Kinetic Studies of the Inhibition of a Human Liver 3α-Hydroxysteroid : Dihydrodiol Dehydrogenase Isozyme by Bile Acids and Anti-inflammatory Drugs

Abstract

We have investigated the steady-state kinetics for a cytosolic 3α-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 μM, whereas indomethacin exhibited noncompetitive inhibition (K_i<24 μM). 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.