Lemna enzyme activity

The importance of zinc for a normal functioning of the Cu-Zn-SOD was shown in Lemna gibba. In zinc-deficient culture media the activity of Cu-Zn-SOD was strongly inhibited whereas in copper-deficient media little change was found in the enzyme activity (Vaughan et al., 1982). In extracts of zinc-deficient plants, restoration of enzyme activity was possible by supplying zinc to the enzyme assay medium. 

Phosphohomoserine serves as a precursor of both threonine and methionine in higher plants, and regulation of its utilization in both branches of the pathway would be expected. This appears to occur, in part, by 5-adenosylmethio-nine activation of threonine synthase (5). Results obtained with partially purified Lemna threonine synthase (Giovanelli et al, 1984) indicate that the enzyme is essentially inactive in the absence of -adenosylmethionine, which cooperatively activates the enzyme at concentrations of less than 100 /iM. Conceptually, methionine could be synthesized and converted to S-adenosylmethionine prior to enzyme activation and the synthesis of threonine. Both orthophosphate and AMP inhibit Lemna threonine synthase in vitro, but the physiological significance of these effects is uncertain (Giovanelli et al, 1986). 

Brunold and Schmidt (1978) and Ellis (1969) have reported that sulfate, cysteine, and methionine do not significantly affect ATP sulfurylase activity in Lemna. Reuveny and Filner (1977), however, reported that while the enzyme in cultured tobacco cells is not induced by sulfate, it is repressed by sulfate, cysteine, and methionine, and derepressed by sulfur starvation. 

The final reaction in the biosynthesis of threonine involves a /8-y rearrangement and the loss of phosphate from O-phosphohomoserine (Fig. 2). Threonine synthases have been isolated from Lemna (Schnyder et al., 1975) radish, sugarbeet (Madison and Thompson, 1975), peas (Schnyder et al., 1975 Thoen et al., 1978b), and barley (Aames, 1978). None of these enzymes has been extensively characterized but a requirement for pyridoxyl-5 -phosphate was demonstrated after partial purification of the barley and pea enzymes. Unlike several other enzymes associated with threonine synthesis, the activity of threonine synthase was not stimulated by monovalent cations. However, all of the plant enzymes are strongly activated by 5-adeno-sylmethionine (Section III,B,5). 

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). 

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