Inhibition of activated Hageman factor (factor XIIa) by an inhibitor of the plasminogen activation (PA inhibitor)

Structures of human plasma β-factor XIIa cocrystallized with potent inhibitors

Activated factor XIIa (FXIIa) is a serine protease that has received a great deal of interest in recent years as a potential target for the development of new antithrombotics. Despite the strong interest in obtaining structural information, only the structure of the FXIIa catalytic domain in its zymogen conformation is available. In this work, reproducible experimental conditions found for the crystallization of human plasma β-FXIIa and crystal growth optimization have led to determination of the first structure of the active form of the enzyme. Two crystal structures of human plasma β-FXIIa complexed with small molecule inhibitors are presented herein. The first is the noncovalent inhibitor benzamidine. The second is an aminoisoquinoline containing a boronic acid-reactive group that targets the catalytic serine. Both benzamidine and the aminoisoquinoline bind in a canonical fashion typical of synthetic serine protease inhibitors, and the protease domain adopts a typical chymotrypsin-like serine protease active conformation. This novel structural data explains the basis of the FXII activation, provides insights into the enzymatic properties of β-FXIIa, and is a great aid toward the further design of protease inhibitors for human FXIIa.

FXII

The plasma protein FXII (Hageman factor) has been shown to be linked with the plasma defence systems of coagulation, fibrinolysis, kallikrein-kinin and complement. It can be activated by surface contact activation and in solution. Surface contact activation is a complex phenomenon involving negatively charged surfaces, FXII, high molecular weight kininogen and plasma kallikrein. Fluid-phase activation can be effected by a variety of serine proteases. In both types of activation the FXII zymogen is converted to active enzymes. FXII levels in plasma are low or undetectable in both inherited deficiencies and in a variety of clinical conditions. FXII levels can also be elevated in some clinical conditions. Although discovered as a clotting protein FXII appears to play an important role in the kallikrein-kinin and fibrinolytic systems and also has effects on cells. Recent studies suggest that therapeutic blockade of activation of FXII can be of benefit in certain clinical conditions.

The fibrinolytic system in man

The fibrinolytic system comprises a proenzyme, plasminogen, which can be activated to the active enzyme plasmin, that will degrade fibrin by different types of plasminogen activators. Inhibition of fibrinolysis may occur at the level of plasmin or at the level of the activators. Fibrinolysis in human blood seems to be regulated by specific molecular interactions between these components. In plasma, normally no systemic plasminogen activation occurs. When fibrin is formed, small amounts of plasminogen activator and plasminogen adsorb to the fibrin, and plasmin is generated in situ. The formed plasmin, which remains transiently complexed to fibrin, is only slowly inactivated by alpha 2-antiplasmin, while plasmin, which is released from digested fibrin, is rapidly and irreversibly neutralized. The fibrinolytic process, thus, seems to be triggered by and confined to fibrin. Thrombus formation may occur as the result of insufficient activation of the fibrinolytic system and (or) the presence of excess inhibitors, while excessive activation and/or deficiency of inhibitors might cause excessive plasmin formation and a bleeding tendency. Evidence obtained in animal models suggests that tissue-type plasminogen activator, obtained by recombinant DNA technology, may constitute a specific clot-selective thrombolytic agent with higher specific activity and fewer side effects than those currently in use.

The fibrinolytic system in man

The existence of a system in the human body capable of inducing the dissolution of endogenous pathologically formed thrombi was appreciated in ancient times. Considered in detail in this article are the data that have elucidated the physiologic regulation of which plasmin formation is dependent on, the plasma concentration of plasminogen, availability of activators of plasminogen in the plasma and surrounding tissue environment, the concentration of naturally present inhibitors, and the existence of fibrin in the circulation. Important in this rapidly progressive scientific discipline is consideration of the factors which control the synthesis of the components of this proteolytic enzyme system. Recently abundant information has indicated that this plasminogen-plasmin proteolytic enzyme system can be utilized therapeutically. Knowledge of the mechanisms of this system has permitted identification of agents that can be exogenously administered to releave thrombotic obstruction to blood flow in the venous (pulmonary emboli, deep vein thrombosis) and arterial (peripheral and central vessels) circulatory systems. Particularly important is the demonstration that thrombolytic agents can directly attack and alleviate the immediate cause of acute myocardial infarction. As a result of the innovations in the present decade, it is evident that the plasminogen system can be advantageously employed to reverse the pathologic effects of all thrombotic diseases.

A new type of congenital dysfibrinogenaemia with defective fibrin lysis--Dusard syndrome: possible relation to thrombosis

Congenital dysfibrinogenaemia is described in three members of a family presenting with recurrent thrombosis and in two other young members not yet affected. An abnormality in the polymerization of fibrin monomers was noted. In addition, the pathological fibrin clots were found to be less sensitive to degradation by a post venous occlusion euglobulin solution than normal fibrin. After fibrin clot incubation with lys-plasminogen at different concentrations, the biological activity of plasminogen in patient fibrin clot on S 2251 after SK-addition, was less than that observed with normal fibrin. It is speculated that defective in vivo thrombolysis might explain the recurrent thrombosis observed in this family. This finding represents a new concept in understanding thromboembolic diseases.

Expression of the catalytic activity of plasminogen activator under physiologic conditions

We have investigated the factors governing the plasminogen-dependent fibrinolysis catalyzed by the serine proteinase, plasminogen activator (EC 3.4.21.-), under physiologic conditions. We found that live rabbit fibroblasts digested much less fibrin than predicted by cell-free assay of the secreted plasminogen activator. The reduced catalytic activity of plasminogen activator expressed by cells growing on fibrin was regulated by the salt concentration of culture medium. The plasminogen activators of cells from several mammalian species were inhibited by physiologic salt concentrations (0.15 M NaCl) in cell-free assays. CaCl2 and KCl, but not D-glucose, were also effective inhibitors. The catalytic activity of purified human urokinase and of plasmin was unaffected by increased ionic strength. Plasminogen activators secreted both spontaneously and in response to stimulation by the tumor promoter, 12-O-tetradecanoyl-phorbol-13-acetate, were inhibited by 0.15 M NaCl. Physiologic salt concentration appeared to function by interacting with plasminogen activator, or plasminogen, and a third component, possibly a reversible inhibitor. One consequence of this regulation of plasminogen activator under physiologic conditions is the limitation of plasminogen-dependent fibrin degradation by living cells.