Active-site directed inactivation of rat ovarian 20 alpha-hydroxysteroid dehydrogenase

Mechanism based inhibition of hydroxysteroid dehydrogenases

Steroid hormone action can be regulated not only at the receptor level but also by the enzymes that are responsible for the synthesis and degradation of biologically active steroids. Traditionally the pharmacological intervention of steroid hormone action has focused on the development of steroidal and nonsteroidal hormone receptor agonists and antagonists with appropriate pharmacokinetics. Recently, the development of selective inhibitors/inactivators of steroid metabolizing enzymes has gained momentum. This review will concentrate on the development of mechanism-based inhibitors for one class of steroid hormone transforming enzymes, the hydroxysteroid dehydrogenases.

Evidence that enzyme-generated aromatic Michael acceptors covalently modify the nucleotide-binding site of 3 alpha-hydroxysteroid dehydrogenase

The non-steroidal allylic and acetylenic alcohols 1-(4'-nitrophenyl)prop-2-en-1-ol (I) and 1-(4'-nitrophenyl)prop-2-yn-1-ol (II) are oxidized by homogeneous 3 alpha-hydroxysteroid dehydrogenase to the corresponding alpha beta-unsaturated ketones 1-(4'-nitrophenyl)prop-2-en-1-one (III) and 1-(4'-nitrophenyl)prop-2-yn-1-one (IV), which then inactivate the enzyme selectively with high affinity; low effective partition ratios are observed for the parent alcohols [Ricigliano & Penning (1989) Biochem. J. 262, 139-149]. Inactivation of 3 alpha-hydroxysteroid dehydrogenase by compound (I) displays an NAD+ concentration optimum. Scavenging experiments indicate that the enzyme-generated inactivators (III) and (IV) alkylate the enzyme via a release-and-return mechanism. Several lines of evidence suggest that compounds (III) and (IV) covalently modify the NAD(P)(+)-binding site. First, micromolar concentrations of NAD(P)H offer substantial protection against enzyme inactivation mediated by Michael acceptors (III) and (IV). In these protection studies Kd measurements for NAD(P)H approached those measured by fluorescence titration of free enzyme. Secondly, under initial-velocity conditions compounds (III) and (IV) act essentially as competitive inhibitors of NAD+ binding, and as mixed competitive or non-competitive inhibitors against androsterone binding. Thirdly, enzyme inactivated with either compound (III) or compound (IV) fails to bind to NAD+ affinity columns (e.g. Affi-gel Blue). Under the same conditions of chromatography native enzyme and enzyme affinity-labelled at the steroid-binding site with 17 beta-bromoacetoxy-5 alpha-dihydrotestosterone is retained on the affinity column. A kinetic scheme that represents the inactivation of the homogeneous dehydrogenase by the enzyme-generated alkylators (III) and (IV) is presented.

Synthesis and evaluation of non-steroidal mechanism-based inactivators of 3 alpha-hydroxysteroid dehydrogenase

Two non-steroidal mechanism-based inactivators for 3 alpha-hydroxysteroid dehydrogenase (3 alpha-HSD) of rat liver have been synthesized: 1-(4'-nitrophenyl)-2-propen-1-ol (I), and 1-(4'-nitrophenyl)-2-propyn-1-ol (II). Both of these compounds inactivate homogeneous 3 alpha-HSD in a time- and concentration-dependent manner only in the presence of NAD+. Analysis of the pseudo-first-order inactivation data gave a Kd of 1.2 mM for the allylic alcohol and a t1/2 (time required to promote a 50% loss of enzyme activity) for the enzyme of less than 10 s at saturation. Similar inactivation studies with the acetylenic alcohol gave a Kd of 1.5 mM and a t1/2 for the enzyme of 9.9 min at saturation. The allylic alcohol and acetylenic alcohol are oxidized stereoselectively by the enzyme, yielding a Km of 2.0 mM and a Vmax. of 0.58 mumol/min per mg for the allylic alcohol and a Km of 0.75 mM and a Vmax. of 0.29 mumol/min per mg for the acetylenic alcohol. Effective partition ratios (kcat./kinact.) are low for both alcohols: for the allylic alcohol, 5.3; and for the acetylenic alcohol, 141. H.p.l.c. indicates that the Michael acceptors 1-(4'-nitrophenyl)-2-propen-1-one (III) and 1-(4'-nitrophenyl-2-propyn-1-one (IV) are the products of the enzymic oxidation of the corresponding alcohols. The latter compound (IV) was trapped as its monothioether adducts before h.p.l.c. analysis. The Michael acceptors III and IV inactivate the 3 alpha-HSD in the absence of NAD+ at a rate too high to accurately measure and titrate the enzyme in a stoichiometric manner. Enzyme inactivated by I and NAD+, II and NAD+, III or IV is not re-activated by gel filtration or dialysis, implying a stable covalent bond has been formed between the enzyme and the inactivators. A screen of five other HSDs, and two aliphatic alcohol dehydrogenases, indicates that alcohol I is a selective inactivator of rat liver 3 alpha-HSD. It is concluded that 3 alpha-HSD generates non-steroidal alkylating agents (III and IV) that potently inactivate the enzyme with low effective partition coefficients. This report of non-steroidal mechanism-based inactivators of 3 alpha-HSD may provide a precedent for the development of related compounds to act as suicide substrates of other HSDs.

Purification and characterization of a novel form of 20 alpha-hydroxysteroid dehydrogenase from Clostridium scindens

We have purified a steroid-inducible 20 alpha-hydroxysteroid dehydrogenase from Clostridium scindens to apparent homogeneity. The final enzyme preparation was purified 252-fold, with a recovery of 14%. Denaturing and nondenaturing polyacrylamide gradient gel electrophoresis showed that the native enzyme (Mr, 162,000) was a tetramer composed of subunits with a molecular weight of 40,000. The isoelectric point was approximately pH 6.1. The purified enzyme was highly specific for adrenocorticosteroid substrates possessing 17 alpha, 21-dihydroxy groups. The purified enzyme had high specific activity for the reduction of cortisone (Vmax, 280 nmol/min per mg of protein; Km, 22 microM) but was less reactive with cortisol (Vmax, 120 nmol/min per mg of protein; Km, 32 microM) at pH 6.3. The apparent Km for NADH was 8.1 microM with cortisone (50 microM) as the cosubstrate. Substrate inhibition was observed with concentrations of NADH greater than 0.1 mM. The purified enzyme also catalyzed the oxidation of 20 alpha-dihydrocortisol (Vmax, 200 nmol/min per mg of protein; Km, 41 microM) at pH 7.9. The apparent Km for NAD+ was 526 microM. The initial reaction velocities with NADPH were less than 50% of those with NADH. The amino-terminal sequence was determined to be Ala-Val-Lys-Val-Ala-Ile-Asn-Gly-Phe-Gly-Arg. These results indicate that this enzyme is a novel form of 20 alpha-hydroxysteroid dehydrogenase.