The mechanism of reduction of cholesterol to coprostanol by Eubacterium coprostanoligenes ATCC 51222 was studied by incubating the bacterium with a mixture of α- and β-isomers of [4-3H, 4-14C] cholesterol. Coprostanol, isolated after incubation of [4-3H, 4- 14C] cholesterol in a growth medium under anaerobic conditions, retained 97% of the tritium originally present in cholesterol. The majority of this tritium (64%), however, was in the C-6 ...