Cysteine protease inhibitors in various milk preparations and its importance as a food

The content of cysteine-protease inhibitors in milk preparations were reported and their contributions to bacterial growth inhibition and protection of bone resorption were discussed. Mammalian milk contains large amount of cysteine protease inhibitors, such as lactoferrin, β-casein and cystatins. Lactoferrin content in human-milk was much higher than that in cow-milk and the papain inhibition of human-milk was stronger than that of cow-milk. Since lactoferrin content in colostrum was about two to three times higher than that in mature-milk, both of the cysteine-protease inhibition and the bacterial growth inhibition by colostrum were much stronger than that by mature-milk. β-Casein content in colostrum was much lower than that in mature-milk. Lactoferrin showed strong inhibition to papain and cathepsin L, but urease to cathepsin B and H. The papain inhibition by lactoferrin was lost by 70 °C heat-treatment. Since cathepsin L inhibitors suppress bone collagen degradation, content of lactoferrin in milk is important to protect bone resorption. Lactoferrin inhibited growth of Streptococcus epidermidis, furthermore, the active site of cysteine-protease inhibition, synthesized peptide Y679-K695, of lactoferrin, also showed the growth inhibition.