Fifty years of research on the plasma kallikrein-kinin system: from protein structure and function to cell biology and in-vivo pathophysiology.

Thromb Haemost 2007; 98: 77–83 For forty years my laboratory has been studying the kallikrein-kininogen-kinin system (KKS). I was challenged by my mentor Dr. Sol Sherry in whose laboratory I was completing my postdoctoral fellowship in 1966–1967 to find out what major plasma esterase was being activated by exposure of plasma to glass beads. Although both factors XII and XI were candidates, neither accounted for the high enzyme levels. I then purified plasma kallikrein for the first time. Based on these results, we derived a synthetic substrate assay which we used to assay prekallikrein in human and rodent plasma. We next investigated the natural substrate of plasma kallikrein. Kininogens were described as the proteins (1) from which proteases released the peptides bradykinin (BK) and lysyl-bradykinin (Lys-BK) (Fig. 1). By 1967, two forms of purified human plasma kininogens had been described: Low-molecular-weight kininogen (LK) and high-molecular-weight kininogen (HK) (2). Both are substrates for the proteolytic release of Lys-BK and BK by tissue and plasma kallikreins, respectively. HK is a β globulin, 120 kDa polypeptide with a plasma concentration about 80 μg/ml (670 nM). LK is a β globulin with a plasma concentration of 60 μg/ml and a molecular weight of 68 kDa. HK, along with prekallikrein and factor (F)XII, is a component of the plasma KKS, also known as the intrinsic activation system of coagulation or the contact system. The KKS has generally been known to be activated by contact with negatively charged macromolecules, leading to binding and activation of FXII (FXIIa); activation of prekallikrein (PK) to kallikrein (Kal) by FXIIa; cleavage of HK by kallikrein with release of bradykinin (BK); and formation of cleaved kininogen (HKa) (Figs. 1 and 2). By 1982, the entire sequence of HK and LK as well as the nucleotide sequence of the kininogen gene was known (3). From 1975 onward this view expanded thanks to the detection of healthy individuals with total or partial deficiency of HK (1, 4) in my laboratory and that of Wuepper, as well as studies of kininogen-deficient rats (5) and the development of different detection assays and more sophisticated techniques. The primary sequence of HK was determined (Fig. 3). HK is a multifunctional, multidomain protein (Fig. 1); and each HK domain may have distinct functions (6). We learned that PK exists as a biomolecular complex with HK in normal plasma (7). This association explains the reason why Ms. Williams (who lacked HK and LK) had low plasma activity of PK (40% of normal) which was discovered by adding purified HK to her plasma. We discovered that FXI also complexes with HK. HK accelerates the reciprocal activation of FXII by kallikrein and the activation of FXI.The association of HK with kallikrein and FXIa (Fig. 4) protects both enzymes from plasma inhibitors such as C1 (C1-INH) inhibitor and α2-macroglobulin (8, 9). The KKS activation properties are regulated by HK's light chain, which contains both a surfacebinding domain 5 (HKD5) and a zymogen binding domain 6 (HKD6) (10).