Molecular Pharmacology, Vol 8, 667-680, Copyright © 1972 by the American Society for Pharmacology and Experimental Therapeutics

Genetic Expression of Aryl Hydrocarbon Hydroxylase Induction

IV. Interaction of Various Compounds with Different Forms of Cytochrome P-450 and the Effect on Benzo[a]pyrene Metabolism in Vitro

¹ Section on Developmental Pharmacology, Laboratory of Biomedical Sciences, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, Maryland 20014

The phenomena of increased type II binding, measured by pyridine interaction with oxidized cytochrome P-450 in vitro, and of decreased type I binding, determined by hexobarbital combination with oxidized P-450 in vitro, are related to aryl hydrocarbon hydroxylase induction by aromatic hydrocarbons in genetically responsive mice and do not occur in 3-methylcholanthrene-treated genetically nonresponsive mice. A method for determining specific binding between P-450 and compounds absorbing in the 350-450 nm region is described.

Various lipophilic compounds preferentially inhibit the hydroxylase activity from control or 3-methylcholanthrene-treated genetically nonresponsive mice, whereas other compounds selectively block the 3-methylcholanthrene-induced enzyme activity. By observing the preferential inhibition of one or the other form of hydroxylase activity, one may be able to determine the form of cytochrome P-450 with which a given compound binds. Hence we suggest that -naphthoflavone, -naphthoflavone, 2,5-diphenyloxazole, and lindane (-hexachlorocyclohexane) interact with a spectrally distinct type a species of P-450, the formation of which is associated with hydroxylase induction by aromatic hydrocarbons. Phenylimidazoles, 2-diethylaminoethyl-2,2-diphenyl valerate HCl (SKF 525-A), metyrapone, 2,2-bis(p-chlorophenyl)-1,1,1-trichloroethane, 17-estradiol , ⁹-tetrahydrocannabinol, cholecalciferol, pyridine, n-octylamine, and aniline inhibit aryl hydrocarbon hydroxylase activity by competing with benzo[a]pyrene at a type b P-450 active site. A third class of compounds inhibits both type a and b hydroxylase activities equally, and a fourth class of compounds does not affect either form of the enzyme system. Hexachlorobenzene, - and -naphthoflavone, 2,5-diphenyloxazole, lindane, and derivatives of 2-phenylbenzothiazole interact differently with the hepatic enzyme in phenobarbital-treated mice and in control mice. In microsomes from mouse kidney and from rat liver or kidney, the control and 3-methylcholanthrene- or phenobarbital-inducible hydroxylase activities are preferentially inhibited by many of these same compounds in the same manner, indicating that the two forms of the enzyme active site are probably the same in the liver and kidney of the mouse and rat.

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