We are investigating the mechanism by which Hageman factor becomes activated upon binding to negatively charged surfaces. We have found that it autodigests and autoactivates when a trace of active enzyme is present. The reaction is then accelerated further by activation by kallikrein, an enzymatic process which requires a cofactor called high molecular weight (HMW) kininogen. Both of these steps are needed to give a normal reaction rate. We are now trying to determine whether native Hageman factor possesses intrinsic activity, whether an active site is created upon binding to surfaces in the absence of cleavage, or whether traces of cleaved (and activated) Hageman factor exists in plasma. Two active forms of Hageman factor are known, an 80,000 dalton two-chain enzyme (HFa), and a 28,000 dalton fragment (HFf). We are determining the sequence of events that lead to formation of these molecular species as well as the chain structure of HFf. HMW-kininogen acts as a cofactor for Hageman factor activation by placing prekallikrein on the surface so that it can be converted to kallikrein and facilitating dissociation of kallikrein from the surface so that the effective concentration available for Hageman factor activation is augmented.