Isoform-selective mechanism-based inhibition of human cytochrome P450 1A2 by furafylline

Biotransformation reactions catalyzed by human cytochrome P450 1A2 (P450 1A2) appear to play a significant role in both the metabolic clearance of drugs and the activation of environmental contaminants and drugs to toxic or carcinogenic species. Furafylline is a potent and selective inhibitor of P450 1A2 activity in human liver microsomes [Sesardic, D., Boobis, A., Murray, B., Murray, S., Segura, J., De La Torre, R., and Davies, D. (1990) Br. J. Clin. Pharmacol. 29, 651-663] which may be of great utility in defining the role of P450 1A2 in metabolic processes. We have investigated the hypothesis that furafylline is a mechanism-based inhibitor of P450 1A2. Key findings consistent with this hypothesis are the following: (1) Furafylline causes a time- and cofactor-dependent loss of P450 1A2 activity which does not return upon dialysis. (2) The loss of activity is associated with a reduction of P450 spectral content which is in turn proportional in amount to P450 1A2-associated catalytic activity in uninhibited microsomes from 7 individual livers. (3) The inactivation of P450 1A2 is characterized by a Ki of 23 microM, a kinact of 0.87 min-1 and a furafylline depletion-based partition ratio of approximately 3-6 metabolic events per inactivating event. (4) The processing of the C-8 methyl group of furaylline is involved in inactivation as demonstrated by the observation of a deuterium isotope effect of approximately 2.0 on kinact and no effect on Ki when the C-8 methyl group protons of furafylline are replaced with deuterium atoms.(ABSTRACT TRUNCATED AT 250 WORDS)