Substrate type of induction of microsomal monooxygenases in the liver by phenobarbital

The possibility of successive induction of microsomal monooxygenases by the introduction of phenobarbital against a background of maximal induction with 3-methylcholanthrene was studied. It was found that in such a system, despite the absence of a polycyclic hydrocarbon, phenobarbital activates a further increase in the content of CO-binding hemoprotein in the form of cytochrome P-448. In addition, there is a substantial increase in the low-spin form (type b) of the hemoprotein without an increase in the concentration of its high-spin form (type a). An increase in the rate of NADPH-dependent reduction of CO-binding hemoprotein and N-demethylation of amino-pyrine is observed. The criterion of functional specificity of the hemoprotein is not the position of the CO peak of its reduced form, but the ratio of the content of high- and low-spin types of cytochrome. The possibility of induction with phenobarbital under conditions when the step of binding of the inducer to the microsomal hemoprotein is absent was demonstrated, since cytochrome P-448 lacks binding sites for phenobarbital. It is suggested that in the mechanism of phenobarbital induction of microsomal monooxygenases, activation of the genome and subsequent protein synthesis are accomplished by the substrate itself, and not by products of its primary metabolism in the microsomes.