Kinetics of the action of chymosin (rennin) on a peptide bond of bovine alpha s1-casein. Comparison of the behaviour of this substrate with that of beta- and kappa o-caseins.

The action of chymosin on the Phe23-Phe24 bond of bovine alpha s1-casein, in citrate buffer (pH 6.2) at 30 degrees C, was followed by reversed-phase HPLC quantification of residual alpha s1-casein or fragment 24-199 after different time periods and at different substrate concentrations. This allowed determination of the Michaelian parameters for the reaction under study which were compared with those previously obtained for the action of chymosin on beta- and kappa o-casein under identical reaction conditions. The whole efficiency of the three reactions, as estimated by kcat/Km, was 1.8, 20.6 and 1405.0 for alpha s1-, beta- and kappa o-caseins, respectively. The specificity of chymosin is discussed in the light of these results and of the known sequences of the 3 caseins.