Effect of gallic acid on peptides released by trypsin digestion of bovine α-casein

In this study, the effects of gallic acid (GA) on trypsin digestion of commercial α-casein (α-CN), which contains αs1-CN and αs2-CN, and the peptides released during digestion were investigated. Gallic acid showed no effect on the initial rate of digestion. However, the apparent degree of hydrolysis achieved its maximum value after 1 h, then decreased in the presence of GA, suggesting the cross-linking between peptides once released from α-CN during digestion. In the presence of GA, three peaks derived from αs1-CN disappeared and three new peaks appeared in high-performance liquid chromatography (HPLC) analysis. In these peptides, two Met residues corresponding to the Met135 and Met196 in αs1-CN were oxidized to Met sulfoxide residues. The oxidation of Met196 was quicker than that of Met135. The inhibitory activity of TTMPLW (αs1-CN 193–199) against angiotensin I-converting enzyme was reduced slightly by the oxidation of its Met residue.