HYDROLYSIS OF BOVINE AND CAPRINE CASEINS BY RENNET AND PLASMIN IN MODEL SYSTEMS

The proteolytic activity of both calf rennet and plasmin combined on b ovine and caprine caseins was studied under various pH conditions by e lectrophoresis. Electrophoretic studies of the pH 4,6-insoluble fracti on showed that both enzyme preparations (rennet and plasmin) hydrolyze d caseins, giving the typical breakdown products derived from individu al caseins (CN): alpha(s1)-I-CN and beta-I to beta-III from a(s1)-CN a nd beta-CN, respectively, by rennet action and gamma-CNs from beta-CN by plasmin action. These breakdown products were more or less evident in the hydrolysates depending on the pH conditions used, and they were subsequently hydrolyzed during the different hydrolysis times. Isolat ed gamma-CNs were resistant to the rennet action, showing that althoug h these breakdown products contain the chymosin-susceptible bonds of b eta-CN, presumably these bonds are inaccessible in gamma-CNs. However, beta-CN-derived products by the rennet action (i.e., beta-I, etc.) we re largely hydrolyzed by the plasmin action to yield peptides in the e lectrophoretic zone of gamma-CNs. Bovine alpha(s1)-I-CN and para-kappa -CN, the first proteolytic products from a(s1)-CN and kappa-CN, respec tively, by rennet, were degraded by plasmin, indicating that these bre akdown products are susceptible to further proteolysis by plasmin in s olution.