A novel enzyme system for the reduction of 3-oxo bile acids in human red blood cells

7α,12α-Dihydroxy-3-oxo- and 3,7,12-trioxo-5β-cholanoic acids labeled with 18O atoms were incubated with human red blood cells, and the biotransformation products were separated and characterized by gas chromatography-mass spectrometry as the pentafluorobenzyl ester-trimethylsilyl and -dimethylethylsilyl ether derivatives with the negative ion chemical ionization mode. The reduced products, 3β,7α,12α-trihydroxy-5β-cholanoic acid for the former, and 3α-hydroxylated dioxo bile acid together with 3β-hydroxylated 7,12-dioxo-5β-cholanoic acid for the latter, were identified as metabolites. When 3-oxo bile acid was incubated with human blood denatured at 70°C for 2 min, no metabolites were formed. The enzymic reduction activity has been localized in the red blood cell fraction.