Thiosulfonate reductase

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... 

Fig. 2. Summary of the free and bound pathways of sulfate assimilation in plants. Some related reactions and points of entry of several forms of inorganic sulfur are also shown. The reaction sequence catalyzed by (1) ATP sulfurylase, (2) APS sulfotransferase, (3) thiosulfonate reductase, and (4) cysteine synthase constitutes the bound sulfate assimilation pathway. The synthesis of OAS is catalyzed by (5) serine transacetylase. The reaction sequence (I), (6)-(9)or (1), (2), (10), (8), (9) constitutes the free pathway reactims (7) and (10) are nonenzymatic, (6) is catalyzed by APS sulfotransferase, (8) by sulfite reductase, and (9) by cysteine synthase. APS and PAPS are interrelated via (11) APS kinase and (12) NDP phophohydrolase. APS can be hydrolyzed via (13) APS sulfohydrolase or (14) APS cyclase.

Our current understanding of the assimilation of inorganic sulfur into cysteine is summarized in Fig. 2. In spinach the enzymes ATP sulfurylase, APS kinase, APS sulfotransferase, thiosulfonate reductase, sulfite reductase, and cysteine synthase are known to be associated with chloroplasts. The subcel-lular localization of the other enzymes shown in Fig. 2 is either uncertain or unknown. The association of the enzymes of the bound pathway with mitochondria in Euglena (Brunold and Schifif, 1976) appears to be a special case. 

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. 

The findings of Chambers and Trudinger (1971) suggest that Eq. (3) may be catalyzed in bacteria by certain forms of cysteine synthase. 5-sulfocysteine can be converted to cysteine either by enzymatic reduction with NADPH (Nakamura and Sato, 1965), or by chemical reduction with GSH (Woodin and Segel, 1968). Thiosulfate is a natural constituent of many plants (Wilson and Reuveny, 1976) and accumulates in a ChloreUa mutant blocked in the thiosulfonate reductase step (SchiflFand Hodson, 1970 Schmidt et ai, 1974). However, in reviews of the subject, Schiff and Hodson (1970, 1973) concluded that thiosulfate lies on a side branch rather than on the main pathway for sulfate reduction by ChloreUa. 

Sulfate adenylyltransferase (ATP sulfurylase) 2 adenosine 5 -sulfatophosphate kinase 3 adenosine 5 -sulfatophosphate sulfo-transferase 4 thiosulfonate reductase 5 cysteine synthase... 

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