Reduction of 7-ketolithocholic acid by human liver enzyme preparations in vitro

The formation of chenodeoxycholic and ursodeoxycholic acids from 7-ketolithocholic acid by human liver preparations was examined in vitro. Liver preparations were incubated with 7-ketolithocholic acid at pH 5.5 in a sodium-potassium-phosphate buffer containing NADPH or NADH. The products formed were analyzed by gas chromatography and gas chromatography-mass spectrometry. Results showed that chenodeoxycholic and ursodeoxycholic acids could be formed from 7-ketolithocholic acid by human liver enzyme(s). The enzyme(s) required NADPH but not NADH as coenzyme and was localized largely in the microsomes. The conjugated 7-ketolithocholic acid, especially the taurine conjugated, was predominantly reduced to chenodeoxycholic acid, whereas the unconjugated 7-ketolithocholic acid was not reduced well to either chenodeoxycholic acid or ursodeoxycholic acid. Thus the reduction of 7-ketolithocholic acid by human liver enzyme(s) was found to be dependent on whether the substrate was conjugated or not.