Plasminogen activator activities of equimolar complexes of streptokinase with variant recombinant plasminogens

Rapid binding of plasminogen to streptokinase in a catalytic complex reveals a three-step mechanism

Rapid kinetics demonstrate a three-step pathway of streptokinase (SK) binding to plasminogen (Pg), the zymogen of plasmin (Pm). Formation of a fluorescently silent encounter complex is followed by two conformational tightening steps reported by fluorescence quenches. Forward reactions were defined by time courses of biphasic quenching during complex formation between SK or its COOH-terminal Lys(414) deletion mutant (SKΔK414) and active site-labeled [Lys]Pg ([5-(acetamido)fluorescein]-D-Phe-Phe-Arg-[Lys]Pg ([5F]FFR-[Lys]Pg)) and by the SK dependences of the quench rates. Active site-blocked Pm rapidly displaced [5F]FFR-[Lys]Pg from the complex. The encounter and final SK ·[5F]FFR-[Lys]Pg complexes were weakened similarly by SK Lys(414) deletion and blocking of lysine-binding sites (LBSs) on Pg kringles with 6-aminohexanoic acid or benzamidine. Forward and reverse rates for both tightening steps were unaffected by 6-aminohexanoic acid, whereas benzamidine released constraints on the first conformational tightening. This indicated that binding of SK Lys(414) to Pg kringle 4 plays a role in recognition of Pg by SK. The substantially lower affinity of the final SK · Pg complex compared with SK · Pm is characterized by a ∼ 25-fold weaker encounter complex and ∼ 40-fold faster off-rates for the second conformational step. The results suggest that effective Pg encounter requires SK Lys(414) engagement and significant non-LBS interactions with the protease domain, whereas Pm binding additionally requires contributions of other lysines. This difference may be responsible for the lower affinity of the SK · Pg complex and the expression of a weaker "pro"-exosite for binding of a second Pg in the substrate mode compared with SK · Pm.

New insights into the role of Plg-RKT in macrophage recruitment

Plasminogen (PLG) is the zymogen of plasmin, the major enzyme that degrades fibrin clots. In addition to its binding and activation on fibrin clots, PLG also specifically interacts with cell surfaces where it is more efficiently activated by PLG activators, compared with the reaction in solution. This results in association of the broad-spectrum proteolytic activity of plasmin with cell surfaces that functions to promote cell migration. Here, we review emerging data establishing a role for PLG, plasminogen receptors and the newly discovered plasminogen receptor, Plg-RKT, in macrophage recruitment in the inflammatory response, and we address mechanisms by which the interplay between PLG and its receptors regulates inflammation.

Bacterial plasminogen receptors utilize host plasminogen system for effective invasion and dissemination

In order for invasive pathogens to migrate beyond the site of infection, host physiological barriers such as the extracellular matrix, the basement membrane, and encapsulating fibrin network must be degraded. To circumvent these impediments, proteolytic enzymes facilitate the dissemination of the microorganism. Recruitment of host proteases to the bacterial surface represents a particularly effective mechanism for enhancing invasiveness. Plasmin is a broad spectrum serine protease that degrades fibrin, extracellular matrices, and connective tissue. A large number of pathogens express plasminogen receptors which immobilize plasmin(ogen) on the bacterial surface. Surface-bound plasminogen is then activated by plasminogen activators to plasmin through limited proteolysis thus triggering the development of a proteolytic surface on the bacteria and eventually assisting the spread of bacteria. The host hemostatic system plays an important role in systemic infection. The interplay between hemostatic processes such as coagulation and fibrinolysis and the inflammatory response constitutes essential components of host defense and bacterial invasion. The goal of this paper is to highlight mechanisms whereby pathogenic bacteria, by engaging surface receptors, utilize and exploit the host plasminogen and fibrinolytic system for the successful dissemination within the host.

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.

Plasminogen substrate recognition by the streptokinase-plasminogen catalytic complex is facilitated by Arg253, Lys256, and Lys257 in the streptokinase beta-domain and kringle 5 of the substrate

Streptokinase (SK) conformationally activates the central zymogen of the fibrinolytic system, plasminogen (Pg). The SK.Pg* catalytic complex binds Pg as a specific substrate and cleaves it into plasmin (Pm), which binds SK to form the SK.Pm complex that propagates Pm generation. Catalytic complex formation is dependent on lysine-binding site (LBS) interactions between a Pg/Pm kringle and the SK COOH-terminal Lys(414). Pg substrate recognition is also LBS-dependent, but the kringle and SK structural element(s) responsible have not been identified. SK mutants lacking Lys(414) with Ala substitutions of charged residues in the SK beta-domain 250-loop were evaluated in kinetic studies that resolved conformational and proteolytic Pg activation. Activation of [Lys]Pg and mini-Pg (containing only kringle 5 of Pg) by SK with Ala substitutions of Arg(253), Lys(256), and Lys(257) showed decreases in the bimolecular rate constant for Pm generation, with nearly total inhibition for the SK Lys(256)/Lys(257) double mutant. Binding of bovine Pg (BPg) to the SK.Pm complex containing fluorescently labeled Pm demonstrated LBS-dependent assembly of a SK.labeled Pm.BPg ternary complex, whereas BPg did not bind to the complex containing the SK Lys(256)/Lys(257) mutant. BPg was activated by SK.Pm with a K(m) indistinguishable from the K(D) for BPg binding to form the ternary complex, whereas the SK Lys(256)/Lys(257) mutant did not support BPg activation. We conclude that SK residues Arg(253), Lys(256), and Lys(257) mediate Pg substrate recognition through kringle 5 of the [Lys]Pg and mini-Pg substrates. A molecular model of the SK.kringle 5 complex identifies the putative interactions involved in LBS-dependent Pg substrate recognition.

Rapid-reaction kinetic characterization of the pathway of streptokinase-plasmin catalytic complex formation

Binding of the fibrinolytic proteinase plasmin (Pm) to streptokinase (SK) in a tight stoichiometric complex transforms Pm into a potent proteolytic activator of plasminogen. SK binding to the catalytic domain of Pm, with a dissociation constant of 12 pm, is assisted by SK Lys(414) binding to a Pm kringle, which accounts for a 11-20-fold affinity decrease when Pm lysine binding sites are blocked by 6-aminohexanoic acid (6-AHA) or benzamidine. The pathway of SK.Pm catalytic complex formation was characterized by stopped-flow kinetics of SK and the Lys(414) deletion mutant (SKDeltaK414) binding to Pm labeled at the active site with 5-fluorescein ([5F]FFR-Pm) and the reverse reactions by competitive displacement of [5F]FFR-Pm with active site-blocked Pm. The rate constants for the biexponential fluorescence quenching caused by SK and SKDeltaK414 binding to [5F]FFR-Pm were saturable as a function of SK concentration, reporting encounter complex affinities of 62-110 nm in the absence of lysine analogs and 4900-6500 and 1430-2200 nm in the presence of 6-AHA and benzamidine, respectively. The encounter complex with SKDeltaK414 was approximately 10-fold weaker in the absence of lysine analogs but indistinguishable from that of native SK in the presence of 6-AHA and benzamidine. The studies delineate for the first time the sequence of molecular events in the formation of the SK.Pm catalytic complex and its regulation by kringle ligands. Analysis of the forward and reverse reactions supports a binding mechanism in which SK Lys(414) binding to a Pm kringle accompanies near-diffusion-limited encounter complex formation followed by two slower, tightening conformational changes.

The conformational switch from the factor X zymogen to protease state mediates exosite expression and prothrombinase assembly

Zymogens of the chymotrypsin-like serine protease family are converted to the protease state following insertion of a newly formed, highly conserved N terminus. This transition is accompanied by active site formation and ordering of several surface loops in the catalytic domain. Here we show that disruption of this transition in factor X through mutagenesis (FXa(I16L) and FXa(V17A)) not only alters active site function, but also significantly impairs Na(+) and factor Va binding. Active site binding was improved in the presence of high NaCl or with saturating amounts of factor Va membranes, suggesting that allosteric linkage exists between these sites. In line with this, irreversible stabilization of FXa(I16L) with Glu-Gly-Arg-chloromethyl ketone fully rescued FVa binding. Furthermore, the K(m) for prothrombin conversion with the factor Xa variants assembled into prothrombinase was unaltered, whereas the k(cat) was modestly reduced (3- to 4-fold). These findings show that intramolecular activation of factor X following the zymogen to protease transition not only drives catalytic site activation but also contributes to the formation of the Na(+) and factor Va binding sites. This structural plasticity of the catalytic domain plays a key role in the regulation of exosite expression and prothrombinase assembly.

Coupling of Conformational and Proteolytic Activation in the Kinetic Mechanism of Plasminogen Activation by Streptokinase

Binding of streptokinase (SK) to plasminogen (Pg) induces conformational activation of the zymogen and initiates its proteolytic conversion to plasmin (Pm). The mechanism of coupling between conformational activation and Pm formation was investigated in kinetic studies. Parabolic time courses of Pg activation by SK monitored by chromogenic substrate hydrolysis had initial rates (v(1)) representing conformational activation and subsequent rates of activity increase (v(2)) corresponding to the rate of Pm generation determined by a specific discontinuous assay. The v(2) dependence on SK concentration for [Lys]Pg showed a maximum rate at a Pg to SK ratio of approximately 2:1, with inhibition at high SK concentrations. [Glu]Pg and [Lys]Pg activation showed similar kinetic behavior but much slower activation of [Glu]Pg, due to an approximately 12-fold lower affinity for SK and an approximately 20-fold lower k(cat)/K(m). Blocking lysine-binding sites on Pg inhibited SK.Pg* cleavage of [Lys]Pg to a rate comparable with that of [Glu]Pg, whereas [Glu]Pg activation was not significantly affected. The results support a kinetic mechanism in which SK activates Pg conformationally by rapid equilibrium formation of the SK.Pg* complex, followed by intermolecular cleavage of Pg to Pm by SK.Pg* and subsequent cleavage of Pg by SK.Pm. A unified model of SK-induced Pg activation suggests that generation of initial Pm by SK.Pg* acts as a self-limiting triggering mechanism to initiate production of one SK equivalent of SK.Pm, which then converts the remaining free Pg to Pm.

Resolution of Conformational Activation in the Kinetic Mechanism of Plasminogen Activation by Streptokinase

Streptokinase (SK) activates plasminogen (Pg) by specific binding and nonproteolytic expression of the Pg catalytic site, initiating Pg proteolysis to form the fibrinolytic proteinase, plasmin (Pm). The SK-induced conformational activation mechanism was investigated in quantitative kinetic and equilibrium binding studies. Progress curves of Pg activation by SK monitored by chromogenic substrate hydrolysis were parabolic, with initial rates (v(1)) that indicated no transient species and subsequent rate increases (v(2)). The v(1) dependence on SK concentration for [Glu]Pg and [Lys]Pg was hyperbolic with dissociation constants corresponding to those determined in fluorescence-based binding studies for the native Pg species, identifying v(1) as rapid SK binding and conformational activation. Comparison of [Glu]Pg and [Lys]Pg activation showed an approximately 12-fold higher affinity of SK for [Lys]Pg that was lysine-binding site dependent and no such dependence for [Glu]Pg. Stopped-flow kinetics of SK binding to fluorescently labeled Pg demonstrated at least two fast steps in the conformational activation pathway. Characterization of the specificity of the conformationally activated SK.[Lys]Pg* complex for tripeptide-p-nitroanilide substrates demonstrated 5-18- and 10-130-fold reduced specificity (k(cat)/K(m)) compared with SK.Pm and Pm, respectively, with differences in K(m) and k(cat) dependent on the P1 residue. The results support a kinetic mechanism in which SK binding and reversible conformational activation occur in a rapid equilibrium, multistep process.

Streptokinase Triggers Conformational Activation of Plasminogen through Specific Interactions of the Amino-terminal Sequence and Stabilizes the Active Zymogen Conformation

Cleavage of Arg(561)-Val(562) in plasminogen (Pg) generates plasmin (Pm) through a classical activation mechanism triggered by an insertion of the new amino terminus into a binding pocket in the Pg catalytic domain. Streptokinase (SK) circumvents this process and activates Pg through a unique nonproteolytic mechanism postulated to be initiated by the intrusion of Ile(1) of SK in place of Val(562). This hypothesis was evaluated in equilibrium binding and kinetic studies of Pg activation with an SK mutant lacking Ile(1) (SK(2--414)). SK(2--414) retained the affinity of native SK for fluorescein-labeled [Lys]Pg and [Lys]Pm but induced no detectable conformational activation of Pg. The activity of SK(2--414) was partially restored by the peptides SK(1--2), SK(1--5), SK(1--10), and SK(1--15), whereas Pg(562--569) peptides were much less effective. Active site-specific fluorescence labeling demonstrated directly that the active catalytic site was formed on the Pg zymogen by the combination of SK(1--10) and SK(2--414), whereas sequence-scrambled SK(1-10) was inactive. The characterization of SK(1--10) containing single Ala substitutions demonstrated the sequence specificity of the interaction. SK(1--10) did not restore activity to the further truncated mutant SK(55-414), which was correlated with the loss of binding affinity of SK(55--414) for labeled [Lys]Pm but not for [Lys]Pg. The studies support a mechanism for conformational activation in which the insertion of Ile(1) of SK into the Pg amino-terminal binding cleft occurs through sequence-specific interactions of the first 10 SK residues. This event and the preferentially higher affinity of SK(2--414) for the activated proteinase domain of Pm are thought to function cooperatively to trigger the conformational change and stabilize the active zymogen conformation.

Streptokinase binds to human plasmin with high affinity, perturbs the plasmin active site, and induces expression of a substrate recognition exosite for plasminogen

Binding of streptokinase (SK) to plasminogen (Pg) conformationally activates the zymogen and converts both Pg and plasmin (Pm) into specific Pg activators. The interaction of SK with Pm and its relationship to the mechanism of Pg activation were evaluated in equilibrium binding studies with active site-labeled fluorescent Pm derivatives and in kinetic studies of SK-induced changes in the catalytic specificity of Pm. SK bound to fluorescein-labeled and native Pm with dissociation constants of 11 +/- 2 pm and 12 +/- 4 pm, which represented a 1,000-10,000-fold higher affinity than determined for Pg. Stoichiometric binding of SK to native Pm was followed by generation of a two-fragment form of SK cleaved at Lys(59) (SK'), which exhibited an indistinguishable affinity for labeled Pm, while a truncated, SK(55-414) species had a 120-360-fold reduced affinity. Binding of SK to native Pm was accompanied by a >50-fold enhancement in specificity for activation of Pg, which was paralleled by a surprising 2.6-10-fold loss of specificity of Pm for 8 of 11 tripeptide-pNA substrates. Further studies with Pm labeled at the active site with 2-anilinonaphthalene-6-sulfonic acid demonstrated directly that binding of SK to Pm resulted in expression of a new substrate binding exosite for Pg on the SK.Pm complex. It is concluded that SK activates Pg in part by preferential binding to the active zymogen conformation. High affinity binding of SK to Pm enhances Pg substrate specificity principally through emergence of a substrate recognition exosite.

Function of the central domain of streptokinase in substrate plasminogen docking and processing revealed by site-directed mutagenesis

The possible role of the central beta-domain (residues 151-287) of streptokinase (SK) was probed by site-specifically altering two charged residues at a time to alanines in a region (residues 230-290) previously identified by Peptide Walking to play a key role in plasminogen (PG) activation. These mutants were then screened for altered ability to activate equimolar "partner" human PG, or altered interaction with substrate PG resulting in an overall compromised capability for substrate PG processing. Of the eight initial alanine-linker mutants of SK, one mutant, viz. SK(KK256.257AA) (SK-D1), showed a roughly 20-fold reduction in PG activator activity in comparison to wild-type SK expressed in Escherichia coli (nSK). Five other mutants were as active as nSK, with two [SK(RE248.249AA) and SK(EK281.282AA), referred to as SK(C) and SK(H), respectively] showing specific activities approximately one-half and two-thirds, respectively, that of nSK. Unlike SK(C) and SK(H), however, SK(D1) showed an extended initial delay in the kinetics of PG activation. These features were drastically accentuated when the charges on the two Lys residues at positions 256 and 257 of nSK were reversed, to obtain SK(KK256.257EE) [SK(D2)]. This mutant showed a PG activator activity approximately 10-fold less than that of SK(D1). Remarkably, inclusion of small amounts of human plasmin (PN) in the PG activation reactions of SK(D2) resulted in a dramatic, PN dose-dependent rejuvenation of its PG activation capability, indicating that it required pre-existing PN to form a functional activator since it could not effect active site exposure in partner PG on its own, a conclusion further confirmed by its inability to show a "burst" of p-nitrophenol release in the presence of equimolar human PG and p-nitrophenyl guanidino benzoate. The steady-state kinetic parameters for HPG activation of its 1:1 complex with human PN revealed that although it could form a highly functional activator once "supplied" with a mature active site, the Km for PG was increased nearly eightfold in comparison to that of nSK-PN. SK mutants carrying simultaneous two- and three-site charge-cluster alterations, viz., SK(RE24249AA:EK281.282AA) [SK(CH)], SK(EK272.273AA;EK281.282AA) [SK(FH)], and SK(RE248.249AA;EK272.273AA:EK281.282AA+ ++) [SK(CFH)], showed additive/synergistic influence of multiple charge-cluster mutations on HPG activation when compared to the respective "single-site" mutants, with the "triple-site" mutant [SK(CFH)] showing absolutely no detectable HPG activation ability. Nevertheless, like the other constructs, the double- and triple-charge cluster mutants retained a native like affinity for complexation with partner PG. Their overall structure also, as judged by far-ultraviolet circular dichroism, was closely similar to that of nSK. These results provide the first experimental evidence for a direct assistance by the SK beta-domain in the docking and processing of substrate PG by the activator complex, a facet not readily evident probably because of the flexibility of this domain in the recent X-ray crystal structure of the SK-plasmin light chain complex.

Engineering of plasmin-resistant forms of streptokinase and their production in Bacillus subtilis: streptokinase with longer functional half-life

The short in vivo half-life of streptokinase limits its efficacy as an efficient blood clot-dissolving agent. During the clot-dissolving process, streptokinase is processed to smaller intermediates by plasmin. Two of the major processing sites are Lys59 and Lys386. We engineered two versions of streptokinase with either one of the lysine residues changed to glutamine and a third version with both mutations. These mutant streptokinase proteins (muteins) were produced by secretion with the protease-deficient Bacillus subtilis WB600 as the host. The purified muteins retained comparable kinetics parameters in plasminogen activation and showed different degrees of resistance to plasmin depending on the nature of the mutation. Muteins with double mutations had half-lives that were extended 21-fold when assayed in a 1:1 molar ratio with plasminogen in vitro and showed better plasminogen activation activity with time in the radial caseinolysis assay. This study indicates that plasmin-mediated processing leads to the inactivation of streptokinase and is not required to convert streptokinase to its active form. Plasmin-resistant forms of streptokinase can be engineered without affecting their activity, and blockage of the N-terminal cleavage site is essential to generate engineered streptokinase with a longer in vitro functional half-life.

Plasminogen activation by streptokinase via a unique mechanism

The mechanism of human plasminogen (HPlg) activation by streptokinase (SK)-type activator was investigated with recombinant truncated SK peptides. An enzyme-substrate intermediate of HPlg.SK. HPlg ternary complex was demonstrated by a sandwich-binding experiment. Formation of the ternary complex was saturable, HPlg-specific, and inhibited by 6-aminocaproic acid. Three interaction sites between SK and HPlg were demonstrated. SK-(220-414) bound to HPlg with two binding sites: one to the micro-HPlg region, the catalytic domain of HPlg, and one to the kringle 1-5 region, with Kd values of 1.50 x 10(-7) and 2.44 x 10(-6) M, respectively. SK-(16-251) bound to a single site on the kringle 1-5 region of HPlg with a Kd of 4.09 x 10(-7) M. SK-(220-414) and SK-(16-251) competed for binding on the same or nearby location on the human kringle 1-5 domain. Combination of SK-(220-414) and SK-(16-251), but not either peptide alone, could effectively activate HPlg. In addition, SK-(16-251) dose-dependently enhanced the activation of HPlg by SK-(16-414), while the HPlg activation by SK-(16-414) was inhibited by SK-(220-414). We conclude that the HPlg that binds to the COOH-terminal domains of SK functions as an enzyme to catalyze the conversion of substrate HPlg that binds to the NH2-terminal domain of SK to human plasmin.

Enzymatic properties of phage-displayed fragments of human plasminogen

Two low-molecular-mass forms of human plasminogen, plasminogen-(543-791)-peptide (micro-plasminogen), comprising the serine protease domain, and plasminogen-(444-791)-peptide (mini-plasminogen), which in addition contains kringle 5, were displayed on filamentous phage by fusion to the N-terminus of the minor coat protein pIII, to levels of 0.5 molecules micro-plasminogen-pIII/phage particle and 0.1 molecules mini-plasminogen-pIII/phage particle. The proenzymes, quantitatively activated by urokinase, showed catalytic efficiencies that were virtually identical to their soluble counterparts, and activity remained associated with the phage as demonstrated by phage ELISA and biopanning with human alpha2-antiplasmin or the inhibitor Phe-Pro-Arg-CH2Cl. Micro-plasminogen-pIII was activated by streptokinase and staphylokinase, two non-enzymatic plasminogen activators, to the same extent as by urokinase. Activated forms of mini-plasminogen-pIII micro-plasminogen-pIII and mini-plasminogen dissolved 125I-labelled fibrin films in a dose-dependent time-dependent manner, with 50% lysis in 20 h requiring 0.52, 3.2 and 0.46 nM active plasmin, respectively. Thus, proenzyme moieties derived from plasminogen can be successfully displayed on phage with maintenance of their enzymatic properties. The micro-plasminogen and mini-plasminogen phage-display systems may be useful to study mechanisms of plasminogen activation.

Activation of Human Plasminogen by Staphylokinase. Direct Evidence That Preformed Plasmin Is Necessary for Activation to Occur

To directly determine whether the mechanism of activation of human plasminogen (HPg) by staphylokinase (Sak) required formation of an active complex of Sak and HPg, recombinant (r) variants of HPg were examined that allowed dissection of the steps involved in this activation. The rate of activation of wild-type (wt) r-HPg by equimolar levels of Sak was enhanced when small amounts of human plasmin (HPm) were included, suggesting that a Sak-HPm complex was a more effective plasminogen activator than a putative Sak-HPg complex. Incubation of equimolar Sak with a cleavage site resistant mutant of HPg (r-[R561 A]HPg) did not result in generation of amidolytic activity of the complex, in contrast to a similar experiment with streptokinase (SK) in place of Sak, where substantial amidolytic activity was generated. This result supplies evidence that an active complex of Sak and HPg does not form, as is the case with SK. Another mutant, r-[D646E]HPg, which, upon activation, would lead to a form of HPm defective in enzymatic activity, is also not converted to its two-chain form by Sak, but is converted to the inactive two-chain form by urokinase, a direct plasminogen activator, and by equimolar complexes of SK or Sak with wtr-HPm. This shows that the active site of HPm is the functional plasminogen activator entity in the Sak-HPm complex. These results show that the mechanism of activation of HPg by Sak proceeds in a distinctly different manner than the similar activation by SK. Although SK does not require the presence of HPm for this activation, a necessary condition for the activation by Sak is formation of a small amount of HPm generated via another activation pathway. These different mechanisms have significant implications in production of the fibrinolytic state by these two indirect bacterial plasminogen activators.

Activation of Human Plasminogen by Staphylokinase. Direct Evidence That Preformed Plasmin Is Necessary for Activation to Occur

To directly determine whether the mechanism of activation of human plasminogen (HPg) by staphylokinase (Sak) required formation of an active complex of Sak and HPg, recombinant (r) variants of HPg were examined that allowed dissection of the steps involved in this activation. The rate of activation of wild-type (wt) r-HPg by equimolar levels of Sak was enhanced when small amounts of human plasmin (HPm) were included, suggesting that a Sak-HPm complex was a more effective plasminogen activator than a putative Sak-HPg complex. Incubation of equimolar Sak with a cleavage site resistant mutant of HPg (r-[R561 A]HPg) did not result in generation of amidolytic activity of the complex, in contrast to a similar experiment with streptokinase (SK) in place of Sak, where substantial amidolytic activity was generated. This result supplies evidence that an active complex of Sak and HPg does not form, as is the case with SK. Another mutant, r-[D646E]HPg, which, upon activation, would lead to a form of HPm defective in enzymatic activity, is also not converted to its two-chain form by Sak, but is converted to the inactive two-chain form by urokinase, a direct plasminogen activator, and by equimolar complexes of SK or Sak with wtr-HPm. This shows that the active site of HPm is the functional plasminogen activator entity in the Sak-HPm complex. These results show that the mechanism of activation of HPg by Sak proceeds in a distinctly different manner than the similar activation by SK. Although SK does not require the presence of HPm for this activation, a necessary condition for the activation by Sak is formation of a small amount of HPm generated via another activation pathway. These different mechanisms have significant implications in production of the fibrinolytic state by these two indirect bacterial plasminogen activators.

Identification of a hydrophobic exosite on tissue type plasminogen activator that modulates specificity for plasminogen

A wide variety of important biological processes, including both the formation and dissolution of blood clots, depend on specific cleavage of individual target proteins by serine proteases. For example, tissue type plasminogen activator (t-PA), a trypsin-like enzyme that catalyzes the rate-limiting step of the endogenous fibrinolytic cascade, has only one known substrate in vivo, a single peptide bond (Arg561-Val562) in the proenzyme plasminogen. We have previously suggested that the specificity of t-PA for plasminogen is mediated in part by direct protein-protein interactions between the protease domain of t-PA and plasminogen that are distinct from those occurring within t-PA's active site. We demonstrate in this study that residues 420-423 of t-PA, which form a fully solvent-exposed, hydrophobic region of a surface loop mapping near one edge of the active site of t-PA, form, or are essential for the integrity of, an important, secondary site of interaction between t-PA and plasminogen that significantly modulates the rate of plasminogen activation in the absence, but not the presence, of fibrin. Identification of this secondary site of interaction between t-PA and plasminogen provides new insight into molecular details of the evolution of stringent substrate specificity by t-PA and suggests a novel strategy to enhance the fibrin dependence of plasminogen activation by t-PA. While the activity of wild type t-PA is stimulated by fibrin by a factor of approximately 650, the activity of two variants characterized in this study, t-PA/R275E,P422G and t-PA/R275E,P422E, is stimulated by a factor of approximately 39,000 or 61,000, respectively. It is therefore possible that, compared with wild type t-PA, the two variants would display enhanced "clot selectivity" in vivo due to reduced activity in the circulation but full activity at a site of fibrin deposition.

Converting tissue type plasminogen activator into a zymogen. Important role of Lys156

In stark contrast to most other members of the chymotrypsin family of serine proteases, tissue type plasminogen activator (t-PA) is not synthesized and secreted as a true zymogen. Instead, single-chain t-PA exhibits very significant catalytic activity. Consequently, the zymogenicity, or ratio of the catalytic efficiencies of the mature, two-chain enzyme and the single-chain precursor, is only 3-9 for t-PA. Both we and others have previously proposed that Lys156 may contribute directly to this exceptional property of t-PA by forming interactions that selectively stabilize the active conformation of the single-chain enzyme. To test this hypothesis we created variants of t-PA in which Lys156 was replaced by a tyrosine residue. As predicted, the K156Y mutation selectively suppressed the activity of the single-chain enzyme and thereby substantially enhanced the enzyme's zymogenicity. In addition, however, this mutation produced a very dramatic increase in the ability of single-chain t-PA to discriminate among distinct fibrin co-factors. Compared with wild type t-PA, one of the variants characterized in this study, t-PA/R15E,K156Y, possessed substantially enhanced response to and selectivity among fibrin co-factors, resistance to inhibition by plasminogen activator inhibitor type 1, and significantly increased zymogenicity. The combination of these properties, and the maintenance of full activity in the presence of fibrin, suggest that the R15E,K156Y mutations may extend the therapeutic range of t-PA.

Molecular Pathogenesis of Type I Congenital Plasminogen Deficiency: Expression of Recombinant Human Mutant Plasminogens in Mammalian Cells

We previously reported the genetic abnormality in a Japanese family with type I congenital plasminogen deficiency caused by a Ser572 to Pro572 mutation. To characterize the molecular pathogenesis of the disease in this family, we expressed recombinant human wild-type and mutant (rS572P) plasminogens in COS-1 cells. Activation-resistant wild-type and mutant plasminogen stable transfectants in CHO-K1 cells also were established. Transient transfection and metabolic labeling experiments followed by immunoprecipitation analysis showed that the mutant plasminogen was secreted from COS-1 cells in reduced amounts, compared with the wild type. Endo H digestion of the wild-type and mutant plasminogen showed no shift in their migrations on sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis, indicating that both contain complex type oligosaccharide structures and could therefore be secreted. Furthermore, the secretion of activation-resistant mutant plasminogen was significantly reduced. Pulse-chase experiments and Northern blot analysis showed that the impaired secretion of the mutant plasminogen was the consequence of the accumulation of the mutant protein inside the cells but not of reduced plasminogen mRNA. Immunocytochemical staining of stable transfectants also revealed that CHO-K1 cells expressing the activation-resistant mutant plasminogen stained mainly in the perinuclear area, suggesting delayed processing of the mutant protein in the intracellular transport pathway. We conclude that the impaired secretion of mutant plasminogen, due to intracellular accumulation, is the molecular pathogenesis of type I congenital plasminogen deficiency caused by a Ser572 to Pro572 mutation.

Molecular Pathogenesis of Type I Congenital Plasminogen Deficiency: Expression of Recombinant Human Mutant Plasminogens in Mammalian Cells

We previously reported the genetic abnormality in a Japanese family with type I congenital plasminogen deficiency caused by a Ser572 to Pro572 mutation. To characterize the molecular pathogenesis of the disease in this family, we expressed recombinant human wild-type and mutant (rS572P) plasminogens in COS-1 cells. Activation-resistant wild-type and mutant plasminogen stable transfectants in CHO-K1 cells also were established. Transient transfection and metabolic labeling experiments followed by immunoprecipitation analysis showed that the mutant plasminogen was secreted from COS-1 cells in reduced amounts, compared with the wild type. Endo H digestion of the wild-type and mutant plasminogen showed no shift in their migrations on sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis, indicating that both contain complex type oligosaccharide structures and could therefore be secreted. Furthermore, the secretion of activation-resistant mutant plasminogen was significantly reduced. Pulse-chase experiments and Northern blot analysis showed that the impaired secretion of the mutant plasminogen was the consequence of the accumulation of the mutant protein inside the cells but not of reduced plasminogen mRNA. Immunocytochemical staining of stable transfectants also revealed that CHO-K1 cells expressing the activation-resistant mutant plasminogen stained mainly in the perinuclear area, suggesting delayed processing of the mutant protein in the intracellular transport pathway. We conclude that the impaired secretion of mutant plasminogen, due to intracellular accumulation, is the molecular pathogenesis of type I congenital plasminogen deficiency caused by a Ser572 to Pro572 mutation.

Converting tissue-type plasminogen activator into a zymogen

In striking contrast to most other members of the chymotrypsin family of serine proteases, tissue-type plasminogen activator (t-PA) is not synthesized and secreted as a true zymogen. The zymogenicity, or ratio of the catalytic efficiencies of the mature, two-chain enzyme and the single-chain precursor, is only 5-10 for t-PA. This exceptional property of t-PA, however, is not shared by urokinase (u-PA), a plasminogen activator that is very closely related to t-PA. The molecular basis of this important functional distinction between these two intimately related serine proteases has not been previously investigated. Based on observation of the recently described structures of the protease domains of two-chain t-PA and u-PA, and molecular modeling of the corresponding single-chain enzymes, we propose that the presence or absence of an acidic residue at position 144 (chymotrypsin numbering system) is the primary determinant of the distinct zymogenicities of the two enzymes. Consistent with this hypothesis, mutation of histidine 144 of t-PA to an acidic residue, as in u-PA, selectively suppressed the activity of single-chain t-PA and thereby significantly enhanced the enzyme's zymogenicity. A variant of t-PA containing an aspartate residue at position 144, for example, exhibited a zymogenicity of 150, compared to a value of 9 for wild type t-PA and 250 for u-PA.

The domain organization of streptokinase: nuclear magnetic resonance, circular dichroism, and functional characterization of proteolytic fragments

Streptococcus equisimilis streptokinase (SK) is a bacterial protein of unknown tertiary structure and domain organization that is used extensively to treat acute myocardial infarction following coronary thrombosis. Six fragments of SK were generated by limited proteolysis with chymotrypsin and purified. NMR and CD experiments have shown that the secondary and tertiary structure present in the native molecule is preserved within all fragments, except the N-terminal fragment SK7. NMR spectra demonstrate the presence in SK of three structurally autonomous domains and a less structured C-terminal "tail." Cleavage within the N-terminal domain generates an N-terminal fragment, SK7, which remains noncovalently associated with the remainder of the molecule; in isolation, SK7 adopts an unfolded conformation. The abilities of these fragments to induce active site formation within human plasminogen upon formation of their heterodimeric complex were assayed. The lowest mass SK fragment exhibiting Plg-dependent activator activity was shown to be SK27 (mass 27,000, residues 147-380), which contains both central and C-terminal domains, although this activity was reduced approximately 6,000-fold relative to that of full-length SK. The activity of a 36,000 mass fragment, SK36 (residues 64-380), which differs from SK27 in possessing a portion of the N-terminal domain, was reduced to 0.1-1.0% of that of SK. Other fragments (masses 7,000, 11,000, 16,000, 17,000, 25,000, and 26,000), representing either single domains or single domains extended by portions of other domains, were inactive. However, SK7 (residues 1-63), at a 100-fold molar excess concentration, greatly potentiated the activities of SK27 and SK36, by up to 50- and > 130-fold, respectively. These findings demonstrate that all of SK's three domains are essential for native-like SK activity. The central and C-terminal domains mediate plasminogen-binding and active site-generating functions, whereas the N-terminal domain mediates an activity-potentiating function.

Analogs of human plasminogen that are labeled with fluorescence probes at the catalytic site of the zymogen. Preparation, characterization, and interaction with streptokinase

Fluorescent analogs of the proteinase zymogen, plasminogen (Pg), which are specifically inactivated and labeled at the catalytic site have been prepared and characterized as probes of the mechanisms of Pg activation. The active site induced non-proteolytically in Pg by streptokinase (SK) was inactivated stoichiometrically with the thioester peptide chloromethyl ketone. N alpha-[(acetylthio)acetyl]-(D-Phe)-Phe-Arg-CH2Cl; the thiol group generated subsequently on the incorporated inhibitor with NH2OH was quantitatively labeled with the fluorescence probe, 2-((4'-iodoacetamido)anilino)naphthalene-6-sulfonic acid; and the labeled Pg was separated from SK. Cleavage of labeled [Glu]Pg1 by urokinase-type plasminogen activator (uPA) was accompanied by a fluorescence enhancement (delta Fmax/Fo) of 2.0, and formation of 1% plasmin (Pm) activity. Comparison of labeled and native [Glu]Pg1 as uPA substrates showed that activation of labeled [Glu]Pg1 generated [Glu]Pm1 as the major product, while native [Glu]Pg1 was activated at a faster rate and produced [Lys]Pm1 because of concurrent proteolysis by plasmin. When a mixture of labeled and native Pg was activated, to include plasmin-feedback reactions, the zymogens were activated at equivalent rates. The lack of potential proteolytic activity of the Pg derivatives allowed their interactions with SK to be studied under equilibrium binding conditions. SK bound to labeled [Glu]Pg1, and [Lys]Pg1 with dissociation constants of 590 +/- 110 and 110 and 11 +/- 7 nM, and fluorescence enhancements of 3.1 +/- 0.1 and 1.6 +/- 0.1, respectively. Characterization of the interaction of SK with native [Glu]Pg1 by the use of labeled [Glu]Pg1 as a probe indicated a approximately 6-fold higher affinity of SK for the native Pg zymogen compared to the labeled Pg analog. Saturating levels of epsilon-aminocaproic acid reduced the affinity of SK for labeled [Glu]Pg1 by approximately 2-fold and lowered the fluorescence enhancement to 1.8 +/- 0.1, whereas the affinity of SK for labeled [Lys]Pg1 was reduced by approximately 98-fold with little effect on the enhancement. These results demonstrate that occupation of lysine binding sites modulates the affinity of SK for Pg and the changes in the environment of the catalytic site associated with SK-induced conformational activation. Together, these studies show that the labeled Pg derivatives behave as analogs of native Pg which report functionally significant changes in the environment of the catalytic site of the zymogen.

ATP-regulated activity of the plasmin-streptokinase complex: a novel mechanism involving phosphorylation of streptokinase

Streptokinase, an extracellular protein produced by Streptococci, is capable of activating the human fibrinolytic zymogen plasminogen. The rate of amidolytic activity of the plasminogen-streptokinase complex is greatly diminished by micromolar concentrations of ATP and heparin oligosaccharides. In addition, the plasminogen activator activity of the plasminogen-streptokinase complex is also inhibited by these effectors. ATP and heparin oligosaccharides show structural similarity, suggesting that the inhibition is caused by binding of these molecules to a common newly formed binding pocket in streptokinase, which appears after interaction with plasminogen. Addition of the bivalent cations Ca2+ and Mg2+ reverses the inhibition caused by ATP and heparin. In the presence of ATP and bivalent cations, the complex between plasminogen and streptokinase develops an autophosphorylating activity whose target is the sequence LTSRPAHG in the 4.5 kDa streptokinase N-terminal peptide, which is an early autolysis peptide. This streptokinase N-terminal peptide, which is essential for streptokinase activating activity, may serve, once phosphorylated, in mechanisms related to the pathogenicity of Streptococci. These studies suggest a critical role for plasminogen in regulating the activity of the streptokinase molecule.

Variants of tissue-type plasminogen activator with substantially enhanced response and selectivity toward fibrin co-factors

Unlike most proteases, tissue-type plasminogen activator (t-PA) is not synthesized as an inactive precursor or zymogen. Instead, the single-chain "proenzyme" form of t-PA possesses very significant catalytic activity. Recent investigations of the molecular basis of the unusually high enzymatic activity of single-chain t-PA have focused attention upon Asp-194, a residue that is invariant among chymotrypsin-like enzymes. The critical role of this residue in securing the active conformation of mature chymotrypsin-like enzymes has been discussed extensively. Subsequent work, however, has indicated that this conserved residue can also form interactions that dramatically influence the catalytic activity of serine protease zymogens. While Asp-194 forms interactions that suppress the activity of the zymogen chymotrypsinogen, it may, by contrast, directly promote the catalytically active conformation of single-chain t-PA. To test the hypothesis that Asp-194 promotes the activity of both single- and two-chain t-PA and therefore plays opposing roles in single-chain t-PA and chymotrypsinogen, and also to examine whether this invariant residue plays an essential role in the stimulation of t-PA by fibrin, we used site-directed mutagenesis to construct the following variants of t-PA: t-PA/D194E, t-PA/D194N, t-PA/R15E,D194E, and t-PA/R15E,D194N. In the absence of fibrin, the activity of enzymes carrying a mutation at position 194 was reduced by factors of 1000-2000 compared to wild type t-PA. Similar reductions of activity were observed for both single- and two-chain variants, suggesting an important role for Asp-194 in both forms of the enzyme. The mutated enzymes, however, displayed a dramatically enhanced response to fibrin monomers. While the activity of wild type t-PA was stimulated by fibrin monomers by a factor of 960, the corresponding stimulation factor for the mutated enzymes varied from 498,000-1,050,000.

Structure and function of microplasminogen: I. Methionine shuffling, chemical proteolysis, and proenzyme activation

We have cloned and expressed microplasminogen (mPlg), consisting of the N-terminal undecapeptide of human glu-Plg spliced to its proenzyme domain. This truncated (approximately 28 kDa) proenzyme retained the distinctive catalytic activities of the larger parent. Replacement of M residues followed by M shuffling permitted subsequent scission by site-directed chemical proteolysis (in CNBr/formic acid) without impairing any of the protein's characteristic properties. Activation of chymotrypsinogen-related zymogens occurs by limited proteolysis; the newly liberated, highly conserved N-terminus (VVGG) forms a salt bridge with an aspartyl residue immediately upstream of the active site serine. The role of both of these elements in mPlg activation was probed using protein engineering and site-directed proteolysis to alter the length and amino acid composition of the N-terminus, and to replace the aspartate. All modifications affected both Km and Kcat. The results identify some structural parameters of the N-terminus required for proenzyme activation.

Variants of tissue-type plasminogen activator which display substantially enhanced stimulation by fibrin

Unlike most proteases, tissue-type plasminogen activator (t-PA) is secreted from cells as an active, single chain "proenzyme" whose catalytic efficiency is comparable with that of the corresponding mature, two-chain enzyme. We have previously suggested that the absence of the "zymogen triad" (Asp194-His40-Ser32; chymotrypsin numbering) contributes to this unusually high enzymatic activity of single chain t-PA. Consistent with this prediction, the single chain form of a variant of t-PA containing the zymogen triad displayed dramatically reduced activity toward synthetic substrates. Activation cleavage of this variant, however, resulted in a mature, two-chain enzyme with full catalytic activity. To further examine the functional significance of the zymogen triad, we used site-specific mutagenesis to construct a variant of t-PA, t-PA/R275E,A292S,F305H, that contained this triad but could not be converted into its two-chain form by plasmin. Characterization of this variant demonstrated that the presence of the zymogen triad specifically suppressed plasminogen activation by single chain t-PA in the absence of fibrin. In addition, these studies indicated that, like wild type t-PA, zymogen activation of this variant could be accomplished by binding to the co-factor fibrin. The combination of full activity in the presence of fibrin and reduced activity in its absence resulted in novel variants of t-PA that displayed dramatically enhanced stimulation by fibrin. While the presence of fibrin increased the catalytic efficiency of t-PA toward plasminogen by a factor of approximately 520, this stimulation factor increased to 130,000 for t-PA/R275E,A292S,F305H. Plasmin-resistant, zymogen-like variants of t-PA, therefore, may represent thrombolytic enzymes with enhanced "clot selectivity."

The activation-resistant conformation of recombinant human plasminogen is stabilized by basic residues in the amino-terminal hinge region

Fully activable recombinant human plasminogen (rPlg) was expressed in mammalian cells employing either recombinant vaccinia virus or stable lines coexpressing alpha 2-plasmin inhibitor. A panel of eight variants of rPlg was constructed, in which progressively up to 6 basic amino acid residues in the hinge region of rPlg between the NH2-terminal acidic domain ("proactivation peptide") and kringle 1 were substituted by neutral residues. Analysis of the cleavage rates of these variants by plasmin revealed that the peptide bond at Arg68 is most susceptible, followed by Lys62 and Lys77. A variant with all 6 basic residues substituted was cleaved at Lys20. Three of these variants, PlgB (R68A, R70A), PlgF (R68A, R70A, K77H, K78H), and PlgG (R61A, K62A, R68A, R70A, K77H, K78H), as well as rPlg, were analyzed in more detail. The conformation of these plasminogens was analyzed by monitoring the change in intrinsic fluorescence upon binding of lysine analogs. This revealed that rPlg exhibits the native tight Glu1-plasminogen conformation, whereas PlgB, PlgF, and Plg G display an open conformation similar to Lys78-plasminogen, leading to an increased affinity for lysine analogs. This allowed a direct study of the impact of the activation-resistant conformation on the properties of Glu1-plasminogen. The open conformation of rPlg variants leads to an increased rate of activation by urokinase-type plasminogen activator and streptokinase and increased binding to a fibrin clot. Fibrin clot lysis mediated by tissue-type plasminogen activator was accelerated for the variants as a result of a lower Km for tissue-type plasminogen activator-mediated plasminogen activation, resulting from the increased affinity of rPlg (variants) for intact fibrin. We conclude that the basic residues in the extremely plasmin susceptible hinge region of plasminogen are directly involved in maintaining the activation resistant Glu1-plasminogen conformation.

Substitution of arginine 719 for glutamic acid in human plasminogen substantially reduces its affinity for streptokinase

In isolation human plasminogen possesses no enzymatic activity, yet upon formation of an equimolar complex with the bacterial protein streptokinase, it acquires a plasminogen activator function. The region(s) of plasminogen and of streptokinase which mediate complex formation has (have) not been previously published. Here it is reported that a single-residue substitution (Arg719-->Glu) in the serine protease domain of full-length Glu-plasminogen substantially reduces its affinity for streptokinase. The plasminogen variant displays no other significant differences from the wild-type molecule with respect to activation by two-chain urokinase-type plasminogen activator, recognition by monoclonal antibodies, or ability to undergo conformational change. It is concluded that Arg719 in human plasminogen is an important determinant of the streptokinase binding site, although further sites are likely to contribute both to the affinity of plasminogen for streptokinase and to mechanisms by which the active site is formed within the complex.

Breaching the conformational integrity of the catalytic triad of the serine protease plasmin: localized disruption of a side chain of His-603 strongly inhibits the amidolytic activity of human plasmin

Site-directed mutagenesis has been used to construct a cDNA that encodes a recombinant variant human plasminogen (hPg) containing a Pro-611-->Ile mutation (MrhPg). The mutein was expressed in recombinant baculovirus-infected Spodoptera frugiperda cells (IPLB-SF-21AE), and purified. After activation of this zymogen to its corresponding form of the serine protease plasmin (MrhPm), this latter enzyme was essentially inactive toward an amide plasmin substrate, most likely from alteration of the spatial relationships of the active-site His-603 to its partners of the catalytic triad, Asp-646 and Ser-741. Partial amidolytic activity of MrhPm was restored as a consequence of imidazole addition to the assay medium, due to an increase in the catalytic constant kcat of the enzyme. The serine protease inhibitor, diisopropylphosphofluoridate, when preincubated with MrhPm, did not inhibit restoration of its amidolytic activity with imidazole, whereas diisopropylphosphofluoridate did inhibit the amidolytic activity of MrhPm in the presence of imidazole. This result implies that His-603 directly influences the nucleophilic character of Ser-741. When imidazole as pretreated with alpha-N-tosyl-L-lysine chloromethyl ketone, the ability of this imidazole solution to restore amidolytic activity to MrhPm was eliminated, suggesting that N alpha-(p-tosyl)lysine chloromethyl ketone directs into the binding pocket a derivatized form of imidazole, which is ineffective as an His-603 substitute. These results indicate that the conformational reorientation of His-603 results in a malfunctional catalytic triad in the serine protease MrhPm, thus leading to an inactive enzyme despite the presence of all three essential amino acids of the catalytic triad. Addition of extramolecular imidazole restores a portion of the amidolytic activity of this mutant enzyme. These data also argue for an enzyme mechanism in which the active-center His-603 residue directly influences the nucleophilicity of the active-site Ser 741 residue.

Comparative recombinant protein production of eight insect cell lines

A recombinant Autographa californica baculovirus expressing secreted alkaline phosphatase (SEAP) gene was used to evaluate the expression of a secreted glycoprotein in eight insect cell lines derived from Spodoptera frugiperda, Trichoplusia ni, Mamestra brassicae and Estigmene acrea. Because cell density was found to influence protein production, SEAP production was evaluated at optimal cell densities for each cell line on both a per cell and per milliliter basis. On a per cell basis, the T. ni-derived BTI-TN-5B1-4 cells produced a minimum of 20-fold more SEAP than the S. frugiperda-derived Sf9 or Sf2l cell lines and a minimum of 9-fold more than any of the other cell lines growing in serum-containing medium. On a per milliliter basis, BTI-TN-5B1-4 cells produced a minimum of fivefold more SEAP than any of the other cell lines tested. Using cell lines that were adapted to serum-free medium, SEAP yields were the same or better than their counterparts in serum-containing medium. At 3 days postinoculation, extracellular SEAP activity ranged from 59 to 85% of total SEAP activity with cell lines grown in serum-free and serum-containing media.

The use of bovine fibrin-streptokinase films for the determination of recombinant human plasminogen

Plasminogen is a key component of the haemostatic system in man and the plasma-derived protein molecule has been actively investigated. Within the last few years cDNA and the gene encoding plasminogen have been cloned and the protein has been expressed in a number of eukaryotic or prokaryotic systems. Yields of expressed plasminogen are frequently low. Currently available assays for plasminogen generally rely on the determination of antigen or utilize tripeptide substrates for measuring functional activity, and they have certain limitations. Assays employing relevant protein substrates offer an alternative way to measure function and overcome the drawbacks associated with the other tests. The use of fibrin films for the assay of low levels of recombinant plasminogen has not been described fully before. The two fibrin film-based assays described in this paper are significant additions to the array of assays available for plasminogen molecules.