Yeast sulfite reductase (EC 184.108.40.206) has the unique property that it is inactivated at low ionic strength. The effects of ionic strength on various enzyme activities were investigated and the characterization of the inactivation was carried out. The multiple activities shown by this enzyme were all nullified by exposing the enzyme to low ionic strength, except for reduced methyl viologen-sulfite reductase activity, which was increased rather than decreased. The Km values for NADPH and sulfite were not significantly affected by ionic strength. When the enzyme was reduced with NADPH at low ionic strength, the height of the peak at 455 nm was decreased to one-half of the fully-reduced level, while the height of the peak at 587 nm was unchanged. These results, together with the experiment using an FMN-depleted apoenzyme, indicate that the inactivation at low ionic strength was caused by the interception of the electron flow between FAD and FMN. The inactivation at low ionic strength was reversible, but the restoration of the activity was dependent on the incubation period at low ionic strength and the protein concentration. The kinetics and the effect of glycerol and/or 2-mercaptoethanol on the inactivation and the restoration showed that further change, including oxidation of SH groups, should occur in the enzyme through prolonged incubation at low ionic strength. Changes in the enzyme structure related to these results are described in the subsequent paper.
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