The conversion of testosterone to testololactone has been studied in soluble and partially purified enzyme systems derived from Penicillium lilacinum. This transformation is carried out by two steroid-induced enzyme systems: a diphosphopyridine nucleotide-linked 17β-hydroxysteroid dehydrogenase that catalyzes the reversible interconversion of testosterone and 4-androstene-3,17-dione; and a lactonizing enzyme system that converts 4-androstene-3,17-dione to testololactone. The lactonization reaction shows an absolute requirement for reduced triphosphopyridine nucleotide and for molecular oxygen. Tracer studies with oxygen have shown that the single oxygen atom incorporated during the formation of the lactone is derived from molecular oxygen and is located in the ethereal and not the carbonyl group linked to C-17.
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