Equilibrium dialysis studies established that 1-[4′-(3H)-phenyl]imidazole (PI) was bound to hepatic microsomal suspensions from control, phenobarbital (PB)- and 3-methylcholanthrene (3MC)-treated rats and that the binding was directly related to the cytochrome P-450 content. Computer-assisted Scatchard plot analysis of the binding data indicated the existence of two major types of microsomal binding sites in both control and induced rats, one with a high affinity (Ka ∼ 1.5 × 107 M-1) and the other with a low affinity (Ka ∼ 5 × 105 M-1) for PI. The binding of PI to the highly purified, individual cytochrome P-450s that constituted the major forms from the PB- and β-naphthoflavone (βNF)-induced rats exhibited affinities similar to the high and low affinity binding sites observed in microsomal suspensions. The two types of PI binding sites were characteristics of two classes of cytochrome P-450, and the major cytochrome induced by PB and 3MC (or βNF) were each associated with one of these two classes. In concurrence with this, it was shown that, although PI was an excellent inhibitor of aromatic hydrocarbon hydroxylase (AHH) activity in PB-induced rats, it exhibited little or no inhibitory activity towards AHH activity in 3MC-induced animals.
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