMPi hydroxylamines and their one-electronoxidation products, the nitroxide radicals, as intermediates in this pathway. Nitroxide radicals could beconverted to their corresponding 2,2-DMPy metabolites by "inactivated CYP3A4", as well as by a numberof other heme proteins and hemin, suggesting that this is a heme-catalyzed process. The conversion ofnitroxide radicals to the 2,2-DMPy products by CYP3A4 or hemin was accompanied by the generationof acetone in incubations, providing evidence that the three-carbon unit from 2,2,6,6-TMPi was lost asacetone. With one model 2,2,6,6-TMPi nitroxide radical, evidence for an alternate pathway, which resultedin the formation of an intermediate that incorporated two oxygen atoms from water of the incubationmedium before collapsing to the 2,2-DMPy product, was also obtained. To account for both pathways, amechanism involving interaction of the nitroxide radicals with heme iron (Fe^III), followed by a homolyticscission of the N-O bond and transfer of the nitroxide oxygen to heme iron to form a perferryl-oxygencomplex, is proposed. The nitrogen-centered 2,2,6,6-TMPi radical thus formed then precipitates thecontraction of the piperidine ring via C2-C3 bond cleavage, and the resulting product further oxidizes toan exocyclic iminium ion (by the perferryl-oxygen complex); the latter may undergo capture by waterfrom the incubation medium and eliminate the three-carbon unit via N-dealkylation. It remains to bedetermined whether this novel interaction of nitroxide radicals with heme iron has any relevance in regardto the known biological properties of these stable radical species.