PROTEOLYSIS OF BOVINE CASEINS BY CATHEPSIN-D - PRELIMINARY-OBSERVATIONS AND COMPARISON WITH CHYMOSIN

Proteolysis of alpha(s1-), alpha(s2), beta- and kappa-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 alpha(s1)- than on beta-casein; alpha(s1)-casein was optimally hydrolyzed at pH 4.0. Proteolysis of beta-casein was more sensitive to inhibition by NaCl than was alpha(s1)-casein. Comparison of the proteolysis of individual caseins with that by chymosin (E.C. 3.4.23.4) showed that the hydrolysis of alpha(s1)-casein by the two enzymes was very similar, but the specificities on alpha(s2)-casein differed. The initial stage of beta-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 kappa-casein slowly in solution and the HPLC profiles of hydrolysates were similar to those produced by chymosin, cathepsin D showed poor milk clotting ability.