CYTOCHROME-P450 AND ITS INTERACTIONS WITH THE HEME BIOSYNTHETIC-PATHWAY

An important factor in disruption of hepatic heme biosynthesis by porp hyrinogenic drugs appears to be interaction with one or several cytoch rome P450 isozymes. Clarification of the nature of the interaction bet ween porphyrinogenic drugs and cytochrome P450 isozymes, as well as id entification of the isozymes involved, will be helpful in extrapolatin g the results of animal experimentation to humans. Administration of g riseofulvin to mice results in accumulation in the liver of two N-alky lated protoporphyrins (PPs). The major N-alkyl PP, N-griseofulvin PP, which is devoid of ferrochelatase-inhibitory activity, was shown to be the precursor of N-methyl PP, which is a potent ferrochelatase inhibi tor. N-Griseofulvin PP was present predominantly as the N-C regioisome r rather than the anticipated N-A regioisomer. Progesterone (PG) 6 bet a-hydroxylase and androstenedione (AD) 6 beta-hydroxylase, diagnostic markers for cytochrome P450 3Al/2 activity in rat liver, were identifi ed in chick embryo liver. The in ovo administration of xycarbonyl-1,4- dihydro-2,6-dimethyl4-ethylpyridine (4-ethyl DDC) and -(2,4,6-trimethy lphenyl)thioethyl]-4-methylsydnone (TTMS) caused inactivation of chick embryo hepatic PG and AD 6 beta-hydroxylases. Ascorbate was shown to inhibit uroporphyrin accumulation in cultures of chick embryos treated with porphyrinogenic drugs and in the livers of an ascorbate-requirin g rat strain treated with a porphyrinogenic combination of chemicals. Ascorbate appears to act by inhibiting oxidation of uroporphyrinogen t o uroporphyrin. Avian hepatocytes were cultured in 48-well plates and directly assayed within the wells for the activity of 7-ethoxyresorufi n O-deethylation (EROD), and for porphyrin and protein concentration b y a fluorescence plate reader. Uroporphyrin was the main porphyrin to accumulate in response to 2,3,7,8-tetrachlorodibenzo;p-dioxin (TCDD) i n chick embryos, pheasants, ducks, and herring gulls; heptacarboxylate d porphyrin predominated in turkey hepatocytes. The EROD-inducing pote ntial of complex mixtures of polyhalogenated aromatic hydrocarbons fro m herring gull eggs was assessed and compared with that of TCDD. A nuc lear heme pool appears to regulate the transcription of the cytochrome P450 2B1/2B2 genes in rat Liver, by modulating binding of a transcrip tion factor(s) that appears to be involved in the transcriptional acti vation by phenobarbitone.