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 was required for achieving maximum inhibition of nitrate reductase activity. Furthermore, the reaction(s) taking place during the preincubation period was thermo-dependent. High activities of protease and peroxidase were detected in the fraction of inhibitor I. It seemed that the inhibition was not caused by the rapid bin ding of the inhibitors to the enzyme, and that proteolytic activity played a role in the inhibitory activity of at least inhibitor I.
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