Inhibition of Aminopeptidase by Cow's Milk κ-Casein

The aminopeptidase inhibitory action of milk κ-casein was studied. By increasing the κ-casein concentration to 0.1%, nearly half the aminopeptidase activity was inhibited, thus showing a marked effect. However, when sialidase was allowed to react on the κ-casein, which removed sialic acid, even at the 0.1% concentration, the enzyme activity was hardly affected and aminopeptidase was not inhibited. This suggested that the presence of sialic acid was necessary for enzyme inhibition. In an examination of the enzyme inhibitory type of κ-casein, sialic acid acted as a competitive inhibitor with Ki value of 1×10-1M. It was assumed that the κ-casein cornbined with the active center of the enzyme or with nearby points to inhibit its activity.