Relationship between the newly identified bile acid binder and bile acid oxidoreductases in human liver

Gel filtration fractions of human hepatic cytosol obtained from an autopsy liver were examined for elution of bile acid oxidoreductases. Several enzymes including 3 alpha-hydroxysteroid dehydrogenase (3 alpha-HSD), 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD), 3-ketosteroid reductase, and dihydrodiol dehydrogenase eluted mainly in the 30,000-40,000 Mr fractions known to contain the newly identified bile acid binder (Stolz, A., Sugiyama, Y., Kuhlenkamp, J., and Kaplowitz, N. (1984) FEBS Lett. 177, 31-35). These enzyme activities could be resolved into six peaks of dihydrodiol dehydrogenase activity on chromatofocusing, some of which also had oxidoreductase activity with bile acids. Using equilibrium dialysis, the major lithocholate-binding activity was found to coelute with 3 beta-HSD, completely separate from 3 alpha-HSD. Reexamination of the surgical liver specimen originally used to purify the bile acid binder confirmed these results. The peak fraction from chromatofocusing, which exhibited the bulk of binding activity with bile acids, had 3 beta-HSD activity, whereas other fractions had 3 alpha-HSD. Anti-serum to the previously purified binder identified a single 36-kDa protein in both liver specimens and exclusively in the chromatofocusing fractions containing both the binding and 3 beta-HSD activity. However, upon further purification of the binder from this fraction, 3 beta-HSD activity was separated from the binder, but the homogeneous protein retained dihydrodiol dehydrogenase activity. Thus, in contrast to the rat in which the major bile acid binder is identical to 3 alpha-HSD, in human liver the bile acid binder is distinct from 3 alpha-HSD and copurifies with a different oxidoreductase that has dihydrodiol dehydrogenase activity but no activity with bile acids.