Association of the Ah locus with specific changes in metyrapone and ethylisocyanide binding to mouse liver microsomes.

The genetic trait of "responsiveness," which refers to the capacity for induction of cytochrome P-448 and numerous monooxygenase activities by certain aromatic hydrocarbons, is known to segregate almost exclusively as a single autosomal dominant gene among progeny of appropriate crosses originating from the responsive C57BL/6 and the nonresponsive DBA/2 inbred mouse strains. In this report the allele for responsiveness is shown to be associated with (a) increases in the apparent KS values for metyrapone bound to reduced P-450; (b) increases in the ethylisocyanide difference ratio (deltaA455-490/deltaA430-490);(c) increases in the deltaA455-490 per mg of microsomal protein but not in the deltaA430-490 per mg of protein from the reduced P-450-ethylisocyanide complex; (d) an approximately 2-nm hypsochromic shift in the spectral maximum in the 446 nm region for the reduced P-450-metyrapone complex; (e) an approximately 2-nm hypsochromic shift of the absorption maximum in the 455 nm region, but not of the maximum in the 430 nm region, for the reduced P-450-ethylisocyanide complex; and (f) larger increases in the deltaA455-490 than in the deltaA430-490 per mg of microsomal protein for the reduced P450-ethylisocyanide complex as a function of increasing pH. All of these phenomena are felt to be associated with the genetically regulated induction of liver microsomal cytochrome P-448 by polycyclic aromatic compounds. Whereas increases in the total hepatic P-450 content appear to be expressed almost exclusively as a single autosomal dominant trait, the increase in apparent KS value for metyrapone bound to reduced P-450 appears to be expressed additively. The reason for this finding is unclear. The increase in apparent KS value for metyrapone in 3-methylcholanthrene-treated rats is known to occur even when the induction process is presumably blocked by treating the rat concomitantly with cycloheximide. Several lines of evidence in this report indicate that, although total P-450 content does not increase in C57BL/6N mice treated with 3-methylcholanthrene plus cycloheximide, hepatic P-448 induction does occur; P-448 induction does not occur in DBA/2N mice under these same conditions. These results indicate that cytochrome P-448 induction is relatively resistant to the inhibition of protein synthesis and that a responsive animal treated with 3-methylcholanthrene plus cycloheximide cannot be considered experimentally the same as a genetically nonresponsive animal treated with 3-methylcholanthrene alone.