ENZYMATIC-ACTIVITIES OF ACTIVATED AND ZYMOGEN FORMS OF HUMAN HAGEMAN-FACTOR (FACTOR-XII)

Abstract

Pro-Phe-Arg chloromethylketone (PPACMK) at 5.26 .mu.M inactivated the amidolytic activity of native human Hageman factor with an apparent 1st-order rate constant of 0.75/min. The activated forms of Hageman factor, Hfa and HFf, were also inactivated by PPACMK with rate constants 0.82 and 0.72/min. These numbers indicated that the activity detectable in native Hageman factor was due to contamination with activated species. Uncleaved Hageman factor reacted slowly with 40 mM diisopropyl fluorophosphate with concomitant loss of its procoagulant activity. Incubation of native Hageman factor with PPACMK did not destroy its procoagulant activity, even in the presence of the activator dextran sulfate, but PPACMK inhibited autoactivation of Hageman factor, suggesting that no active site is formed in uncleaved, surface-bound Hageman factor. The activation of prekallikrein by Hageman factor under initial-rate conditions occurred after a lag and was prevented by an inhibitor of Hageman factor from corn. The kinetics of prekallikrein activation and the effects of inhibitors provide evidence that the amidolytic and proteolytic activities of human Hageman factor reside in the activated forms derived by limited proteolysis of the native molecule.