APS sulfotransferase

Fig. 40. A proposed unified scheme of sulfate assimilation in algae. Adenylyl sulfate (APS) transfers the sulfo group via APS-sulfotransferase to form Car-S-SOr (Car = carrier), which is reduced further by thio-sulfonate reductase to Car-S-S which yields the thiol group of cysteine. In addition, if sulfite is released from Car-S-SOa (i.e., by thiol or from mutated sites) or if it enters the cell from outside, it can be reduced via a separate sulhte reductase. From Abrams and Schiff (57i).

Further evidence for implicating APS as the substrate for reduction was the discovery of an APS-sulfotransferase in Chlorella and the chloroplasts of higher plants (Schmidt, 1972a,b, 1975a,b) which catalyzes the reaction ... 

Studies of enzyme activity in CMorella mutants suggests that bound sulfite, formed from APS via APS sulfotransferase, is an intermediate in the incorporation of sulfate into cysteine (Schmidt et al., 1974). Mutants, ... 

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. 

Fig. 3. Regulation of the bound pathway for the assimilation of sulfate into cysteine and associated processes. Carrier refers to an endogenous thiol of uncertain identity in higher plants. Enzymes associated with the sulfate assimilation pathway and the synthesis of O-acetylseiine are (1) high-ailinity sulfate uptake mechanism, (2) ATP-sulfurylase, (3) adenosine S -phosphosulfate (APS) sulfotransferase, (4) organic thiosulfate reductase, (5) cysteine synthase, and (6) serine transacetylase. Cysteine sulfhydrase (7), an enzyme of cysteine catabolism, and nitrate reductase (8), the first enzyme of the nitrate assimilation pathway, are also shown. Inhibitory control of the pathways is shown by discontinuous lines (----) and enhancement by continuous lines (------).

Cysteine has a strong inhibitory effect on the level of extractable activity of APS sulfotransferase in Lemna (Brunold and Schmidt, 1978) and Phaseolus... 

The level of APS sulfotransferase activity in Lemna is also influenced by the level of sulfate supplied. Brunold et al. (1987) found that when plants were grown on 0.88 mM sulfate and then transferred to 8.8 fiM sulfate the specific activity of the enzyme increased by 100%. This effect was reversed when the plants were returned to the original concentration of sulfate. However, it is not clear whether low concentrations of sulfate per se or low concentrations of a product formed from sulfate (e.g., cysteine) causes derepression of APS sulfotransferase. 

The level of sulfotransferase in Lemna and in Phaseolus vulgaris is also subject to strong inhibition by gaseousH2S(Brunoldand Schmidt, 1976,1978 Wyss and Brunold, 1979). However, the extractable acti vity of cysteine synthase is not similarly affected. Removal of H2S firom the gas phase results in rapid restoration of activity which, based on a study of labeling of the enzyme (von Arb and Brunold, 1980), was attributed to synthesis ofthe enzyme de novo. HjS also inhibits the level of APS sulfotransferase in cell suspension cultures of Nicotiana sylvestris in this tissue neither the ATP-sulfiirylase or cysteine synthase activity was affected by H2S or cysteine (Brunold etal., 9Sl). Importantly, the inhibition of APS sulfotransferase by H2S was correlated with an enhanced level of cysteine, suggesting that the H2S inhibition could have been mediated via this reaction product. Uptake of exogenous sulfate was also inhibited by H2S in this system (Brunold et al., 1981). 

Grill et al, 1985). Nussbaum et al. (1988) found that administration of cadmium to the roots of maize enhanced the rate of sulfate incorporation into the insoluble oi nic sulfur fraction and also enhanced the extractable activities of ATP-sulfiirylase and APS sulfotransferase (but not nitrate reductase) about fourfold. 

The internal regulation of the sulfate assimilation pathway and its coordination with the nitrate assimilation pathway are summarized in Fig. 3. It shows that cysteine is a negative effector of serine transacetylase and that it also controls the level of APS sulfotransferase. The inhibitory effects of HjS on the level of APS sulfotransferase are probably mediated via cysteine, though HjS itself at high concentrations inhibits cysteine synthase. 

In tobacco cell cultures the extractable levels of ATP-sulfurylase and cysteine synthase are very low when the cells are subject to nitrogen stress but increase rapidly upon alleviation of the stress, suggesting that a product of nitrogen assimilation derepresses the levels of these enzymes. In Lemna (and possibly in cultured Rosa cells) it appears that this role is fulfilled by APS sulfotransferase and that ATP-sulfurylase and cysteine play unimportant roles in coordinating the sulfate assimilation pathway with the nitrate assimilation pathway. A further regulatory mechanism known to occur in cultured tobacco cells is that excessively high concentrations of cysteine induce the synthesis of cysteine desulfiiydrase (see Section VI). 

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