Heat shock from 25°C to 40°C of Hansen& anomala cells resulted in a rapid and reversible mactivation of the NADPH-nitrate reductase (NR) activity. The inactive enzyme retained partial activity with the non-physiological co-substrates, reduced methyl viologen and reduced flavin mononucleotide. The inactive NR pelleted after centrifugation at 12,000 x g for 30 min and was associated with mitochondria. In untreated cells around 10% of the total NR is inactive and associated with mitochondria, while the active enzyme is soluble. In vitro, inactive NR could be partially dissociated from the mitochondria by incubating them at pH 11.5 or in the presence of 15 mM CHAPS.

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