Forty years ago, Coulter and Talalay (A. W. Coulter and P. Talalay, J. Biol. Chem. 243:3238–3247, 1968) established the oxygenase-dependent pathway for the degradation of testosterone by aerobes. The oxic testosterone catabolic pathway involves several oxygen-dependent reactions and is not available for anaerobes. Since then, a variety of anaerobic bacteria have been described for the ability to degrade testosterone in the absence of oxygen. Here, a novel, oxygenase-independent testosterone catabolic pathway in such organisms is described. Steroidobacter denitrificansDSMZ18526 was shown to be capable of degrading testosterone in the absence of oxygen and was selected as the model organism in this study. In a previous investigation, we identified the initial intermediates involved in an anoxic testosterone catabolic pathway, most of which are identical to those of the oxic pathway demonstrated in Comamonas testosteroni. In this study, five additional intermediates of the anoxic pathway were identified. We demonstrated that subsequent steps of the anoxic pathway greatly differ from those of the established oxic pathway, which suggests that a novel pathway for testosterone catabolism is present. In the proposed anoxic pathway, a reduction reaction occurs at C-4 and C-5 of androsta-1,4-diene-3,17-dione, the last common intermediate of both the oxic and anoxic pathways. After that, a novel hydration reaction occurs and a hydroxyl group is thus introduced to the C-1α position of C19steroid substrates. To our knowledge, an enzymatic hydration reaction occurring at the A ring of steroid compounds has not been reported before.
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