Plants are rich in bioactive compounds such as phenolics, vitamins, fatty acids, proteins and bioactive peptides (BPs). BPs are linear or cyclic chains that can include post-translational modifications, unusual amino acids and stabilising motifs. Their molecular structure places them in a unique chemical space between small molecules and proteins. BPs play a significant role in plant and human health and fitness, yet their diversity across various grain samples remains unknown. The extraction of these short peptides from cereal grains can be challenging due to the presence of significant non-protein components. Therefore, complementary protocols have been used to maximise peptide extraction from cereal grains. Additionally, identifying the most promising candidate peptides from large-scale peptidomics experiments can be overlooked due to the incompleteness of background proteome databases, especially for non-model crop species.
In the present study, we investigated four wholemeal grain flour samples, including sorghum, wheat, oat and quinoa, using mass spectrometry-based peptidomics and bioinformatics analysis to discover bioactive peptides. Six food-grade extraction buffers were applied with different digestive enzymes, such as trypsin, chymotrypsin, pepsin, and thermolysin, to capture many bioactive peptides present within the grain samples. The resulting data files were searched against species-specific databases built from the protein sequences translated from publicly available multiple genomic, transcriptomic and UniProt protein sequences. The detected peptides were searched against a comprehensive bioactive peptide database prepared from the collected bioactive peptide sequences from published datasets and databases using CLC Genomics Workbench software (version 24.1). Our result shows that bioactive peptides in cereal grains are primarily represented by small, cysteine-rich proteins with antimicrobial and anti-hypertensive activities and the extraction methods strongly affect the enrichment levels of identified bioactive peptide subclasses. These peptides can be released upon digestion with enzymes in the gastrointestinal tract, with peptides fulfilling antihypertensive and cardiovascular properties primarily enriched by enzymes in the jejunal digestion. Future studies will focus on synthesis of the candidate peptides from the grain samples and performing bioassays to validate their functional properties.