A plasma protein, molecular weight of 110 kD, that normally exists in plasma in a 1:1 complex with prekallikrein. HMWK is split by plasma kallikrein to produce bradykinin. The complex is a cofactor in the activation of coagulation factor xii. The product of this reaction, XIIa, in turn activates prekallikrein to kallikreins. (From Stedman, 26th ed)