Several compounds were examined for their inhibitory effects on xylose reductase from the yeastPichia stipitis NRC 2548. Mercuric chloride, cupric chloride, menadione sodium bisulfite, and sodium bisulfite inhibited enzyme activity in a sigmoidal dose-dependent manner, whereas quercetin and rutin were observed to have nonsigmoidal dose-response curves. Diphenylhydantoin, hydantoin, and valproic acid had no effect on xylose reductase activity. Mercuric chloride was the most potent inhibitor tested, with an IC50 (the concentration that inhibited enzyme activity by 50%) of 4.7xl0-6M. Three distinct inhibition patterns were observed amongst selected inhibitors. Mercuric chloride and quercetin were noncompetitive inhibitors of xylose reductase with respect to substrate and cofactor. Sodium bisulfite was an uncompetitive inhibitor with respect to substrate and cofactor, whereas menadione sodium bisulfite was a competitive inhibitor with respect to substrate, but noncompetitive to the cofactor.