Abstract Three macromolecular inhibitors of nitrate reductase were isolated from an extract of chinese cabbage roots by DEAE-cellulose column chromatography. These inhibitors, I, II, and III, had an inhibitory etfect on NADH-nitrate reductase activity but had no etfect on the activity of nitrite reductase and NADH-glutamate dehydrogenase from chinese cabbage leaves. Inhibitor land III also strongly inhibited the activity of FMNH.-nitrate reductase and only slightly that of glutamine synthetase, whereas inhibitor II had a weaker inhibitory etfect on FMNH2-nitrate reductase activity and no etfect on glutamine synthetase activity. Although these inhibitors were heat-labile, inhibitor 11 was relatively less sensitive to heat treatment compared to both inhibitors land III. Inhibitory etfect of I, II and III was caused by neither the prest:nce of phenolic compounds or corresponding quinones nor the oxidation of NADH which is required for nitrate reductase activity. Preincubation of the inhibitors with enzyme wa...