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 δ-[3H]-aminolevulinic acid. P450b was isolated by immunoprecipitation and quantitated by rocket immunoelectrophoresis. The isotope disappearance curves revealed a mean half-life (T81050-8/asset/equation/feb20014579382810508-math-si1.gif?v=1&s=8baca5de59ee80976bf40beeec6eec2025a414dd)
) of 12.4 h for the heme moiety and a T81050-8/asset/equation/feb20014579382810508-math-si2.gif?v=1&s=e35c81ad5644fce3e148f78624c0bf9bb8762b66)
of 19.1 h for the apoprotein moeity 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.
