The in vitro binding of metyrapone to dithionite-reduced cytochrome P-450 in hepatic microsomes from rats treated in vivo with thirteen different xenobiotics was studied spectrophotometrically. The proportion of cytochrome P-450 that bound metyrapone increased 1.8-fold to about 78% following treatment with phenobarbitone (PB) and PB-type inducers (trans-stilbene oxide, 2,2′,4,4′-tetrachloro-, 2,2′,4,5,5′-pentabromo- and 2,2′,4,4′,5,5′-hexachlorobiphenyl) but remained unaltered following treatment with 3-methylcholanthrene (MC) and MC-type inducers (benzo[a]pyrene, β-naphthoflavone and 3,3′,4,4′-tetrabromobiphenyl). The simulatenous induction of the PB-inducible and MC-inducible forms of cytochrome P-450 by administering Aroclor 1254 or by coadministering PB with MC increased the proportion of cytochrome P-450 that bound metyrapone to 74 and 78% respectively. PB treatment increased whereas MC treatment decrease the binding affinity for metyrapone by approximately 20-fold. Treatment with isosafrole or metyrapone itself failed to stimulate metyrapone binding. In contrast, pregnenolone-16α-carbonitrile was indistinguishable from PB in its ability to increase the binding capacity and binding affinity for metyrapone. Our results indicate that metyrapone binding is not specific for cytochrome P-450b, the major PB-inducible hemoprotein, as has been proposed [V. Luu-The, J. Cumps and P. Dumont, Biochem. biophys. Res. Commun. 93, 776 (1980)].
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