Common Bean ( L.) Accumulates Most -Methylcysteine as Its γ-Glutamyl Dipeptide.

The common bean () constitutes an excellent source of vegetable dietary protein. However, there are sub-optimal levels of the essential amino acids, methionine and cysteine. On the other hand, accumulates large amounts of the γ-glutamyl dipeptide of -methylcysteine, and lower levels of free -methylcysteine and -methylhomoglutathione. Past results suggest two distinct metabolite pools. Free -methylcysteine levels are high at the beginning of seed development and decline at mid-maturation, while there is a biphasic accumulation of γ-glutamyl--methylcysteine, at early cotyledon and maturation stages. A possible model involves the formation of -methylcysteine by cysteine synthase from -acetylserine and methanethiol, whereas the majority of γ-glutamyl--methylcysteine may arise from -methylhomoglutathione. Metabolite profiling during development and in genotypes differing in total -methylcysteine accumulation showed that γ-glutamyl--methylcysteine accounts for most of the total -methylcysteine in mature seed. Profiling of transcripts for candidate biosynthetic genes indicated that expression is correlated with both the developmental timing and levels of free -methylcysteine accumulated, while homoglutathione synthetase () expression was correlated with the levels of γ-glutamyl--methylcysteine. Analysis of -methylated phytochelatins by liquid chromatography and high resolution tandem mass spectrometry revealed only small amounts of homophytochelatin-2 with a single -methylcysteine. The mitochondrial localization of phytochelatin synthase 2-predominant in seed, determined by confocal microscopy of a fusion with the yellow fluorescent protein-and its spatial separation from -methylhomoglutathione may explain the lack of significant accumulation of -methylated phytochelatins.