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

In a system consisting of purified proteins inositol‐phospholipid‐accelerated activation of prekallikrein by α‐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 inhibitition 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 × 105 M−1 s−1 in the absence and 1.1 × 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.