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

Abstract

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 (1.4 units mL−1) was added separately to β-casein or αs1-casein (5 mg mL–1, in 100 mm Na acetate buffer, pH 6.0, containing 1.5 mm dithiothreitol). Samples were taken over a 24 h 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 αs1-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–Phe24 bond of αs1-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.