The turnover of cytochrome P450b

The turnover of the heme and apocytochrome moieties of phenobarbital-inducible microsomal cytochrome P450 (P450b) was investigated. Adult male Sprague-Dawley rats were treated with phenobarbital for 5 days and injected with [35S]methionine and the heme precursor delta-[3H]-aminolevulinic acid. P450b was isolated by immunoprecipitation and quantitated by rocket immunoelectrophoresis. The isotope disappearance curves revealed a mean half-life (T1/2) of 12.4 h for the heme moiety and a T1/2 of 19.1 h for the apoprotein moiety of P450b. The apparently slower catabolic rate of the apoprotein may be due to reutilization of [35S]methionine and does not exclude synchronous turnover of the two moieties. Our data are consistent with the kinetics of the drug-mediated induction of cytochrome P450b.