Mechanism-based inactivation of cytochromes P4502B1 and P4502B6 by 2-phenyl-2-(1-piperidinyl) propane

2-Phenyl-2-(1-piperidinyl)propane (PPP), an analog of phencyclidine, was te sted for its ability to inactivate cytochrome P450s (P450s) 2B1 and 2B6. PP P inactivated the 7-(benzyloxy)resorufin O-dealkylation activity of liver m icrosomes obtained from phenobarbital-induced rats with a K-I of 11 mu M. T he 7-ethoxy-4-(trifluoromethyl)coumarin O-deethylation activity of purified rat liver P450 2B1 and expressed human P450 2B6 was inactivated by PPP in a reconstituted system containing NADPH-cytochrome P450 reductase and lipid . In the presence of NADPH, the loss of activity was time- and concentratio n-dependent, and followed pseudo first order kinetics. The rate of inactiva tion for P450 2B1 was 0.3 min(-1), and the concentration of PPP required to achieve half-maximal inactivation was 12 mu M. The time for 50% of the P45 0 2B1 to become inactivated at saturating concentrations of PPP was 2.5 min . P450 2B6 was inactivated with a k(inact) of 0.07 min(-1), a K-I of 1.2 mu M, and a t(1/2) of 9.5 min. The inactivated P450s 2B1 and 2B6 lost about 2 5 and 15%, respectively, of their ability to form a CO-reduced complex, sug gesting that the loss of activity was caused by a PPP modification of the a poprotein rather than the heme. The estimated partition ratio for P450s 2B1 and 2B6 with PPP was 31 and 15, respectively. The inactivation was not rev ersible and reductase activity was not affected. Coincubation of P450 2B1 a nd 2B6 with PPP and NADPH in the presence of an alternate substrate protect ed both enzymes from inactivation. The exogenous nucleophile GSH did not af fect the rate of inactivation. PPP-inactivated P450s 2B1 and 2B6 were recog nized on Western blots by an antibody generated to phencyclidine that had b een conjugated to BSA. Stoichiometries of 1.4:1 and 0.7:1 were determined f or the binding of a [H-3]PPP metabolite to P450 2B1 and 2B6, respectively.