Reversible inactivation and binding to mitochondria of nitrate reductase by heat shock in the yeast Hansenula anomala.

Heat shock from 25 degrees C to 40 degrees C of Hansenula anomala cells resulted in a rapid and reversible inactivation 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.