Sulfur-containing Compounds in Lemna perpusilla 6746 Grown at a Range of Sulfate Concentrations

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Abstract
L. perpusilla 6746, grown photoautotrophically at a series of sulfate concentrations ranging from 0.32-1000 .mu.M, was labeled to radioisotopic equilibrium with 35SO42-. S-containing compounds were isolated and purified from the colonies. Radioactivity in each compound was a measure of the amount of that compound present in the tissue. The following compounds were identified and quantitated: inorganic sulfate, glutathione, homocyst(e)ine, cyst(e)ine, methionine, S-methylmethionine sulfonium, S-adenosylmethionine, S-adenosylhomocysteine, cystathionine, chloroform-soluble (presumed to be sulfolipid), protein cyst(e)ine and protein methionine. .gamma.-Glutamylcyst(e)ine, erythro- and threo-thiothreonine and S-methylcysteine were not detected. No volatile 35S compounds were formed during plant growth at 1000 .mu.M sulfate, nor were significant amounts of 35S compounds excreted into the medium. The amount of each component present in colonies grown over the 3000-fold range of medium sulfate was relatively constant except for inorganic sulfate. This increased about 30-fold from the lowest to the highest medium sulfate concentration. The total soluble sulfur amino acids increased about 1.5- to 2-fold, due primarily to an increased amount of glutathione. Protein cyst(e)ine and protein methionine were the major organic sulfur compounds in Lemna, and the amounts of these compounds remained virtually constant despite the variation in external sulfate concentration. Procedures for the analysis of S-adenosylmethionine, S-methylmethionine sulfonium and S-adenosylhomocysteine are presented.