Kinetic analysis of covalent hybrid plasminogen activators: effect of CNBr-degraded fibrinogen on kinetic parameters of Glu1-plasminogen activation

Preservation of Thrombolytic Activity of Urokinase Modified with Monomethoxypoly(ethylene glycol

A method is described to modify urokinase by covalent binding of monomethoxypoly(ethylene glycol) (mPEG) without impairing its catalytic ac tivity towards high molecular weight substrates. The urokinase active site is protected by an inhibitor, benzamidine, bound to Sepharose during the mPEG modification in order to avoid binding mPEG chains to the active site or to the surrounding area. The mPEG modified urokinase had increased activity towards small molecular weight substrates (acetyl-Gly-methyl ester) as com pared to the unmodified enzyme, while the activity towards the high molecular weight plasminogen and the insoluble substrate fibrin clot was preserved. This did not occur when the enzyme was modified in the absence of active site pro tection. The polymer modification increased the enzyme's thermostability and the stability in plasma in vitro and prolonged in vivo retention after in travenous injection in rats.

Multiple exosites distributed across the three domains of streptokinase co-operate to generate high catalytic rates in the streptokinase-plasmin activator complex

To examine the global function of the key surface-exposed loops of streptokinase, bearing substrate-specific exosites, namely, the 88-97 loop in the α domain, the 170 loop in the β domain, and the coiled-coil region (Leu321-Asn338) in the γ domain, mutagenic as well as peptide inhibition studies were carried out. Peptides corresponded to the primary structure of an exosite, either individual or stoichiometric mixtures of various disulfide-constrained synthetic peptide(s) inhibited plasminogen activation by streptokinase. Remarkably, pronounced inhibition of substrate plasminogen activation by the preformed streptokinase-plasmin activator complex was observed when complementary mixtures of different peptides were used compared to the same overall concentrations of individual peptides, suggesting co-operative interactions between the exosites. This observation was confirmed with streptokinase variants mutated at one, two, or three sites simultaneously. The single/double/triple exosite mutants of streptokinase showed a nonadditive, synergistic decline in kcat for substrate plasminogen activation in the order single > double > triple exosite mutant. Under the same conditions, zymogen activation by the various mutants remained essentially native- like in terms of nonproteolytic activation of partner plasminogen. Multisite mutants also retain affinity to form 1:1 stoichiometric activator complexes with plasmin when probed through sensitive equilibrium fluorescence studies. Thus, the present results strongly support a model of streptokinase action, wherein catalysis by the streptokinase-plasmin complex operates through a distributed network of substrate-interacting exosites resident across all three domains of the cofactor protein.

Streptococcus uberis plasminogen activator (SUPA) activates human plasminogen through novel species-specific and fibrin-targeted mechanisms

Bacterial plasminogen (Pg) activators generate plasmin to degrade fibrin blood clots and other proteins that modulate the pathogenesis of infection, yet despite strong homology between mammalian Pgs, the activity of bacterial Pg activators is thought to be restricted to the Pg of their host mammalian species. Thus, we found that Streptococcus uberis Pg activator (SUPA), isolated from a Streptococcus species that infects cows but not humans, robustly activated bovine but not human Pg in purified systems and in plasma. Consistent with this, SUPA formed a higher avidity complex (118-fold) with bovine Pg than with human Pg and non-proteolytically activated bovine but not human Pg. Surprisingly, however, the presence of human fibrin overrides the species-restricted action of SUPA. First, human fibrin enhanced the binding avidity of SUPA for human Pg by 4-8-fold in the presence and absence of chloride ion (a negative regulator). Second, although SUPA did not protect plasmin from inactivation by α(2)-antiplasmin, fibrin did protect human plasmin, which formed a 31-fold higher avidity complex with SUPA than Pg. Third, fibrin significantly enhanced Pg activation by reducing the K(m) (4-fold) and improving the catalytic efficiency of the SUPA complex (6-fold). Taken together, these data suggest that indirect molecular interactions may override the species-restricted activity of bacterial Pg activators; this may affect the pathogenesis of infections or may be exploited to facilitate the design of new blood clot-dissolving drugs.

Identification of a new exosite involved in catalytic turnover by the streptokinase-plasmin activator complex during human plasminogen activation

With the goal of identifying hitherto unknown surface exosites of streptokinase involved in substrate human plasminogen recognition and catalytic turnover, synthetic peptides encompassing the 170 loop (CQFTPLNPDDDFRPGLKDTKLLC) in the beta-domain were tested for selective inhibition of substrate human plasminogen activation by the streptokinase-plasmin activator complex. Although a disulfide-constrained peptide exhibited strong inhibition, a linear peptide with the same sequence, or a disulfide-constrained variant with a single lysine to alanine mutation showed significantly reduced capabilities of inhibition. Alanine-scanning mutagenesis of the 170 loop of the beta-domain of streptokinase was then performed to elucidate its importance in streptokinase-mediated plasminogen activation. Some of the 170 loop mutants showed a remarkable decline in k(cat) without any alteration in apparent substrate affinity (K(m)) as compared with wild-type streptokinase and identified the importance of Lys(180) as well as Pro(177) in the functioning of this loop. Remarkably, these mutants were able to generate amidolytic activity and non-proteolytic activation in "partner" plasminogen as wild-type streptokinase. Moreover, cofactor activities of the 170 loop mutants, pre-complexed with plasmin, against microplasminogen as the substrate showed a similar pattern of decline in k(cat) as that observed in the case of full-length plasminogen, with no concomitant change in K(m). These results strongly suggest that the 170 loop of the beta-domain of streptokinase is important for catalysis by the streptokinase-plasmin(ogen) activator complex, particularly in catalytic processing/turnover of substrate, although it does not seem to contribute significantly toward enzyme-substrate affinity per se.

Effects of deletion of streptokinase residues 48-59 on plasminogen activation

Streptokinase (SK) is a thrombolytic agent widely used for the clinical treatment of clotting disorders such as heart attack. The treatment is based on the ability of SK to bind plasminogen (Pg) or plasmin (Pm), forming complexes that proteolytically activate other Pg molecules to Pm, which carries out fibrinolysis. SK contains three major domains. The N-terminal domain, SKalpha, provides the complex with substrate recognition towards Pg. SKalpha contains a unique mobile loop, residues 45-70, absent in the corresponding domains of other bacterial Pg activators. To study the roles of this loop, we deleted 12 residues in this loop in both full-length SK and the SKalpha fragment. Kinetic data indicate that this loop participates in the recognition of substrate Pg, but does not function in the active site formation in the activator complex. Two crystal structures of the deletion mutant of SKalpha (SKalpha(delta)) complexed with the protease domain of Pg were determined. While the structure of SKalpha(delta) is essentially the same as this domain in full-length SK, the mode of SK-Pg interaction was however different from a previously observed structure. Even though mutagenesis studies indicated that the current complex represents a minor interacting form in solution, the binding to SKalpha(delta) triggered similar conformational changes in the Pg active site in both crystal forms.

A catalytic switch and the conversion of streptokinase to a fibrin-targeted plasminogen activator

Plasminogen (Pg) activators such as streptokinase (SK) save lives by generating plasmin to dissolve blood clots. Some believe that the unique ability of SK to activate Pg in the absence of fibrin limits its therapeutic utility. We have found that SK contains an unusual NH(2)-terminal "catalytic switch" that allows Pg activation through both fibrin-independent and fibrin-dependent mechanisms. Unlike SK, a mutant (rSKDelta59) fusion protein lacking the 59 NH(2)-terminal residues was no longer capable of fibrin-independent Pg activation (k(cat)/K(m) decreased by >600-fold). This activity was restored by coincubation with equimolar amounts of the NH(2)-terminal peptide rSK1-59. Deletion of the NH(2) terminus made rSKDelta59 a Pg activator that requires fibrin, but not fibrinogen, for efficient catalytic function. The fibrin-dependence of the rSKDelta59 activator complex apparently resulted from selective catalytic processing of fibrin-bound Pg substrates in preference to other Pg forms. Consistent with these observations, the presence (rSK) or absence (rSKDelta59) of the SK NH(2)-terminal peptide markedly altered fibrinolysis of human clots suspended in plasma. Like native SK, rSK produced incomplete clot lysis and complete destruction of plasma fibrinogen; in contrast, rSKDelta59 produced total clot lysis and minimal fibrinogen degradation. These studies indicate that structural elements in the NH(2) terminus are responsible for SK's unique mechanism of fibrin-independent Pg activation. Because deletion of the NH(2) terminus alters SK's mechanism of action and targets Pg activation to fibrin, there is the potential to improve SK's therapeutic efficacy.

Allosteric regulation of tPA-mediated plasminogen activation by a modifier mechanism: evidence for a binding site for plasminogen on the tPA A-chain

We studied the mechanism responsible for nonlinear double reciprocal plots for tissue type plasminogen activator (tPA)-mediated plasminogen activation reported previously by several groups. We found nonlinear Eadie-Scatchard plots for Glu-plasminogen activation by recombinant single-chain tPA confirming a non-Michaelis-Menten behavior of tPA. In order to characterize this mechanism, enzyme kinetic studies with truncated substrates (Lys- and miniplasminogen) and modified or truncated enzymes (two-chain tPA and tPA B-chain) were performed. Thereby it could be excluded that product-mediated modifications of the enzyme or the substrate are responsible for the nonlinear plots. Linear plots, i.e., Michaelis-Menten kinetics, were only found when tPA B-chain was used as a plasminogen activator, indicating that the tPA A-chain should be responsible for the non-Michaelis-Menten behavior. Binding studies of plasminogen to immobilized tPA A-chain in fact demonstrated a saturable binding of Glu- and miniplasminogen to the A-chain of tPA with a KD approximately 0.1 microM and one binding site per molecule of tPA A-chain. These data suggested a modifier mechanism responsible for the nonlinear plots whereby the substrate plasminogen itself could function as a modifier. When such a mechanism was included into a model for tPA-mediated plasminogen activation, the experimentally obtained data could be fitted into such a model by nonlinear regression analysis with resulting p-values of less than 0.001.

The effect of the carboxy-terminal lysine of urokinase on the catalysis of plasminogen activation

When single-chain pro-UK is activated by plasmin or kallikrein, the Lys158-Ile159 bond is cleaved, leaving a C-terminal lysine on the A-chain (Lys-UK). Two-chain, high molecular weight urokinase (UK) purified from urine, however, has been shown to contain a phenylalanine residue as the C-terminal of the A-chain (Phe-UK). Since C-terminal lysine residues have a strong binding affinity for plasminogen that may promote its activation, we undertook kinetic studies comparing plasminogen activation by Lys- and Phe-UK. A two-stage method was employed in order to minimize factors known to interfere with plasminogen activation and plasmin determination. The Lys-UK was prepared by plasmin activation of pro-UK purified from human fetal kidney cell culture medium. The Phe-UK was prepared by carboxypeptidase B (CpB) treatment of Lys-UK. Removal of the C-terminal lysine of Lys-UK by CpB produced small but significant increases in the Michaelis constants for the activation of both Glu- and Lys-plasminogen. The apparent Michaelis constants for Glu-plasminogen activation by Lys- and Phe-UK were 3.7 microM +/- .36 microM and 5.9 microM +/- .70 microM, respectively and the Michaelis constants for Lys-plasminogen activation by Lys- and Phe-UK were 5.4 microM +/- .72 microM and 15.2 microM +/- 1.4 microM, respectively. The catalytic efficiency (kcat/Km) of Lys-UK was approximately 2-fold greater than that of Phe-UK for the activation of either Glu- or Lys-plasminogen. When the fibrinolytic activities of Lys- and Phe-UK were compared in a plasma milieu no significant differences were detected. In conclusion, the findings indicate that the C terminal lysine on the A-chain of UK significantly promotes the catalysis of plasminogen in a purified system. However, the higher catalytic efficiency of Lys-UK was not found to induce significant acceleration of clot lysis at pharmacological concentrations in plasma.

Kinetic studies on novel plasminogen activators. Demonstration of fibrin enhancement for hybrid enzymes comprising the A-chain of plasmin (Lys-78) and B-chain of tissue-type plasminogen activator (Ile-276) or urokinase (Ile-159)

The activation of plasminogen by two novel hybrid enzymes, constructed from the A-chain of plasmin and the B-chains of tissue-type plasminogen activator (t-PA) or urokinase, was compared with the activation by the parent enzymes. Basal kinetic constants for 'Lys-plasminogen' (human plasminogen with N-terminal lysine) and 'Glu-plasminogen' (human plasminogen with N-terminal glutamic acid) activation were similar to those of the parent activators. The Km for plasminogen turnover for both hybrid enzymes was considerably decreased in the presence of both soluble fibrin and a mimic, a CNBr digest of fibrinogen. These enhancements and the related apparent negative co-operativity are similar to the behaviour of t-PA itself. The results are discussed with regard to the molecular features involved in the mechanism of fibrin stimulation.