It has been proposed that FX11 zymogen can cleave pre- kallikrein without prior conversion into one of its proteo- lytically cleaved forms FXlla or FXllf. We treated FX11 with pro-phe-arg-chloromethyl ketone (PPACMK) to remove all measurable amidolytic activity due to traces of FXlla or FXllf. Prekallikrein (PK) and IgG were similarly treated to remove all traces of kallikrein. FX11 was diluted to 0.016 μg/ml, using 3.8mg/ml IgG as the diluent, and mixed 1:1 with 215μg/ml PK. The rate of

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... ion of kallikrein activity was neglible to begin with but increased with time to approach the rate of activation obtained with FXlla or FXllf at equivalent concentrations. When 10μg/ml dextran sulphate was present the rate of PK activation was maximal from the beginning. This experiment was repeated in the presence of a specific inhibitor of activated FX11 derived from corn; O.lmg/ml inhibitor prevented the activation of PK. This indicates that the PK activator is the same species that reacts with the inhibitor, namely FXlla or FXllf. In another experiment, 1.67μg/ml of PPACMK-treated FX11 and 2.6 μg/ml PPACMK-treated PK were mixed in glass cuvettes containing 500yM chromogenic substrates specific for either FXlla or kallikrein. The resulting curves of absorbance vs time showed that both FX11 and PK became activated in accelerating reactions after short lag phases; corn inhibitor blocked this reaction too. Thus the rapid activation and the lack of a lag phase seen in the experiment with dextran sulphate was not due to a substrate-induced formation of an active site in FX11 zymogen but was a result of the very rapid activation of surface bound FX11 by traces of kallikrein. The kallikrein may have been generated by traces of FXlla which are always present even after treatment of FX11 with inhibitors. This amount of FXlla may also be present in vivo and provides a potential mechanism for the initiation of contact activation.