Determination of partition ratios for allylisopropylacetamide during suicidal processing by a phenobarbital-induced cytochrome P-450 isozyme from rat liver.

Allylisopropylacetamide is shown to be a suicide substrate for the phenobarbital-inducible cytochromes P-450. In phenobarbital-induced rat liver microsomes about 70% of the cytochrome P-450-mediated N,N-dimethylaniline N-demethylase activity is sensitive to allylisopropylacetamide inactivation; the residual 30% of the N-demethylase activity is incapable of allylisopropylacetamide turnover and insensitive to allylisopropylacetamide inactivation. The partition number for inactivation of the susceptible population of cytochrome P-450 indicates turnover of 201 molecules of allylisopropylacetamide per molecule of P-450 inactivated. A purified phenobarbital-induced isozyme of cytochrome P-450, when reconstituted with purified rat liver cytochrome P-450 reductase, is also inactivated by allylisopropylacetamide in a suicide fashion with a corrected partition ratio of 184 turnovers of allylisopropylacetamide per inactivation event. This partition number is corrected for the competing O2-dependent autoinactivation of cytochrome P-450 which we have previously shown to occur with the purified isozyme (Loosemore, M., Light, D. R., and W#alsh, C. (1980) J. Biol. Chem. 255, 9017-9020). The 201 product molecules of cytochrome P-450-mediated turnover of allylisopropylacetamide in either the microsomal or purified enzyme system are probably the epoxide, are reactive toward alkylation of cellular nucleophiles, and covalently modify protein and exogenous calf thymus DNA molecules.