Isotope effect studies on the mechanism of the cytochrome P-450IIA1-catalyzed formation of delta 6-testosterone from testosterone.

Testosterone metabolism by cytochrome P-450IIA1 results in four metabolites: 6 alpha-hydroxytestosterone; 7 alpha-hydroxytestosterone; 17 beta-hydroxy-4,6-androstadiene-3-one (delta 6-T); and 17 beta-hydroxy-4,6-androstadiene-3-one-6,7-oxide. The epoxide is formed upon further oxidation of delta 6-T, and its formation is in competition with the dissociation of delta 6-T from the active site. The analysis of the KM and Vmax values, as well as the product ratios for testosterone and three selectively deuterated analogs, strongly suggest that delta 6-testosterone formation occurs primarily by initial hydrogen atom abstraction at the 6 alpha-position followed by abstraction of the 7 alpha-hydrogen atom.