Microsomal monooxygenase systems contain multiple isozymes of cytochrome P-450 which contribute differentially to the oxidative metabolism of endogenous and exogenous substrates; isozyme differences in Km, Vmax, regioselectivity and stereoselectivity are common. Hence, modulation of the relative amounts of various P-450 isozymes can have pronounced effects on chemical metabolism and toxicity. For this reason we are studying isozyme selectivity and tissue/cell selectivity of suicide inhibitors of cytochrome P-450. The suicide inhibition by 1-aminobenzotriazole (ABT) and some of its novel N-alkylated derivatives, which we synthesized and characterized, is being studied in rabbit lung and liver. Although ABT is a potent suicide inhibitor, it shows little P-450 isozyme selectivity. N-benzyl-ABT, on the other hand, is both potent and highly selective (but still not specific; it destroys isozymes 2 and 6 associated enzyme activity but does not significantly affect isozyme 5 catalyzed activity). In a related project, the chemical nature of the alkyl-benzene metabolites, which selectively destroy pulmonary (versus hepatic) cytochrome P-450, and the biochemical nature (e.g., isozyme specificity, involvement of other enzymes and cofactors) of the pathways involved are being elucidated. Results to date demonstrate at least two distinct pathways for metabolic activation, one relying only on the presence of NADPH for suicide destruction, the second relying both on the presence of NADPH and alcohol dehydrogenase.
