Combining isotope labelling with high resolution liquid chromatography-tandem mass spectrometry to study sulfur amino acid metabolism in seeds of common bean (Phaseolus vulgaris)

Sulfur plays a crucial role in plant metabolism, protein biosynthesis, homeostasis and defense mechanisms. Like in other grain legumes, the protein quality of common bean is limited by the sub-optimal concentration of methionine and cysteine. S-Methylcysteine is a non-proteinogenic sulfur amino acid, characteristically found in the Phaseolus as well as Vigna species. In mature seeds it accumulates in the form of its dipeptide γ-glutamyl-S-methylcysteine. In human diet, this sulfur containing amino acid and its dipeptide cannot substitute for cysteine or methionine. Despite the inverse relationship between the concentration of cysteine and methionine versus S-methylcysteine, to date very little is known about the biosynthesis of the latter in common bean. Here, we developed a method, combining stable isotope label tracking with high resolution liquid chromatography-tandem mass spectrometry to investigate the pathways of S-methylcysteine and γ-glutamyl-S-methylcysteine biosynthesis in the developing seed of common bean.