The addition of nitrite, the product of the reaction catalysed by nitrate reductase, to cell suspensions of the yeast caused a reversible inactivation of NADPH-dependent nitrate reductase activity. The haem- and Mo-dependent and Mo-dependent activities of nitrate reductase, determined with the non-physiological electron donors FMNH and reduced methyl viologen respectively, were less affected. A similar inactivation was found with the proton ionophores 2,4-dinitrophenol and carbonyl cyanide -chlorophenylhydrazone. The inactive enzyme was found in the particulate fraction and cosedimented with the mitochondrial fraction. When the NADPH-dependent nitrate reductase activity was restored the enzyme was found in the soluble fraction. The inactivation of nitrate reductase by nitrite, 2,4-dinitrophenol and carbonyl cyanide -chlorophenylhydrazone was dependent on the external pH. The treatment of isolated mitochondria at alkaline pH with Triton X-100 solubilized about 30% of the inactive enzyme.


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