Abstract: In recent years, dipeptidyl peptidase IV (DPP-IV) inhibitory peptides have become an important auxiliary anti-diabetic agent. In this paper, the in vitro digestion characteristics of bovine casein and the change in dipeptidyl peptidase IV (DPP-IV) inhibitory activity during digestion were investigated. Meanwhile, the pattern of change in the typical structure features of DPP-IV inhibitory peptides was analyzed by liquid chromatography-tandem mass spectrometry (LC-MS/MS) in order to elucidate the relevance of the release of characteristic peptides with the mechanism underlying the change in the DPP-IV inhibitory activity of casein hydrolysate during digestion. As a result, we found that the degree of hydrolysis and digestibility increased with the prolongation of digestion time. At the same time, compared to undigested product, the DPP-IV inhibitory effect also increased overall, reaching the highest value of 58.04% at 240 min, which was about 2-fold higher than that at 120 min (23.22%). The degree of hydrolysis and protein digestibility had a certain relationship with the release of DPP-IV inhibitory peptides. Furthermore, the typical structures of DPP-IV inhibitory peptides (i.e., Xaa-Pro/Ala- and Trp-Xaa-) were determined. The pepsin digestion generated 25 target peptides mostly with molecular mass more than 5 kDa, while the pancreatin digestion produced a total of 48 target peptides mostly with molecular mass less than 1 kDa. These results combined with heat map analysis demonstrated that the number and contents of peptides in casein digests were increased with increasing digestion time, which was consistent with the trend of DPP-IV inhibitory activity. Therefore, the DPP-IV inhibitory activity of casein digests during in vitro digestion is closely related to the number and contents of Xaa-Pro/Ala- and Trp-Xaa- peptides.

Key words: casein, peptides, dipeptidyl peptidase IV inhibitory peptides, liquid chromatography-tandem mass spectrometry, simulated gastrointestinal digestion