GAMMA-GLUTAMYLCYSTEINYLSERINE - A NEW HOMOLOG OF GLUTATHIONE IN PLANTS OF THE FAMILY POACEAE

In addition to glutathione (gamma-GluCysGly), many species of the family Poaceae have another tripeptide which has the amino acid sequence-gamma-GluCysSer. This thiol was isolated from etiolated leaves of wheat (Triticum aestivum L. cv. Star). Its structure was elucidated by quantitative amino acid analysis after total hydrolysis and by partial hydrolysis with carboxypeptidase A and gamma-glutamyltranspeptidase. The content of gamma-GluCysSer in the leaves of T. aestivum is increased by incubation with sulfate and is severely diminished by incubation with buthionine sulfoximine, a specific inhibitor of gamma-glutamylcysteine synthetase. Oxidized-gamma-GluCysSer is reduced by yeast glutathione reductase with a rate somewhat lower than for glutathione, but the new tripeptide is not a substrate of glutathione-S-transferase from equine liver. Besides homoglutathione (gamma-GluCys-beta-Ala), a tripeptide found in plants of the order Fabales, the tripeptide gamma-GluCysSer is the second homologue of glutathione detected in plants.