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Responses of Sulfur-Containing Compounds in Lemna paucicostata Hegelm. 6746 to Changes in Availability of Sulfur Sources

Laboratory of General and Comparative Biochemistry, National Institute of Mental Health, Bethesda, Maryland 20205

The steady state concentrations of S-containing compounds formed in Lemna paucicostata Hegelm. 6746 in response to variations in source and concentrations of sulfur were measured. Neither growth rates nor protein accumulation were markedly affected by the various growth conditions. Ignoring complications due to possible compartmentation, the results are consistent with internal pools of both SO₄^2– and cyst(e)ine (or products of their metabolism), but not methionine, being effectors of regulation of high affinity SO₄^2– uptake. As SO₄^2– in the growth medium was increased to 10 mM, down-regulation of high affinity SO₄^2– uptake was more than compensated for by unregulated uptake via the "non-saturating" uptake system. Tissue inorganic SO₄^2– accumulated but formation of reduced sulfur remained constant. Some conversion of L-cystine sulfur to SO₄^2– occurred. Presence of L-cystine in the medium (a) down-regulated high affinity SO₄^2– uptake and (b) decreased the rate of SO₄^2– organification. The net results were decreased (7 µM L-cystine) or normal (14 µM L-cystine) total tissue SO₄^2– and dose-dependent accumulation of soluble cyst(e)ine and glutathione, but not of soluble methionine. L-Methionine was not metabolized to cyst(e)ine or its products. Presence of L-methionine in the medium led to increased total tissue sulfur, accounted for almost wholly by manyfold increases in soluble methionine, AdoMet, and S-methylmethionine sulfonium. Soluble cyst(e)ine increased slightly.

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