Microbial Transformation of Cholesterol into Coprostanol

The conversion of 4‐cholesten‐3‐one into 5β‐cholestan‐3‐one (coprostanone) by microorganisms in caecal contents from rats has been 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 could be precipitated between 25 and 50% saturation with ammonium sulphate. 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‐oxo‐Δ4‐steroid 5β‐reductase was found to involve transfer of hydrogen from the 4B‐position of NADH to the 5β‐position of the steroid.