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In order to resolve conflicting reports about the possible identity of prekallikrein and Factor XII-dependent plasminogen proactivator (FXII-PPA), the γ-globulin fractions of prekallikrein-deficient (Fletcher trait) and of normal plasma were assayed for FXII-PPA. Based on both fibrin plate and clot lysis tests, FXII-PPA in the γ-globulin fractions of prekallikrein-deficient plasmas from 2 unrelated patients was undetectable, i.e. &lt;1% of the FXII-PPA in the normal γ-globulin fraction. However, PPA independent of FXII was detected in both the Fletcher and the normal γ-globulin fractions at 4% of the FXII-PPA present in the normal γ-globulin fraction.  Human plasma prekallikrein was purified 2,000-fold (specific clotting activity 22 units/mg) and was greater than 95% homogeneous on SDS-gels. FXII-PPA was always copurified with prekallikrein and was totally separated from Factor XI. No Factor XII-dependent or Factor XII-independent plasminogen activator activity was detected in purified Factor XI preparations at 40 units/ml. Purified prekallikrein in its precursor form gave 2 protein bands on SDS-gels at 82,000 and 78,000 MW. Upon reduction, a single 85,000 MW band was observed. Kallikrein and plasminogen activator activity were generated upon incubation with purified human Factor XIIa (28,000 MW form). Analysis of this reaction mixture on SDS-gels without reduction showed 2 bands with apparently identical MW’s as the precursor protein bands, whereas reduction showed cleavage of both protein bands.  These results suggest that prekallikrein is identical to FXII-PPA in normal human plasma and that activation of this zymogen by Factor XIIa involves limited proteolytic cleavage.

Human Prekallikrein (Plasminogen Proactivator): Purification, Characterization and Activation by Activated Factor XII