Nitrite causes reversible inactivation of nitrate reductase in the yeast Hansenula anomala.

The addition of nitrite, the product of the reaction catalysed by nitrate reductase, to cell suspensions of the yeast Hansenula anomala 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 FMNH2 and reduced methyl viologen respectively, were less affected. A similar inactivation was found with the proton ionophores 2,4-dinitrophenol and carbonyl cyanide m-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 in vivo the enzyme was found in the soluble fraction. The inactivation of nitrate reductase by nitrite, 2,4-dinitrophenol and carbonyl cyanide m-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.