The inositol‐phospholipid‐accelerated activation of prekallikrein by activated factor XII at physiological ionic strength requires zinc ions and high‐Mr kininogen

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Abstract
In a system consisting of purified proteins inositol-phospholipid-accelerated activation of prekallikrein by .alpha.-factor XIIa was determined by measuring the appearance of kallikrein amidolytic activity towards the chromogenic substrate, H-D-Pro-Phe-Arg-NH-PhNO2 (PhNO2, 4-nitrophenyl). The activation reaction was ionic-strength dependent. In the absence of high-Mr kininogen optimal activity was recorded at I = 50 mM. Searching for conditions, which could change this optimum towards physiological values, high-Mr kininogen was added. This resulted in an inhibition of the activity, with no change in ionic strength optimum. If, however, Zn2+ were added concomitant with high-Mr kininogen, the inhibition was abolished and optimal activity recorded at physiological ionic strength. The optimal Zn2+ concentration was found to be 0.1 mM. Kinetic analysis of the reaction demonstrated that the kcat/Km was 1.2 .times. 105 M-1 s-1 in the absence and 1.1 .times. 106 M-1 s-1 in the presence of Zn2+. Zn2+ were also required for inositol-phospholipid-accelerated initiation of the contact activation in whole plasma.