An L-cysteine Dependent Nitrate Reductase Inactivating Factor in Synchronous Chlorella sorokiniana

Nitrate reductasee activity of cell-free Chlorella systems is inactivated in the presence of specific thiols. Out of 14 different thiols tested only L-cysteine and to a certain extent D-cysteine catalyzed an inactivation of the Chlorella nitrate reductase. This systeme was active only with reduced L-cysteine, since oxidized cystine had no effect, demonstrating that the reduced thiol group is necessary. A factor not identical with the nitrate reductase itself seems involved in this inactivation system. Evidence for this is that purified nitrate reductase is not inactivated by L-cysteine; however, when purified nitrate reductase was added to extracts inactivated by L-cysteine it was inactivated as well. The highest rate of degradation was found at a pH around 8. The data suggest that the inactivation factor from Chlorella is a proteinase, which has to be activated by L-cysteine. These results are discussed in relation to the regulation of assimilatory nitrate reduction.