AUTOACTIVATION OF HUMAN PLASMA PREKALLIKREIN

Abstract

Incubation of purified human plasma prekallikrein with sulfatides or dextran sulfate resulted in spontaneous activation of prekallikrein as judged by the appearance of amidolytic activity towards the chromogenic substrate H-D-pro-phe-arg-p-nitroanilide (S 2302). The time course of generation of amidolytic activity was sigmoidal with an apparent lag phase followed by a rapid activation until finally a plateau was reached. Soybean trypsin inhibitor completely blocked prekallikrein activation whereas corn, limabean and ovomucoid trypsin inhibitor did not. The Ki of the reversible inhibitor, benzamidine, for autoactivation (240 uM) was identical to the Ki of benzamidine for kallikrein. Thus, spontaneous prekallikrein activation and kallikrein showed the same specificity for a number of serine protease inhibitors, indicating that prekallikrein is activated by its own enzymatically active form, kallikrein. Immunoblotting analysis showed that, concomitant with the appearance of amidolytic activity, prekallikrein was cleaved. However, prekallikrein was not quantitatively converted into two-chain kallikrein since other polypeptide products were visible on the gels. This accounts for the observation that in amidolytic assays not all prekallikrein present in the reaction mixture was measured as active kallikrein. Kinetic analysis showed that prekallikrein activation can be described by a second-order reaction mechanism in which prekallikrein is activated kallikrein. The apparent second order rate constant was 27000 M-ls-1 (pH 7.2, 50 uM sulfatides, ionic strength 1=0.06, at 37°C). Autocatalytic prekallikrein activation was strongly dependent on the ionic strength, since there was a considerable decrease in the rate of the reaction at high salt concentrations. Our data support a prekallikrein autoactivation mechanism in which surface-bound kallikrein activates surface-bound prekallikrein. The rate constant of autoactivation is considerably lower than the rate constants reported for Factor Xlla dependent prekallikrein formation. Autocatalytic prekallikrein activation may, however, contribute to kallikrein formation during the initiating phase of contact activation.