Casein Enhances Stability of Peptides in Intestinal Lumen: Role of Digested Products of Casein

Purpose. To investigate the inhibitory activity of casein on proteases in detail, the effect of digested products of casein itself on trypsin and chymotrypsin in rat small intestine was examined. Methods. Male Sprague-Dawley rats weighing 200-300 g were used as the animal model. The luminal content of the jejunum was prepared, and the enzymatic activities of trypsin and chymotrypsin were determined using a specific substrate for each protease. Then, the effect of enzymatic digested products of casein on them was examined. Results. The inhibitory activity of trypsin-digested casein against trypsin decreased as its digestion proceeded, but its inhibitory activity against chymotrypsin came to be more effective. On the other hand, the inhibitory activity of chymotrypsin-digested casein against chymotrypsin decreased with the degree of digestion, but no change in the inhibitory activity against trypsin was observed. Even the completely digested products of casein with trypsin or chymotrypsin showed inhibitory activities against the two proteases. Conclusions. It was suggested that not only the intact casein but also the products digested with trypsin or chymotrypsin contribute to the inhibitory effect of casein on the proteases in the intestinal lumen.