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.

β-Conglycinin, a major protein in soybeans, shows improvement effect of lipid metabolism. Moreover, this protein influences the processing properties of soybeans. β-Conglycinin is a hetero-trimer constituted by α, α', and β 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,N], NILEASYDTK[C,N], and NPIYSNNFGK[C,N]. This method exhibited linear relationships (r > 0.99) in the concentration range of 1.2-300 fmol/μL for LQSGDALR[C,N] and NILEASYDTK[C,N], and of 4.7-300 fmol/μL for NPIYSNNFGK[C,N]. As a result, the content of these subunits in β-conglycinin-rich and both α and α' subunit-deficient soybean cultivars was successfully determined. This quantitative assay is promising for the evaluation of the food functionality and processing properties of soybeans.