Steroids as Substrates for Cholesterol: Oxygen Oxidoreductase, with Special Reference to 3β-Hydroxy Bile Acids

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Abstract
Cholesterol: oxygen oxidoreductase [EC 1.1.3.6] from Brevibacterium sterolicum (ATCC 21387) was found to catalyze the oxidation of steroids such as sterols, steroid hormones, and bile acids having a free C-3β hydroxyl group. However, the enzyme was inactive towards estradiol and estriol and had a weak activity towards steroids with functional groups adjacent to the 3β- group on the steroid nucleus. Variation in the length of the side chain of 3β steroids had no marked effect on the activity. 3β-Hydroxy bile acids with δ4 or δ5 were oxidized to almost the same extent as cholesterol. In contrast, 3β-hydroxy bile acids without δ4 or δ5 were oxidized only to the extent of 1.4–2.1%. 3β-Hydroxychol-4 or 5-enoic acid was oxidized in the same way as cholesterol. This enzyme is useful as a simple tool for identification of 3β-hydroxy groups of bile acids.