Metabolism of prostaglandins and xenobiotics by adrenal microsomal monooxygenase in the guinea pig

The possibility that prostaglandins could serve as substrates for the guinea pig adrenal microsomal monooxygenase was investigated. The binding of PGE₁ to adrenal microsomes was found to exhibit a reverse type I spectral change. Also PGE₁ diminished the magnitude of type I spectrum elicited by cortisol binding to adrenal microsomes. The incubation of ³H]PGE₁ or of ³H]PGE₂ with adrenal microsomes supplemented with NADPH yielded primarily the respective 19-hydroxy metabolite. The enzymatic activity catalyzing this hydroxylation appears to be a typical monooxygenase, requiring NADPH for activity and being strongly inhibited by metyrapone, SKF 525A, and cytochrome c. Carbon monoxide at a ratio of 9:1 to oxygen moderately inhibited the hydroxylation of PGE₁. Whereas the liver catalyzed the hydroxylation of PGE₁ and PGA₁ equally well, the adrenal microsomes preferentially catalyzed the hydroxylation of PGE₁. This finding and the observation that α-naphthoflavone is a weak inhibitor of the adrenal PGE₁ hydroxylation points to significant differences between the adrenal and liver prostaglandin hydroxylation activities. Cortisol, which is a substrate for adrenal monooxygenase, strongly inhibited PGE₁ and PGE₂ hydroxylation. By contrast, certain xenobiotics (ethylmorphine, hexobarbital, benzpyrene), which are also metabolized by adrenal microsomes, only slightly inhibited the hydroxylation of PGE₁. Similarly, PGE₁ only weakly inhibited ethylmorphine and benzphetamine demethylation and hexobarbital hydroxylation. These observations suggest that adrenal microsomes contain several monooxygenases with different affinities for prostaglandins and for the different xenobiotic substrates.