SULFUR GAMMA-GLUTAMYL PEPTIDES IN MATURE SEEDS OF COMMON BEANS (PHASEOLUS-VULGARIS L.)

The non-protein fractions of five common bean varieties (Phaseolus vul garis L.) have been studied in order to determine their total contents of gamma-glutamyl-S-methyl-L-cysteine (gamma-Glu-Cys(S-Me)) and of fr ee S-methyl-L-cysteine. After isolation by extraction with 70% ethanol and purification by ion-exchange chromatography on cationic and anion ic supports, the identification of these compounds was achieved by ami ne acid analysis after HCl hydrolysis and by mass spectrometry. gamma- Glu-Cys(S-Me) was present in high levels in all the varieties studied: the average content was 11 mu mol g(-1) seed weight, accounting for a pprox. 50% of the bean methionine content determined by ion-exchange c hromatography. Free S-methyl-L-cysteine constituted up to 20% of the t otal amount of S-methyl-L-cysteine found. The presence of gamma-glutam yl-L-methionine did not seem to interfere in the total methionine cont ent since its concentration in the non-protein bean fractions studied was negligible. These results suggest (1) that the presence of gamma-g lutamyl-S-methyl-L-cysteine in whole beans may overestimate the methio nine content up to 50%, when this is determined by the traditional met hod of cyanogen bromide cleavage followed by gas chromatographic analy sis of the resulting methyl thiocyanate; (2) a routine quantification method for these sulphur compounds in beans on a laboratory scale is n ow possible. (C) 1998 Elsevier Science Ltd. All rights reserved.