Genetic expression of aryl hydrocarbon hydroxylase activity in the mouse

Monooxygenases require NADPH and molecular oxygen during the metabolism of numerous endogenous hydrophobic substrates and carcinogenic and toxic exogenous chemicals. The complexity of these membrane‐bound multicomponent drug‐metabolizing enzyme systems is reviewed. What “aryl hydrocarbon (benzo[a] pyrene) hydroxylase activity” actually represents is reviewed and discussed. At least two forms of the hydroxylase activity exist and we suggest that they are associated with different molecular species of membranebound CO‐binding hemoprotein (i.e., they are associated with different enzyme active‐sites). At least two, and probably more than two, nonlinked loci are responsible for the genetic expression of new cytochrome P 1 450 formation and aryl hydrocarbon hydroxylase induction — and the stimulation of 10 other monooxygenase “activities” — in the mouse treated with certain aromatic hydrocarbons. The individual variability of hydroxylase activity in an inbred and in a randombred strain of mice is illustrated. The basal hydroxylase activity appears to be inherited differently from the aromatic hydrocarbon‐inducible hydroxylase activity. The potent inducer 2,3,7,8‐tetrachlorodibenzo‐ p ‐dioxin can stimulate increases in these hepatic monooxygenase activities and P 1 450 formation in socalled “nonresponsive” mice, whereas inducers such as β‐naphthoflavone and 3‐methylcholanthrene cannot. Thus, the genetically “nonresponsive” mice apparently possess the structural and regulatory genes necessary for expression of these inducible monooxygenase activities and associated new formation of cytochrome P 1 450. We suggest that a mutation has occurred in the “nonresponsive” inbred strains that results in production of an inducer‐binding receptor having a diminished affinity for aromatic hydrocarbons.