On the stereospecificity of microsomal "26"-hydroxylation in bile acid biosynthesis

The stereospecificity of microsomal "26" -hydroxylation in bile acid biosynthesis was studied. Cholesterol was biosynthesized from [2-14C] mevalonate by a rat liver preparation. The cholesterol was converted stepwise into 3alpha, 7alpha, 12alpha-trihydroxy-5beta-cholestan-26-oic acid by microsomal and soluble fractions of rat liver homogenate. The 3alpha, 7alpha, 12alpha-trihydroxy-5beta-cholestan-26-oic acid was decarboxylated chemically and the carbon dioxide was assayed for 14C. The amount of radioactivity in the liberated carbon dioxide was assayed for 14C. The amount of radioactivity in the liberated carbon dioxide was such as to indicate complete stereospecificity of the microsomal "26" -hydroxylase system. The system hydroxylates the methyl group in position C-26 (the 25-pro-R methyl group) and its stereospecificity is opposite that of the mitochondrial "26" -hydroxylase system which hydroxylates the methyl group in position C-27 (the 25-pro-S methyl group).