Homoglutathione (hGSH: γ‐glutamyl‐eysteinyl‐β‐alanine) was purified from seeds of Phaseolus coccineus L. cv. Preisgewinner, using anion‐exchange chromatography and Cu2O precipitation. Quantitative and specific determination of this thiol is possible by high‐performance liquid chromatography (HPLC) after monobromobimane derivatization. The enzymatic recycling assay based on yeast glutathione reductase (EC 1.6.4.2) can also be applied, but only to samples containing either hGSH or glutathione (GSH), since enzyme reaction with hGSH is 2.7 times faster than with GSH. Using the very sensitive HPLC method, the thiol content of leaves, roots and seeds of several legumes was investigated. Although GSH and hGSH were found in all plants analysed, the GSH/hGSH ratio varied greatly within the different tribes as well as within the different organs of plants of one species. In seeds and leaves of Vicieae, only traces of hGSH were found beside the main thiol GSH, whereas in roots the hGSH content exceeded the GSH content. The Trifolieae contained both tripeptides and in the tribe Phaseoleae, hGSH predominated by far.

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In addition to glutathione (y-GluCysGly), many species of the family Poaceae have another tripeptide which has the amino acid sequence y-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 y-glutamyltranspeptidase. The content of y-GluCysSer in the leaves ofT. aestivum is increased by incubation with sulfate and is severely diminished by incubation with buthionine sulfoximine, a specific inhibitor of y-glutamylcysteine synthetase. Oxidized y-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.

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