Absolute quantification of the α, α′, and β subunits of β-conglycinin from soybeans by liquid chromatography/tandem mass spectrometry using stable isotope-labelled peptides

beta-Conglycinin, a major protein in soybeans, shows improvement effect of lipid metabolism. Moreover, this protein influences the processing properties of soybeans. beta-Conglycinin is a hetero-trimer constituted by alpha, alpha', and beta subunits. In this work, a method for the selective quantification of these subunits was developed by means of protein absolute quantification (AQUA) technology using liquid chromatography/tandem mass spectrometry with the stable isotope-labelled internal standard peptides LQSGDALR[C-13(6), N-15(4)], NILEASYDTK[C-13(6), N-15(4)], and NPIYSNNEGK(C-13(6), N-15(4)). This method exhibited linear relationships (r(2) > 0.99) in the concentration range of 1.2-300 fmol/mu L for LQSGDALR[C-13(6), N-15(4)] and NILEASYDTK[C-13(6), N-15(4)], and of 4.7-300 fmol/mu L for NPIYSNNFGK[C-13(6), N-15(4)]. As a result, the content of these subunits in beta-conglycinin-rich and both alpha and alpha' subunit-deficient soybean cultivars was successfully determined. This quantitative assay is promising for the evaluation of the food functionality and processing properties of soybeans.