Hydrolysis of bovine caseins by cathepsin B, a cysteine proteinase indigenous to milk

The presence of the lysosomal cysteine proteinase cathepsin B in milk has been demonstrated recently. The potential significance of this enzyme to proteolysis in milk and dairy products was evaluated by the study of its proteolytic specificity towards the caseins. Bovine cathepsin B (1.4unitsmL ^-1) was added separately to β-casein or α _s1-casein (5mgmL^-1, in 100mMNa acetate buffer, pH 6.0, containing 1.5mM dithiothreitol). Samples were taken over a 24h period and analysed by urea polyacrylamide gel electrophoresis and RP-HPLC; peptides were identified by N-terminal sequencing and mass spectrometry. Cathepsin B cleaved β-casein at 32 sites and α_s1-casein at 35 sites, extensively hydrolysing both proteins. Thus, cathepsin B has a very broad specificity against the caseins, with an apparent preference for bonds incorporating the amino acids Leu, Val, Gln, Pro and Ser. Cathepsin B cleaved some bonds in common with chymosin (including the Phe₂₃-Phe ₂₄ bond of α_s1-casein), plasmin and the cell envelope-associated proteinases of Lactococcus, suggesting that it could be involved in proteolysis in dairy products, particularly if manufactured from high somatic cell count milk.