Reductive biotransformation of 3-oxo bile acids in human blood.

7 alpha,12 alpha-Dihydroxy-3-oxo-5 beta-cholanoic acid labeled with 18O atoms was incubated with human blood, and the biotransformation products were separated and characterized by gas chromatography-mass spectrometry as the pentafluorobenzyl ester-trimethylsilyl and -dimethylethylsilyl ether derivatives. 3 beta,7 alpha,12 alpha-Trihydroxy-5 beta-cholanoic acid was identified as a main metabolite. When 3-oxo bile acid was incubated with human blood denatured at 70 degrees C for 2 min, no metabolites were formed. The enzymic reduction activity proved to be localized in the red blood cell fraction.