Characterization of the Reversible Inactivation of Ankistrodesmus braunii Nitrate Reductase by Hydroxylamine

The photoreversible nature of the regulation of nitrate reductase is one of the most interesting features of this enzyme. As well as other chemicals, NH(2)OH reversibly inactivates the reduced form of nitrate reductase from Ankistrodesmus braunii. From the partial activities of the enzyme, only terminal nitrate reductase is affected by NH(2)OH. To demonstrate that the terminal activity was readily inactivted by NH(2)OH, the necessary reductants of the terminal part of the enzyme had to be cleared of dithionite since this compound reacts chemically with NH(2)OH. Photoreduced flavins and electrochemically reduced methyl viologen sustain very effective inactivation of terminal nitrate reductase activity, even if the enzyme was previously deprived of its NADH-dehydrogenase activity. The early inhibition of nitrate reductase by NH(2)OH appears to be competitive versus NO(3) (-). Since NO(3) (-), as well as cyanate, carbamyl phosphate and azide (competitive inhibitors of nitrate reductase versus NO(3) (-)), protect the enzyme from NH(2)OH inactivation, it is suggested that NH(2)OH binds to the nitrate active site. The NH(2)OH-inactivated enzyme was photoreactivated in the presence of flavins, although slower than when the enzyme was previously inactivated with CN(-). NH(2)OH and NADH concentrations required for full inactivation of nitrate reductase appear to be low enough to potentially consider this inactivation process of physiological significance.