Dipeptidyl Peptidase-IV Inhibitory Activity of Katsuobushi-Derived Peptides in Caco-2 Cell Assay and Oral Glucose Tolerance Test in ICR Mice

Proteolytic products of bonito stock residue inhibit dipeptidyl peptidase-IV (DPP-IV). Here, we isolated, purified, and identified the components of its N5 fraction obtained after using neutral protease from Aspergillus oryzae. A 10% ethanol eluent (N5-2 fraction) from column chromatography was sequenced, yielding 18 peptides. Of these, Glu-Val-Phe, Ala-Val-Phe, and Gly-Val-Phe were identified as novel (IC50 values for DPP-IV inhibition were 525.56, 5466.49, and 2870.87 μM, respectively), whereas Trp-Val is the primary peptide (IC50 value of 36.99 μM, 1359 unit (mL/100 g N5-2 fraction) = (yield (mg)/100 g N5-2 fraction)/IC50 (μg/mL). Furthermore, the N5-2 fraction significantly decreased DPP-IV activity in Caco-2 intestinal epithelial cells (p < 0.05). From the oral glucose tolerance test using ICR mice, the N5-2 fraction significantly attenuated the rise in serum glucose levels compared with the control (p < 0.05) through cell-surface DPP-IV inhibition. We discuss the novelty, significance, and relevance of the findings in this study, as well as its broad applications for prevention of diabetes.