COMPARATIVE INHIBITION OF YEAST GLUTATHIONE-REDUCTASE BY ARSENICALS AND ARSENOTHIOLS

ri(gamma-glutamylcysteinylglycinyl)trithioarsenite (As-III(GS)(3)) is formed in cells and is a more potent mixed-type inhibitor of the reduc tion of glutathione disulfide (GSSG) by yeast glutathione (GSH) reduct ase than either arsenite (As-III) or GSH. The present work examines th e effects of valence and complexation of arsenicals with GSH or L-cyst eine (Cys) upon potency as competitive inhibitors of the reduction of GSH disulfide (GSSG) by yeast GSH reductase. Trivalent arsenicals were more potent inhibitors than their pentavalent analogs, and methylated trivalent arsenicals were more potent inhibitors than was inorganic t rivalent As. Complexation of either inorganic trivalent As or methylar sonous diiodide ((CH3AsI2)-I-III) with Cys or GSH produced inhibitors of GSH reductase that were severalfold more potent than the parent ars enicals. In contrast, dimethylarsinous iodide ((CH3)(2)(AsI)-I-III) wa s a more potent inhibitor than its complexes with either GSH or Cys. C omplexes of CH3AsIII with GSH (CH3AsIII(GS)(2)) or with Cys (CH3AsIII( Cys)(2)) were the most potent inhibitors, with K-i's of 0.009 and 0.01 8 mM, respectively. Inhibition of GSH reductase by arsenicals or arsen othiols was prevented by addition of meso-2,3-dimercaptosuccinic acid (DMSA) to a mixture of enzyme, GSSG, and inhibitor before addition of NADPH. DMSA added to the reaction mixture after NADPH reversed inhibit ion by (CH3)(2)(AsI)-I-III but had little effect on inhibition by (CH3 AsI2)-I-III, CH3AsIII(GS)(2), CH3AsIII(Cys)(2), Or AS(III)(GS)(3). Par tial redox inactivation of the enzyme with NADPH increased the inhibit ory potency of (CH3AsI2)-I-III and (CH3)(2)(AsI)-I-III and changed the mode of inhibition for (CH3AsI2)-I-III from competitive to noncompeti tive. The greater potency of methylated trivalent arsenicals and arsen othiols than of inorganic trivalent As suggests that biomethylation of As could yield species that inhibit reduction of GSSG and alter the r edox status of cells.