Selectivity and kinetics of inactivation of rabbit hepatic cytochromes P450 2B4 and 2B5 by N-aralkylated derivatives of 1-aminobenzotriazole.

The kinetics of mechanism-based inactivation of phenobarbital-inducible rabbit hepatic cytochromes P450 2B4 and 2B5 by N-benzyl-(BBT) and N-alpha-methylbenzyl (alpha MB) 1-aminobenzotriazole were investigated using reconstituted P450 2B4, a stable heterologous expression system, and hepatic microsomes. Low micromolar concentrations of the 1-aminobenzotriazole derivatives caused reversible inhibition as well as rapid inactivation of reconstituted P450 2B4 and recombinant P450 2B4 and 2B5. In contrast, even at a 1000-fold higher concentration, aminobenzotriazole inactivated the expressed P450 2B enzymes less rapidly. Preincubation of phenobarbital-induced hepatic microsomes with BBT and alpha MB resulted in concentration-dependent decreases in marker activities of P450 2B4 and 2B5, benzyloxyresorufin O-debenzylase and androstenedione 15 alpha-hydroxylase, respectively. BBT caused the inactivation of P450 2B4 and 2B5 in hepatic microsomes with apparent Kl values of 1.9 and 2.4 microM and maximal rate constants of 0.29 and 0.18 min-1, respectively. alpha MB inactivated both P450 2B enzymes with similar Kl values (approximately 7 microM) and maximal rate constants only slightly higher for 2B4 compared with 2B5 (0.68 vs. 0.55 min-1). Similar P450 2B selectivity of BBT and alpha MB in both hepatic microsomes and the stable expression system further validates this new expression system and the use of the selective markers identified for 2B4 and 2B5 in hepatic microsomes. The results also provide a mechanistic basis for the high potency of the N-aralkylated 1-aminobenzotriazole derivatives in vivo and suggest that treatments that inactivate 2B4 will also lead to 2B5 inactivation.