KINETIC-STUDIES OF THE INHIBITION OF A HUMAN LIVER 3-ALPHA-HYDROXYSTEROID/DIHYDRODIOL DEHYDROGENASE ISOZYME BY BILE-ACIDS AND ANTIINFLAMMATORY DRUGS

We have investigated the steady-state kinetics for a cytosolic 3 alpha-hydroxysteroid/dihydrodiol dehydrogenase isozyme of human liver and its inhibition by several bile acids and anti-inflammatory drugs such as indomethacin, flufemanic acid and naproxen. Initial velocity and product inhibition studies performed in the NADP(+)-linked (S)-1-indanol oxidation at pH 7.4 were consistent with a sequential ordered mechanism in which NADP(+) binds first and leaves last. The bile acids and drugs, competitive inhibitors with respect to the alcohol substrate, exhibited uncompetitive inhibition with respect to the coenzyme, with K-i values less than 1 mu M, whereas indomethacin exhibited noncompetitive inhibition (K-i<24 mu M). The kinetics of the inhibition by a mixture of the two inhibitors suggests that bile acids and drugs, except indomethacin, bind to overlapping sites at the active center of the enzyme-coenzyme binary complex.