The plasma kallikrein-kinin system counterbalances the renin-angiotensin system

The plasma kallikrein-kinin system (KKS), first recognized over 40 years ago, was originally believed to contribute to physiologic hemostasis. At the time, factor XII (Hageman factor, FXII) and the related proteins prekallikrein (PK) and high molecular–weight kininogen (HK) were known to be essential for efficient surface-activated blood coagulation, as measured in the activated partial thromboplastin time (APTT) test. Indeed, in this test, autoactivation of FXII in glass tubes promotes thrombin formation. According to the then-current “contact activation” hypothesis, FXII activation on a negatively charged surface was thought to initiate hemostasis in a similar manner by a cascade of proteolytic reactions that culminate in thrombin formation. This model was undermined by the failure to identify such a physiologically relevant surface, coupled with evidence that individuals deficient in FXII, PK, or HK are free of bleeding disorders. In addition, the recognition that factor XI, whose deficiency is associated with bleeding, can be activated by thrombin provided a bypass mechanism that obviated a need for FXII to activate factor XI. Only over the last 6 years have alternative explanations for the physiologic role of the KKS and its assembly and activation begun to emerge. Rather than assembling on a negatively charged surface such as that used in the APTT test, the proteins of the plasma KKS are now known to bind a multiprotein receptor complex in the intravascular compartment. As shown in Figure ​Figure1,1, HK, the critical regulator of plasma KKS assembly and activation, binds an endothelial cell surface receptor complex containing cytokeratin 1 (CK1), urokinase plasminogen activator receptor (uPAR), and gC1qR (1–6). Recent studies with cultured human umbilical vein endothelial cells indicate that FXII can also bind to this receptor (7), but that this interaction is highly regulated. Plasma concentrations of HK completely block FXII binding to the multiprotein receptor complex. Further, FXII binding requires a 30-fold higher free Zn2+ concentration than does HK, which can only be achieved in a milieu of activating platelets or other cells (7). Thus, under physiologic conditions, HK binds to this endothelial cell complex but FXII is prevented from doing so. Figure 1 Assembly and activation of the plasma KKS on endothelial cells. Plasma PK circulates in complex with HK. The HK•PK complex binds to a multiprotein receptor complex that consists of cytokeratin 1 (CK1), urokinase plasminogen activator receptor ...