Studies on Partially Purified Pig Liver Steroid Δ4-5β-Reductase Activity

Some properties of partially purified steroid Δ4-5β-reductase activity of pig liver cytosol have been studied using testosterone as substrate. The enzymatic activity was stable for 72 h at 4° when stored in 0.05 M Tris–maleate buffer, pH 7.4 or 8.4; storage at pH 8 at 4° resulted in a 25% decrease in activity in 30 days. The pH optimum in Tris–maleate buffers was 6.4. Enzyme activity was completely inhibited by 0.2 mM p-chloromercuribenzenesulfonate and 0.2 mM p-chloromercuribenzoate. Enzyme activity was reduced by 20% and 45% with 1.0 mM iodoacetamide and 1.0 mM N-ethylmaleimide, respectively. The end products of the enzymatic reaction, NADP+ and 5β-dihydrotestosterone, inhibited the rate of reduction of testosterone. Testosterone Δ4-5β-reductase activity was present in protein of molecular weight 25 000–30 000, as determined by gel filtration.The enzyme preparation reduced a variety of C19 and C21 steroids. The highest activity (twice that for testosterone) was found with aldosterone as substrate.