Study of the sites of plasminogen molecule which are responsible for inhibitory effect of Lys-plasminogen on platelet aggregation

Plasminogen/plasmin system is involved in such important processes as thrombosis, inflammation and cancer. plasmin and plasminogen mediate their action through plasminogen-binding proteins on the cell surface. Lys-plasminogen, but not Glu-plasminogen, shows inhibitory effect on platelet aggregation induced by aDp, collagen and thrombin in preparations of both: platelet-rich plasma and washed platelets. We have shown that the kringle domains of Lys-plasminogen mediate interaction of this proenzyme with platelet-surface proteins. the aim of the work is to study the role of certain kringle domains in the inhibitory effect of Lys-plasminogen and to determine possible plasminogen-binding proteins on the platelet surface. all studied plasminogen fragments (k1-3, k4 and k5) abolished the inhibitory effect of Lys-plasminogen on platelet aggregation. We observed that k5 was more effective than k1-3 and k4. Biotin-labeled Lys-plasminogen, Glu-plasminogen and plasminogen fragment K1-3 possessed the highest affinity for actin, whereas the binding of biotin-labeled mini-plasminogen and k4 to actin was negligible. We have suggested that inhibitory effect of Lys-plasminogen is due to the interaction of kringle domains of this proenzyme with membrane-bound proteins which are exposed on the platelet surface during activation and are involved in thrombus formation

Effect of fibrinogen degradation products on various stages of the fibrinolytic process

Over-activation of the fibrinolytic system may result in proteolytic destruction of fibrinogen. However, the effect of the degradation products formed during fibrinogenolysis on fibrinolytic process and plasminogen/plasmin properties remains unclear. To investigate this effect and its mechanism, the ability of fibrinogen fragments E and D to act on plasminogen and tPA binding, proenzyme activation, fibrin clot lysis and plasmin inhibition by plasma α2-antiplasmin, were studied. It was found that early product fragment EE binds to plasminogen and tissue-type plasminogen activator and enhances plasminogen conversion into plasmin. C-terminal lysine residues of all 3 chains pair and 16 or 23 amino acid residues of Aα- chain are essential for this process. C-terminal lysines of fragment E Aα- and γ-chains and lysine-binding site of tPA kringle 2 are responsible for the interaction between these proteins. Binding of fragment E to plasminogen is provided by N-terminal Aα1–19 and C-terminal Bβ120–122 regions. Late plasmic fibrinogen degradation product fragment EL loses the ability to potentiate plasmin generation but can bind proenzyme and its activator. Fragment D has no binding properties towards plasminogen and tPA. None of fibrinogen fragments protects plasmin from α2-antiplasmin inhibition. It is concluded that at over-activation of the fibrinolytic system and subsequent fibrinogenolysis, the products of fibrinogen degradation, can bind plasminogen and tPA and potentiate generation of plasmin, which will be neutralized under the normal level of the plasmin inhibitor