Characterization of grain-specific peptide markers for the detection of gluten by mass spectrometry.

Global and targeted mass spectrometry-based proteomic approaches were developed to discover, evaluate, and apply gluten peptide markers to detect low parts per million (ppm) wheat contamination of oats. Prolamins were extracted from wheat, barley, rye, and oat flours and then reduced, alkylated, and digested with chymotrypsin. The resulting peptides were subjected to LC-MS/MS analysis and database matching. No peptide markers common to wheat, barley, and rye were identified that could be used for global gluten detection. However, many grain-specific peptide markers were identified, and a set of these markers was selected for gluten detection and grain differentiation. Wheat flour was spiked into gluten-free oat flour at concentrations of 1-100,000 ppm and analyzed to determine the lowest concentration at which the wheat "contaminant" could be confidently detected in the mixture. The same 2D ion trap instrument that was used for the global proteomics approach was used for the targeted proteomics approach, providing a seamless transition from target discovery to application. A powerful, targeted MS/MS method enabled detection of two wheat peptide markers at the 10 ppm wheat flour-in-oat flour concentration. Because gluten comprises approximately 10% of wheat flour protein, the reported wheat gluten-specific peptides can enable detection of approximately 1 ppm of wheat gluten in oats.