Porcine dpp 4

High-performance liquid chromatography (HPLC) grade water was from VWR (Dublin, Ireland). Trifluoroacetic acid, formic acid (FA), tris(hydroxymethyl)aminomethane (Tris), Gly-Pro-pNA, diprotin A (Ile-Pro-Ile), porcine DPP-IV (≥10 U mg⁻¹ protein), mass spectrometry (MS) grade water, and acetonitrile were purchased from Sigma Aldrich (Dublin, Ireland). Synthetic peptides (Ala-Ile-Pro, Ala-Pro-Ala, Ala-Pro-Phe, Ala-Pro-Arg, Ala-Arg-Pro, Ile-Phe-Lys, Ile-Ile-Phe, Ile-Pro-Ala, Ile-Arg-Phe, Ile-Arg-Lys, Lys-Pro-Ala, Lys-Arg-Ile, Arg-Ile-Phe, Arg-Ile-Arg, Arg-Lys-Arg, Phe-Pro-Phe, Phe-Pro-Ile, Phe-Pro-Trp, Ile-Ala-Ile, Ile-Pro-Phe, Ile-Pro-Trp, Trp-Pro-Phe, Trp-Pro-Ile, Trp-Pro-Trp) with a purity > 95% (w/w) were obtained from Genscript (Piscataway, NJ, USA).

R. philippinarum was collected from the Lüsi aquaculture regions of Jiangsu province, China in June 2016, and was examined by Prof. Xihe Wan (Institute of Oceanology and Marine Fisheries, Nantong, China) prior to processing.
Papain (from papaya, 8 × 10⁵ units/g of protein), trypsin (from porcine pancreas, 2.5 × 10⁵ units/g of protein), pepsin (from porcine gastric mucosa, 1 × 10⁴ units/g of protein), flavourzyme (from guanyloribonuclease, 2 × 10⁴ units/g of protein), and alcalase (from Bacillus licheniformis, 2 × 10⁵ units/g of protein) were purchased from YIFEIXUE Biological Tech., Ltd. (Nanjing, China). Gly-Pro-p-nitroanilide, diprotin A (Ile-Pro-Ile), and porcine DPP-IV (>10 units/mg protein) were purchased from Sigma Chemical Co. (St. Louis, MO, USA). Mass spectrometry (MS)-grade water, acetonitrile (ACN), and trifluoroacetic acid (TFA) were purchased from Tedia Company Inc. (Fairfield, CT, USA). All other reagents were obtained from Sigma. All reagents were of analytical grade.