Purity ~95% (SDS-PAGE). Human Factor alpha-XIIa is a serine protease responsible for the activation of Factor XI to XIa in the contact activation system. Human Factor XII and prekallikrein are thought to be involved in a reciprocal activation mechanism in which Factor XIIa activates prekallikrein to kallikrein, which in turn converts Factor XII to XIIa. Factor XIIa activates Factor XI to XIa thereby triggering the Contact Factor cascade. Activity: As reported