Sulphur γ-glutamyl peptides in mature seeds of common beans (Phaseolus vulgaris L.)

The non-protein fractions of five common bean varieties (Phaseolus vulgaris L.) have been studied in order to determine their total contents of γ-glutamyl-S-methyl-L-cysteine (γ-Glu-Cys(S-Me)) and of free S-methyl-L-cysteine. After isolation by extraction with 70% ethanol and purification by ion-exchange chromatography on cationic and anionic supports, the identification of these compounds was achieved by amino acid analysis after HCl hydrolysis and by mass spectrometry. γ-Glu-Cys(S-Me) was present in high levels in all the varieties studied: the average content was 11 μmolg−1 seed weight, accounting for approx. 50% of the bean methionine content determined by ion-exchange chromatography. Free S-methyl-L-cysteine constituted up to 20% of the total amount of S-methyl-L-cysteine found. The presence of γ-glutamyl-L-methionine did not seem to interfere in the total methionine content since its concentration in the non-protein bean fractions studied was negligible. These results suggest (1) that the presence of γ-glutamyl-S-methyl-L-cysteine in whole beans may overestimate the methionine content up to 50%, when this is determined by the traditional method of cyanogen bromide cleavage followed by gas chromatographic analysis of the resulting methyl thiocyanate; (2) a routine quantification method for these sulphur compounds in beans on a laboratory scale is now possible.