CONTACT ACTIVATION AND SINGLE-CHAIN UROKINASE-TYPE PLASMINOGEN ACTIVATOR
Authors: Dooi jewaard G , Binnema D J , Kluft C 
Published: August 2018

For many years it is known that activation of the factor XII (FXII) -prekallikrein (PKK)- kininogen system of coagulation (contact activation) also may be involved in activation of fibrinolysis. Despite the numerous efforts over the past two decades to clarify this process, our current insights in this matter are far from complete. Also the physiological meaning of this possible interlinkage of coagulation and fibrinolysis is still uncertain; clearcut clinical manifestations in patients deficient in FXII or PKK are not found.No doubt, activation of fibrinolysis is a much more complicated process than it originally was thought to be, and it is only recently that the importance of urokinase for fibrinolysis in the circulation became clear. Two pathways of plasminogen (Pig) activation may be distinguished: 1. the extrinsic system, catalysed by t-PA, which upon stimulus is increasingly released from the endothelial cells of the vessel wall and 2. the intrinsic system, catalysed via Pig proactivators which circulate in the blood at a fairly constant level of concentration. The discovery that the virgin 55 kD urokinase molecule in fact is a single-chain proenzym (now denoted by scu-PA, single-chain urokinase-type PA), the notion that 55 kD scu-PA occurs in the blood and that its concentration even among individuals is fairly constant (2.1+/-0.4 ng/ml, n=52), and the observation that the efficacy of scu-PA is fibrin selective, all are recent findings pointing to the involvement of scu-PA in the intrinsic system.Still the relation between contact activation and the activation of scu-PA is obscure. Active KK, for instance, is an effective activator of 55 kD scu-PA, but proteolytic cleavage of scu-PA resulting in an active molecule, is readily achieved in plasma’s deficient in FXII or PKK. In addition, a portion of Pig activator activity which is dependent for its activation on FXII and PKK, is fully recovered in plasma’s artificially depleted in 55 kD scu-PA. Yet, both portions are activated by negatively charged surfaces or dextransulphate (DXS) as a substitute! These observations have led to the concept of two co-ordinative pathways of Pig activation for the intrinsic system: one containing scu-PA, the other containing FXII, PKK and a postulated Pig proactivator (note that the Pig activator activities of FXIIa and KK per se do not account for the latter portion of activity). Until recently in both pathways was a missing link: in the former it was the step between the negatively charged surface and scu-PA, in the latter it was the postulated Pig proactivator between active KK and Pig. This year, however, it became clear that in plasma artificially depleted in u-PA, still a substantial amount of protein immunochemically related to u-PA, can be detected (at least 35 ng/ml), but only after SDS PAGE. Part of this protein is a single-chain 110 kD molecule which in plasma can be converted to a cleaved molecule with Pig activator activity provided the plasma contains FXII and PKK. Although the relation with the 55 kD scu-PA remained unclear, the discovery of this 110 kD PA with latent urokinase antigen, undoubtedly, explains the missing link between KK and Pig. The other missing link still remains unexplained. It could be an in vitro artefact by DXS causing scu-PA catalysed activation of Pig as fibrin clots do. Since subsequently generated plasmin is capable of activation of both scu-PA and FXII, the two intrinsic pathways are thus interlinked via feed-back activation and consequently may be co-operative in function.