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

Abstract

Lemnaperpusilla 6746, grown photoautotrophically at a series of sulfate concentrations ranging from 0.32 to 1,000 pm, was labeled to radioisotopic equilibrium with 36SO42-. Sulfur-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 methi-onine. y-Glutamylcyst(e)ine, erythro-and threo-thiothreonine, and S-meth-ylcysteine were not detected. No volatile 35S compounds were formed during plant growth at 1,000 isM sulfate, nor were significant amounts of 35S compounds excreted into the medium. The amount of each component present in colonies grown over the 3,000-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-methylmethio-nine sulfonium, and S-adenosylhomocysteine are presented. MATERIALS Biological Materials. The conditions for maintenance of stock cultures of L. perpusilla 6746 and for the photoautotrophic growth ofexperimental colonies in both batch and semicontinuous culture in the phytostat are described in the accompanying paper (3). Cultures were examined for the possible presence of contaminating microorganisms (1) at the time of harvest. No contaminants were found in the cultures used here. Chemicals. The method of preparation of 5'-methylthioadeno-sine, either from S-adenosyl-L-methionine or from S-adenosyl-L-[methyl-3Hlmethionine (Amersham/Searle) has been described (5). Sodium [35SJthiosulfate (outer S labeled) was from New England Nuclear and S-[methyl-'4CJmethylmethionine sulfonium was from ICN. The sources ofother chemicals have been described (5). METHODS