Oxidative metabolism of estrogens in rat intestinal mitochondria

Intestinal epithelial cells are capable of metabolizing a wide variety of exogenous substrates. To determine how this metabolic capacity may affect endogenous substances such as steroid hormones, we examined the ability of rat gut epithelial preparations to hydroxylate estradiol at the C-2 position. Utilizing a site-specific tritium exchange assay, an active estrogen 2-hydroxylase system was shown to be localized to gut mitochondria throughout the intestine, with enzymatic activities comparable to the activity in crude hepatic homogenates of non-induced animals (0.2 nmol/min/mg protein). Gas chromatography-mass spectrometry confirmed the formation of C-2 hydroxylated estrogens by these mitochondrial preparations. The enzyme system was shown to involve a saturable monooxygenase, utilizing NADH (preferably) or NADPH in a protein- and time-dependent fashion. The Michaelis-Menten constant for this pathway was approximately 150 microM. Enzyme activity decreased by 20% in the presence of carbon monoxide, and was largely unaffected by organic P450 inhibitors such as alpha-naphthoflavone, metyrapone, and SKF-525A. These studies suggest that intestinal mitochondria are able to contribute to the oxidative metabolism of endogenous estrogens circulating within the enterohepatic pool.