Proteolysis of bovine caseins by cathepsin D: Preliminary observations and comparison with chymosin

Proteolysis of α_s1-, a_s2, β- and κ-caseins by bovine cathepsin D (E.C. 3.4.23.5), an indigenous acid proteinase in milk, was studied by reversed-phase HPLC and urea-PAGE. The results indicate that cathepsin D hydrolyzed all casein fractions and was more active on α_s1- than on β-casein; α_s1-casein was optimally hydrolyzed at pH 4.0. Proteolysis of β-casein was more sensitive to inhibition by NaCl than was α_s1-casein. Comparison of the proteolysis of individual caseins with that by chymosin (E.C. 3.4.23.4) showed that the hydrolysis of α_s1-casein by the two enzymes was very similar, but the specificities on ga_s2-casein differed. The initial stage of β-casein hydrolysis by cathepsin D was similar to that by chymosin and HPLC profiles showed a number of peptides in common. Although cathepsin D hydrolyzed κ-casein slowly in solution and the HPLC profiles of hydrolysates were similar to those produced by chymosin, cathepsin D showed poor milk clotting ability.