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The study observation shows that explosive generation of plasmin in blood or plasma which diminishes streptokinase's therapeutic effects is attributated to the formation of SK-Pg* that is governed by kringle domains.

Thrombosis, the formation of a clot within a blood vessel, underlies a number of life-threatening cardiovascular disorders such as heart attack, ischemic stroke, pulmonary embolism, and deep vein thrombosis. These conditions affect the lives of millions of people worldwide and result in significant morbidity and mortality. It is thus crucial to develop novel methodologies to enhance the detection and treatment of these disorders. Thrombolysis, or the dissolution of blood clots, relies upon the administration of exogenous plasminogen activators (PAs) that lyse fibrin. Yet, there are several drawbacks to the use of current PAs, including significant risks of uncontrolled bleeding and suboptimal efficacy and pharmacokinetics. Nanomaterials are well positioned to address these priority issues in thrombolysis, via the alteration of PA pharmacokinetics and biodistribution. Additionally, due to the multifunctional nature of nanoparticles, these thrombolytics may be targeted to the site of occlusion, effectively concentrating the drug where it is most needed. Herein, we describe the methodology associated with the synthesis of a novel thrombus-targeted fibrinolytic nanoagent. At each step of the synthesis, we analyze the nanomaterials, including their physical properties and their ability to bind to thrombosis targets of interest. The effect of the conjugation of tPA to the nanoparticle surface on the amidolytic and fibrinolytic activities of nanoagents is also investigated. Lastly, the in vivo binding of the targeted thrombolytic to intravascular thrombi is examined., (Copyright © 2012 Elsevier Inc. All rights reserved.)

The mechanism of action of plasminogen (Pg) activators may affect their therapeutic properties in humans. Streptokinase (SK) is a robust Pg activator in physiologic fluids in the absence of fibrin. Deletion of a "catalytic switch" (SK residues 1-59), alters the conformation of the SK alpha domain and converts SKDelta59 into a fibrin-dependent Pg activator through unknown mechanisms. We show that the SK alpha domain binds avidly to the Pg kringle domains that maintain Glu-Pg in a tightly folded conformation. By virtue of deletion of SK residues 1-59, SKDelta59 loses the ability to unfold Glu-Pg during complex formation and becomes incapable of nonproteolytic active site formation. In this manner, SKDelta59 behaves more like staphylokinase than like SK; it requires plasmin to form a functional activator complex, and in this complex SKDelta59 does not protect plasmin from inhibition by alpha(2)-antiplasmin. At the same time, SKDelta59 is unlike staphylokinase or SK and is more like tissue Pg activator, because it is a poor activator of the tightly folded form of Glu-Pg in physiologic solutions. SKDelta59 can only activate Glu-Pg when it was unfolded by fibrin interactions or by Cl(-)-deficient buffers. Taken together, these studies indicate that an intact alpha domain confers on SK the ability to nonproteolytically activate Glu-Pg, to unfold and process Glu-Pg substrate in physiologic solutions, and to alter the substrate-inhibitor interactions of plasmin in the activator complex. The loss of an intact alpha domain makes SKDelta59 activate Pg through classical "fibrin-dependent mechanisms" (akin to both staphylokinase and tissue Pg activator) that include: 1) a marked preference for a fibrin-bound or unfolded Glu-Pg substrate, 2) a requirement for plasmin in the activator complex, and 3) the creation of an activator complex with plasmin that is readily inhibited by alpha(2)-antiplasmin.

Streptokinase (SK) and staphylokinase form cofactor-enzyme complexes that promote the degradation of fibrin thrombi by activating human plasminogen. The unique abilities of streptokinase to nonproteolytically activate plasminogen or to alter the interactions of plasmin with substrates and inhibitors may be the result of high affinity binding mediated by the streptokinase beta-domain. To examine this hypothesis, a chimeric streptokinase, SKbetaswap, was created by swapping the SK beta-domain with the homologous beta-domain of Streptococcus uberis Pg activator (SUPA or PauA, SK uberis), a streptokinase that cannot activate human plasminogen. SKbetaswap formed a tight complex with microplasminogen with an affinity comparable with streptokinase. The SKbetaswap-plasmin complex also activated human plasminogen with catalytic efficiencies (k(cat)/K(m) = 16.8 versus 15.2 microm(-1) min(-1)) comparable with streptokinase. However, SKbetaswap was incapable of nonproteolytic active site generation and activated plasminogen by a staphylokinase mechanism. When compared with streptokinase complexes, SKbetaswap-plasmin and SKbetaswap-microplasmin complexes had altered affinities for low molecular weight substrates. The SKbetaswap-plasmin complex also was less resistant than the streptokinase-plasmin complex to inhibition by alpha(2)-antiplasmin and was readily inhibited by soybean trypsin inhibitor. Thus, in addition to mediating high affinity binding to plasmin(ogen), the streptokinase beta-domain is required for nonproteolytic active site generation and specifically modulates the interactions of the complex with substrates and inhibitors.