CASEIN, A POWERFUL ENHANCER OF THE RATE OF PLASMINOGEN ACTIVATION

Casein was found to be a potent accelerator of the rate of plasminogen activation by both tissue plasminogen activator (t-PA) and urokinase plasminogen activator (u-PA). The effect is measured by the increase in the rate of hydrolysis of the synthetic plasmin substrate, Spectrozyme PL, when the activating mixture contains casein. The effect on two chain human urokinase was studied in detail. Both bovine and human casein are effective; of the three principal fractions of bovine casein, alpha, beta, and kappa, the alpha fraction was found to be most effective. Alpha-casein, immobilized on agarose, bound both u-PA and plasminogen to a significantly greater extent than did plain agarose, or agarose conjugated to irrelevant proteins. The enhancement of the rate of activation by u-PA was much larger when Glu-plasminogen, rather than Lys-plasminogen, was used. In the presence of casein, Glu-plasminogen was activated at the fast rate characteristic of Lys-plasminogen. Since the presence of casein did not result in the proteolytic conversion of the Glu to the Lys form, the effect is interpreted as a change in the conformation of Glu-plasminogen to that of the Lys form, by interaction with casein. Kinetic analysis of the time course of activation at varying Glu-plasminogen concentrations gave a 23-fold decrease in Km in the presence of casein. This, coupled with a 3.45-fold increase in kcat, resulted in an 80-fold increase in kcat/Km, i.e. in the overall efficiency of activation. Indirect evidence suggests that the effect may be related to the micellar structure of casein.