Abstract
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 50-55oC, but secondary structure of plasmin increased with increasing temperature, possibly due to aggregation. Plasmin was reversibly inactivated between 55 and 65oC and was not subject to autoproteolysis. Irreversible inactivation (obeying first-order kinetics) started above 65oC with a temperature dependence typical for protein unfolding. Interaction with cystein caused irreversible (first-order) inactivation of plasmin from 45oC upwards; kinetics suggested a mixed mechanism of unfolding and SH/SS interaction. In agreement with literature, casein protected plasmin from irreversible inactivation while -lactoglobulin increased inactivation. Casein partly compensated the destabilizing effect of -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.