Thiosulfonate reductase Chlorella

Thiosulfonate reductase has been purified approximately 50-fold from Chlorella (Schmidt, 1973). In addition to catalyzing the reduction of the sulfonate moiety of GS-SOj by Fd d, it also catalyzes the reduction of dithionite to free sulfide using methylviologen. Schmidt (1973) found that the protein fraction which was labeled by GS- SOj in the absence of Fd,ed could be separated from thiosulfonate reductase and that the labeled protein in turn could be resolved into unlabeled protein and a labeled low molecular weight factor. When this factor was supplied to purified enzyme and activity measured by the dithionite/methylviologen assay, the activity was enhanced about threefold. No details of the kinetics of the presumed physiological reaction with bound sulfite and Fdrej are currently available. 

Schmidt et al. (1974) have reported that the Chlorella mutant Sat, which fails to grow on sulfate, lacks thiosulfonate reductase activity and fails to catalyze the reduction of either GS- SOs or [ S]APS to free or protein-bound exchangeable However, the mutant was shown to contain... 

The sulfur moiety of cysteine is derived ultimately by the reductive assimilation of inorganic sulfate. Sulfate can be reduced in plants by two pathways. One pathway involves free sulfite as an intermediate which is reduced by sulfite reductase to form free sulfide. The other involves carrier-bound sulfite (carrier-S-SOj) which is reduced by thiosulfonate reductase to yield carrier-bound sulfide (carrier-S-S ). Although the relative physiological importance of the two pathways has not been firmly established, the indispensability of thiosulfonate reductase (even in the presence of sulfite reductase) for sulfate reduction in Chlorella mutants indicates the physiological importance of bound sulfite for this organism (Schmidt et al., 1974). Further details of the reduction of sulfate are presented in Chapter 5. 

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