Effects of alpha 2-plasmin inhibitor on plasminogen activation by staphylokinase/plasminogen complex

Serine-proteases as plasminogen activators in terms of fibrinolysis

OBJECTIVES

This review should give an overview about the natural human plasminogen activators and their various modified variants as well as similar substances isolated from animals, microorganisms and plants. When a blood clot is formed in a blood vessel, it avoids the oxygen supply of the surrounding tissue. A fast fibrinolytic therapy should redissolve the blood vessel and reduce the degradation of the tissue. All proteases that are part of the human blood coagulation and fibrinolytic system belong to the serine protease family. t-PA (tissue plasminogen activator) and u-PA (urokinase plasminogen activator) are the naturally occurring fibrinolytic agents that are also used in therapy.

KEY FINDINGS

Despite many years of research, t-PA is still the gold standard in fibrinolytic therapy. But it has to be given as an infusion, which needs time. Modified fibrinolytic substances are, were, or perhaps will be in the market. They have different advantages over t-PA, but often the disadvantages predominate.

CONCLUSION

Many substances have been developed but an optimal fibrinolytic agent combined with a simple administration is not in therapeutic use to date.

Analysis of plasminogen activation by the plasmin-staphylokinase complex in plasma of alpha2-antiplasmin-deficient mice

Staphylokinase (SAK) expresses plasminogen activator (PA) activity by forming a complex with plasmin; this PA activity is inhibited by alpha2-antiplasmin (alpha2-AP) in plasma. However, SAK's activity is protected against inhibition by alpha2-AP in the presence of fibrin because the plasmin-SAK complex binds to fibrin. In the present study, the interaction between SAK and murine plasminogen was investigated in the plasma of alpha2-AP-deficient (alpha2-AP-/-) mice or plasminogen-deficient (Plg-/-) mice. Although the human plasmin-SAK complex was formed in equimolar mixtures of plasmin and SAK, the murine plasmin-SAK complex was not formed. Human plasminogen was activated by the human plasmin-SAK complex, although equimolar mixtures of murine plasmin and SAK did not activate murine plasminogen. These findings suggest that SAK does not react with murine plasmin. However, the murine plasminogen was activated by the human plasmin-SAK complex, although this activation was approximately 100-fold weaker than human plasminogen. Human and wild-type mouse plasminogens were not activated by the human plasmin-SAK complex in their plasma. In alpha2-AP-/- mouse plasma, murine plasminogen was activated by the human plasmin-SAK complex. Human or murine plasminogen, which had been added to Plg-/- mouse plasma, was not activated by the human plasmin-SAK complex. However, plasma clot lysis by the human plasmin-SAK complex was observed in both human and murine plasma. These findings indicate that: (1) murine plasmin does not react with SAK, (2) human plasmin-SAK complex activates murine plasminogen, (3) this activation is inhibited by murine alpha2-AP, but (4) this activation is not inhibited by murine alpha2-AP in the presence of fibrin.

Effects of fibrin and alpha2-antiplasmin on plasminogen activation by staphylokinase

Staphylokinase obtains plasminogen activating activity by forming a complex with plasminogen. Although the enzymatic activity of staphylokinase is enhanced by fibrin, how fibrin enhances enzymatic activity has not been determined yet. The effects of fibrin, or fibrinogen fragments, on the activation of plasminogen by staphylokinase was investigated using CNBr-digested fibrinogen fragments (FCB-2 and FCB-5) and plasmin-degraded cross-linked fibrin fragments ((DD)E complex, DD fragments and E fragments). Kinetic analysis of the activity of staphylokinase revealed that its plasminogen activating activity, which was expressed as kcat/Km, was enhanced by FCB-2 (10-fold) and FCB-5 (5-fold). These fibrin fragments caused 38-, 30-, and 8.5-fold increases in activity for the DD fragment, (DD)E complex and E fragment, respectively. Although alpha2-antiplasmin inhibited the activation of plasminogen by staphylokinase, FCB-2 abolished its inhibitory effects, and the plasminogen activating activity of staphylokinase was restored. The inhibitory effects of alpha2-antiplasmin on the activation of mini-plasminogen by staphylokinase were less than for Glu- or Lys-plasminogen, and the inhibitory effect of alpha2-antiplasmin was not altered by fibrin or EACA. These findings indicate that the staphylokinase/plasmin(ogen) complex reacts with fibrin even in the presence of alpha2-antiplasmin, and efficient plasminogen activation takes place on the surface of fibrin.

Stabilization of plasmin by lysine derivatives

Plasmin is a serine protease with trypsin-like specificity and is activated from plasminogen by several plasminogen activators. Since plasmin has lysine binding site in its heavy chain, lysine derivatives react with plasmin and then modify its activity. The effects of lysine derivatives such as epsilon-aminocaproic acid (EACA) and tranexamic acid on bovine plasmin activity were investigated. In the absence of lysine derivatives, the bovine plasmin activity which was evaluated as the amidolytic activity was reduced in a time- or temperature-dependent manner. However, the bovine plasmin activity became stable upon adding EACA or tranexamic acid. When plasmin was incubated at 4 degrees C for 1, 3 or 5 days without lysine derivatives, the plasmin activity decreased to 43.9%, 19.9% and 11.9% of the initial activity, respectively. On the other hand, when plasmin was incubated at 37 degrees C for 1, 3 or 5 days with tranexamic acid, its activity remained at 110%, 95.6% and 85.9%, respectively. After bovine plasmin had been incubated for 5 days at 4 degrees C in the absence of tranexamic acid, the plasmin activity declined to less than 20%. However, when bovine plasmin had been incubated for 5 days at 37 degrees C in the presence of tranexamic acid, the residual plasmin activity was more than 80%. A similar effect of EACA on bovine plasmin was observed, but it was weaker than that of tranexamic acid. Reversed-phase HPLC followed by SDS-PAGE demonstrated that bovine plasmin was degraded into several fragments. Amino acid sequencing of these fragments revealed that the Lys77-Arg78 or Arg78-Ile79, Arg342-Met343 and Arg557-Ile558 peptide bonds in the bovine plasmin molecule were cleft, respectively. Only the fragment consisting of the amino acid region from Met343 to the C-terminal amino acid, Asn786, exhibited amidolytic activity. In proportion to inactivation of the bovine plasmin, this fragment disappeared. The above findings suggest that lysine derivatives react with bovine plasmin and then stabilize the activity of plasmin by preventing the degradation of active fragment (Met343-Asn786).

Enhancing effect of platelets on staphylokinase-mediated clot lysis and plasminogen activation

The effects of platelets on clot lysis and plasminogen activation by staphylokinase (SAK) were investigated. At concentrations ranging from 2 x 10(-7) to 1 x 10(-5)g/ml of SAK, the lysis time of platelet-rich plasma clots (PRP-clots) was shorter than that of platelet-poor plasma clots (PPP-clots). This reduction of clot lysis time was observed in a dose-dependent manner on platelet count in PRP. The activation rate of plasminogen by SAK measured by the amidolytic method using S-2251 was enhanced by the addition of washed platelets. These enhancing effects of platelets on clot lysis and plasminogen activation were not altered by pretreatment of platelets with indomethacin and theophylline, but were diminished by platelet disruption. Thus, we concluded that platelets enhance fibrinolytic activity of SAK, and this effect is not due to the release reaction or intracellular contents of platelets, but to the existence of platelet surface in the intact shape as a catalytic site for fibrinolysis.