Microbial Transformation of Cholesterol into Coprostanol: Properties of a 3‐Oxo‐Δ⁴‐Steroid‐5β‐Reductase

The conversion of 4 cholesten 3 one into 5β cholestan 3 one (coprostanone) by microorganisms in caecal contents from rats was studied. The enzyme activity was present in the soluble part of a homogenate obtained by freeze pressing of caecal contents of rats. The major part of the activity was precipitated between 25 and 50% saturation with ammonium sulfate. The 3 oxo Δ⁴ steroid 5β reductase activity required NADH as cofactor and was inhibited by p chloromercuribenzoate. The enzyme preparation also catalyzed the reduction of the Δ⁴ double bond in progesterone and testosterone but did not catalyze reduction of the Δ⁵ double bond in cholesterol, pregnenolone or dehydroepiandrosterone. The rate of reduction of the Δ⁴ double bond in progesterone was more than twice as fast as the rate of reduction of the Δ⁴ double bond in 4 cholesten 3 one. The mechanism of reduction of Δ⁴ double bonds in 3 oxo Δ⁴ steroids by the microbial 3 oxy Δ⁴ steroid 5β reductase was found to involve transfer of hydrogen from the 4B position of NADH to the 5β position of the steroid.