Heat inactivation of bovine plasmin

The thermal behaviour of the protease plasmin was studied in the absence and presence of milk components. A global unfolding of the isolated plasmin occurred at 50-55 degrees C, but secondary structure of plasmin increased with increasing temperature, possibly due to aggregation. Plasmin was reversibly inactivated between 55 and 65 degrees C and was not subject to autoproteolysis. Irreversible inactivation (obeying first-order kinetics) started above 65 degrees C with a temperature dependence typical for protein unfolding. Interaction with cystein caused irreversible (first-order) inactivation of plasmin from 45 degrees C upwards; kinetics suggested a mixed mechanism of unfolding and SH/SS interaction. In agreement with literature, casein protected plasmin from irreversible inactivation while beta-lactoglobulin increased inactivation. Casein partly compensated the destabilizing effect of beta-lactoglobulin. The high heat stability of plasmin in milk products appeared not to be due to a high conformational stability, but to protection by casein towards irreversible inactivation of the unfolded enzyme. (C) 1998 Elsevier Science Ltd. All rights reserved.