Biologic activities of the contact factors in vivo--potentiation of hypotension, inflammation, and fibrinolysis, and inhibition of cell adhesion, angiogenesis and thrombosis.

Three plasma proteins, namely, Factor XII (Hageman factor), prekallikrein (Fletcher factor), and high molecular weight kininogen (Williams-Fitzgerald-Flaujeac factor), have been shown to be required for the activation of the surface-mediated pathway, or “contact system.” In vitro, such as in the activated partial thromboplastin test or on exposure to artificial surfaces, factor XII (FXII) is autoactivated to factor XIIa, which cleaves plasma prekallikrein (PK) by limited proteolysis into the active enzyme, plasma kallikrein (Fig. 1). In contrast, in vivo, high molecular weight kininogen (HK) binds to the endothelial cell surface associated with PK in a noncovalent complex. PK is activated by an endothelial cell cysteine protease to kallikrein which cleaves prourokinase to urokinase and factor XII to factor XIIa (Fig. 2). Thus, on cells, PK activation initiates the contact system, while on artificial surfaces, FXII is the initiating molecule. HK is also the substrate of plasma kallikrein, which cleaves to yield kinin-free HK (HKa) and bradykinin (Fig. 1). Although the genetically determined deficiencies of FXII, PK, and HK display abnormal in vitro plasma coagulation, none of these deficiencies is associated with a hemorrhagic diathesis. However, Factor XII, PK and HK display a wide variety of important interactions with other plasma proteins. In vitro, activated factor XII activates factor XI, factor VII and the C1 component of the complement system. Plasma kallikrein activates prourokinase (Fig. 1), prorenin, and factor XII, and cleaves HK, releasing bradykinin. Each contact protein interacts with blood and vascular cells. Factor XII downregulates a monocyte Fc receptor, releases IL-1 and IL-6 from monocytes and macrophages, and stimulates neutrophils, Plasma kallikrein activates neutrophils and potentiates fibrinolysis on endothelial cells. HK inhibits adhesion of neutrophils to artificial surfaces, enhances cellular fibrinolysis, and inhibits thrombin-induced platelet activation. Knowledge of these biochemical and cellular reactions has led to an understanding of the role of contact proteins in inflammation, fibrinolysis, cell adhesion, platelet activation and angiogenesis, and an appreciation of their participation in various pathophysiologic conditions. The molecular biology, structure and biochemical properties of FXII, PK and HK have recently been comprehensively reviewed. This review will focus on their participation in vivo in pathophysiologic processes including hypotension, inflammation, fibrinolysis, cell adhesion, angiogenesis and thrombosis.