Turnover of hepatic microsomal cytochrome P4501A protein and heme in beta-naphthoflavone-induced Fundulus heteroclitus.

Earlier studies showed that in Fundulus heteroclitus liver, the content of microsomal cytochrome P4501A (CYP1A) protein induced by beta-naphthoflavone is elevated for weeks after its induction, whereas the content of the induced CYP1A mRNA peaks and declines rapidly (2-4 days). This finding suggests an unusually long half-life for teleost CYP1A. We examined directly the half-live of hepatic microsomal CYP1A protein and heme moieties in F. heteroclitus, by measuring the incorporation and disposition of [3H]-leucine and [14C]-5-aminolevulinic acid (ALA) in fish that had been treated with the CYP1A inducer beta-naphthoflavone prior to administration of label. Incorporation of [3H]-leucine into trichlororacetic acid-precipitable (total) microsomal protein and into immunoprecipitated CYP1A protein peaked between 1.5 to 4 hours. Incorporation of [14C]-ALA into total microsomal (heme) protein peaked at 24 hours, and into the heme of CYP1A at 8 hours. Loss of label from total microsomal protein was biphasic, giving half-lives of 8 and 138 hours for bulk protein, and 66 and 141 hours for heme. CYP1A apoprotein had a half-life of 32 to 39 hours based on isotopic loss and 43 hours calculated from the kinetics of enzyme induction. The apparent half-life of CYP1A heme was approximately 100 hours, which was substantially greater than the protein half-life, in contrast to mammalian P450s, in which half-lives for protein and heme are similar. The distinction in heme half-lives could represent fundamental differences between fish and mammals in P450 heme/protein interactions. The protein half-life is inconsistent with a prolonged stability of CYP1A, yet 30% of the [3H] and 40% of the [14C] seen in CYP1A at the peak of incorporation could still be measured after eight days. Mechanisms other than inherently long-lived protein, perhaps stabilization or enhanced translation of a minor mRNA pool, might contribute to persistence of CYP1A.