Peptide-mediated inactivation of recombinant and platelet plasminogen activator inhibitor-1 in vitro.

Plasminogen activator inhibitor-1 (PAI-1), the primary inhibitor of tissue-type plasminogen activator (t-PA) and urokinase plasminogen activator, is an important regulator of the blood fibrinolytic system. Elevated plasma levels of PAI-1 are associated with thrombosis, and high levels of PAI-1 within platelet-rich clots contribute to their resistance to lysis by t-PA. Consequently, strategies aimed at inhibition of PAI-1 may prove clinically useful. This study was designed to test the hypothesis that a 14-amino acid peptide, corresponding to the PAI-1 reactive center loop (residues 333-346), can rapidly inhibit PAI-1 function. PAI-1 (0.7 microM) was incubated with peptide (55 microM) at 37 degrees C. At timed intervals, residual PAI-1 activity was determined by addition of reaction mixture samples to t-PA and chromogenic substrate. The T1/2 of PAI-1 activity in the presence of peptide was 4 +/- 3 min compared to a control T1/2 of 98 +/- 18 min. The peptide also inhibited complex formation between PAI-1 and t-PA as demonstrated by SDS-PAGE analysis. However, the capacity of the peptide to inhibit PAI-1 bound to vitronectin, a plasma protein that stabilizes PAI-1 activity, was markedly attenuated. Finally, the peptide significantly enhanced in vitro lysis of platelet-rich clots and platelet-poor clots containing recombinant PAI-1. These results indicate that a 14-amino acid peptide can rapidly inactivate PAI-1 and accelerate fibrinolysis in vitro. These studies also demonstrate that PAI-1 function can be directly attenuated in a physiologic setting and suggest a novel approach for augmenting fibrinolysis in vivo.

Transient kinetics of heparin-catalyzed protease inactivation by antithrombin III. The reaction step limiting heparin turnover in thrombin neutralization.

The intrinsic protein fluorescence quenching which accompanies the heparin-accelerated inhibition of thrombin (T) by antithrombin III (AT) was resolved into a heparin-independent component associated with formation of the product T-AT complex and a component associated with an AT conformational change linked to heparin dissociation. To determine whether dissociation of heparin from the product T-AT complex limits the rate at which heparin can turn over catalytically, the kinetics of protein fluorescence quenching during the reaction of thrombin with AT X heparin complex (AT X H) were investigated by stopped-flow fluorimetry under pseudo-first order conditions ([AT X H]o much greater than [T]o). Both fluorescence components were quenched in a single exponential reaction with a hyperbolic dependence of the first order rate constant (kobs) on [AT X H]o. An indistinguishable hyperbolic dependence of kobs on [AT X H]o was measured by displacement of p-aminobenzamidine from the T active site, with both signals extrapolating to a limiting rate constant of 5 s-1. These results indicate that heparin dissociation occurs concomitant with T-AT complex formation at the limiting 5 s-1 rate constant. In reasonable agreement with this value, a kcat of 2.3 s-1 was determined for heparin turnover at catalytic concentrations. We conclude that formation of the T-AT complex is the primary rate-limiting step in heparin catalytic turnover and that this reaction is accompanied by a change in conformation of the AT component resulting in facile heparin dissociation.

Transient kinetic studies of substrate inhibition in the horse liver alcohol dehydrogenase reaction.

The rate-limiting step of ethanol oxidation by alcohol dehydrogenase (E) at substrate inhibitory conditions (greater than 500 mM ethanol) is shown to be the dissociation rate of NADH from the abortive E-ethanol-NADH complex. The dissociation rate constant of NADH decreased hyperbolically from 5.2 to 1.4 s-1 in the presence of ethanol causing a decrease in the Kd of NADH binding from 0.3 microM for the binary complex to 0.1 microM for the abortive complex. Correspondingly, ethanol binding to E-NADH (Kd = 37 mM) was tighter than to enzyme (Kd = 109 mM). The binding rate of NAD+ (7 X 10(5) M-1s-1) to enzyme was not affected by the presence of ethanol, further substantiating that substrate inhibition is totally due to a decrease in the dissociation rate constant of NADH from the abortive complex. Substrate inhibition was also observed with the coenzyme analog, APAD+, but a single transient was not found to be rate limiting. Nevertheless, the presence of substrate inhibition with APAD+ is ascribed to a decrease in the dissociation rate of APADH from 120 to 22 s-1 for the abortive complex. Studies to discern the additional limiting transient(s) in turnover with APAD+ and NAD+ were unsuccessful but showed that any isomerization of the enzyme-reduced coenzyme-aldehyde complex is not rate limiting. Chloride increases the rate of ethanol oxidation by hyperbolically increasing the dissociation rate constant of NADH from enzyme and the abortive complex to 12 and 2.8 s-1, respectively. The chloride effect is attributed to the binding of chloride to these complexes, destabilizing the binding of NADH while not affecting the binding of ethanol.

Protein-protein interactions in contact activation of blood coagulation. Characterization of fluorescein-labeled human high molecular weight kininogen-light chain as a probe.

Limited proteolysis of high molecular weight kininogen by kallikrein resulted in the generation of an inactive heavy chain of Mr = 64,000 and active light chains of Mr = 64,000 and 51,000 when analyzed by sodium dodecyl sulfate (SDS)-gel electrophoresis under reducing conditions. Starting with kininogen from outdated plasma, a light chain with an apparent molecular weight of 51,000 on 7.5% SDS gels was purified and characterized. Molecular weights of 28,900 +/- 1,100 and 30,500 +/- 1,600 were obtained by gel filtration of the reduced and alkylated protein in 6 M guanidine HCl and equilibrium sedimentation under nondenaturing conditions in the air-driven ultracentrifuge, respectively. The light chain stained positively with periodic acid-Schiff reagent on SDS gels indicating that covalently attached carbohydrate may be responsible for the anomalously high molecular weight estimated by SDS-gel electrophoresis. A single light chain thiol group reacted with 5,5'-dithiobis-(2-nitrobenzoic acid) (DTNB) in the presence and absence of 6 M guanidine HCl. Specific fluorescent labeling of the thiol group with 5-(iodoacetamido)fluorescein (IAF) occurred without loss of clotting activity. Addition of purified human plasma prekallikrein to the IAF-light chain resulted in a maximum increase in fluorescence anisotropy of 0.041 +/- 0.001 and no change in the fluorescence intensity. Fluorescence anisotropy measurements of the equilibrium binding of prekallikrein to the IAF-light chain yielded an average Kd of 17.3 +/- 2.5 nM and stoichiometry of 1.07 +/- 0.07 mol of prekallikrein/mol of IAF-light chain. Measurements of the interaction of prekallikrein with iodoacetamide-alkylated light chain using the IAF-light chain as a probe gave an average Kd of 16 +/- 4 nM and stoichiometry of 1.0 +/- 0.2 indicating indistinguishable affinities for prekallikrein.

Spectral evidence for tyrosine ionization linked to a conformational change in liver alcohol dehydrogenase ternary complexes.

Resonance energy transfer from Trp-314 to ionized Tyr-286 was proposed (Laws, W. R., and Shore, J. D. (1978) J. Biol. Chem. 253, 8593-8597) as the mechanism for the observed decrease in protein fluorescence of liver alcohol dehydrogenase seen with alkaline pH, or with the formation of a ternary complex with NAD+ and trifluoroethanol. In the present study, ultraviolet difference spectra confirm the presence of ionized tyrosine not only in these two cases but also in the ternary complex with NADH and isobutyramide. Our results indicate that ternary complex formation, with either oxidized or reduced coenzyme, causes a conformational change leading to partial ionization of tyrosine residues in regions of the enzyme far from the active site.

The mechanism of quenching of liver alcohol dehydrogenase fluorescence due to ternary complex formation.

Difference fluorescence emission spectra, reciprocal Stern-Volmer plots, and variable excitation wave-lengths have been used to evaluate the selective quenching of the two tryptophan residues/subunit of liver alcohol dehydrogenase. Trp-15, at the surface of the enzyme, is quenched by KI consistent with a collisional mechanism, and has a blue-shifted excitation and red-shifted emission spectrum when compared with the spectral properties of TRP-314, which is in a hydrophobic milieu at the subunit interface of the dimeric enzyme. With excitation at 295 nm, Trp-314 is 80% quenched by formation of a ternary enzyme.NAD+.trifluoroethanol complex, and the quenching is essentially additive to that caused by KI. Alkaline pH also results in selective quenching of Trp-314. These results, and considerations of the three-dimensional structure of the enzyme, indicate that the quenching of protein fluorescence of liver alcohol dehydrogenase by either ternary complex formation or alkaline pH is due to resonance energy transfer to tyrosinate. Likely candidates as energy acceptors are the Tyr-286 residues are within transfer distance for each Trp-314 residue, as well as being at the surface of the enzyme and 30 A from the active center zinc atom. Alkaline pH directly ionizes this tyrosine residue, while ternary complex formation causes a conformational change resulting in its ionization.

The role of zinc-bound water in liver alcohol dehydrogenase catalysis.

To elucidate the role of zinc-bound water in liver alcohol dehydrogenase catalysis, chelation by 1,10-phenanthroline and 2,2-bipyridine was studied. The rate constants for association of both chelating agents to the active center zinc were pH-dependent with a pKa of 9.2 and preferential binding to a protonated form. The binary complex dissociation rate constants were pH-independent for both chelating agents. In the presence of saturating NAD+, the pKa for the equilibrium binding of 2,2-bipyridine was perturbed to 7.6, similar to the functional group previously shown to be involved in NAD+ binding. The presence of saturating imidazole resulted in pH-independent 2,2-bipyridine binding. These studies provide compelling evidence that the ionizing enzyme functional group involved in coenzyme binding, proton liberation, and conformational states is zinc-bound water. The limiting rate of chelation by 2,2-bipyridine was pH-independent, and no limiting rate was observed in the presence of saturating imidazole. These results indicate that the limiting rate of chelation is due to the rate of dissociation of zinc-bound water. The implications of this regarding the role of zinc in catalytic turnover of liver alcohol dehydrogenase are discussed.

The role of metal in liver alcohol dehydrogenase catalysis. Spectral and kinetic studies with cobalt-substituted enzyme.

The kinetic and spectral properties of native and totally cobalt-substituted liver alcohol dehydrogenase have been compared. Based on titrimetric determinations of enzyme active site concentration, the turnover number at pH 7.0 for cobalt enzyme was the same as for native enzyme. At pH 10, however, the turnover number was slower for cobalt-substituted enzyme, 3.14 s-1 as compared with 4.05 s-1 for native enzyme. A comparison between native and totally cobalt-substituted enzyme showed a blue-shifted enzyme-NADH double difference spectrum and a splitting and red-shifted enzyme-NAD+-pyrazole double difference spectrum in the near-ultraviolet. The 655-nm peak of the cobalt-substituted enzyme was perturbed by the formation of enzyme-NADH binary complex, enzyme-NAD+-trifloroethanol ternary complex, and enzyme-NAD+ binary complex formation. At pH 7.0, the only observable step in the reaction sequence with a significantly different rate constant for cobalt enzyme was the catalytic hydrogen-transferring step. The rate constant for this step is 92 s-1 for totally cobalt-substituted enzyme as compared with 138 s-1 for native liver alcohol dehydrogenase. The results of this study indicate that zinc is involved in catalysis alcohol and NADH.

Subunit dissociation of mitochondrial malate dehydrogenase.

Fluorescence polarization studies of porcine mitochondrial malate dehydrogenase labeled with fluorescein isothiocyanate or fluorescamine indicated a concentration-dependent dissociation of the dimeric molecule with a KD OF 2 X 10(7) N at pH 8.0. These results were confirmed by the concentration dependence of the stability of the enzyme at elevated temperatures and the creation of hybrid molecules with fluorescein and Rhodamine B labeled subunits, in which energy transfer was observed. The binding of NADH resulted in a small shift of the subunit dissociation curve toward monomer, demonstrating that monomer has twice the affinity for reduced coenzyme. NAD+ binding prevented dissociation of the dimer, even at concentrations below 10(-8) N. These results indicate that binding of reduced or oxidized coenzymes results in different conformation changes, which are transferred to the subunit interface.

Parallel procoagulant and anticoagulant pathways for high molecular weight kininogen coagulant function.

High molecular weight kininogen (HK) or its procoagulant light-chain but not the heavy chain potentiated the heparin enhancement of antithrombin III inactivation of plasma kallikrein and factor XIa from 10-50-fold to approximately 1000-fold at I 0.15, pH 7.4, 25 degrees C. This potentiation resulted in antithrombin becoming a predominant inhibitor of kallikrein and factor XIa in heparinized normal but not HK-deficient plasmas. The heparin chain-length and salt dependence of this potentiation suggested an anticoagulant action of HK analogous to its procoagulant action.

The acid stabilization of plasminogen activator inhibitor-1 depends on protonation of a single group that affects loop insertion into beta-sheet A.

The serpin plasminogen activator inhibitor-1 (PAI-1) spontaneously adopts an inactive or latent conformation by inserting the N-terminal part of the reactive center loop as strand 4 into the major beta-sheet (sheet A). To examine factors that may regulate reactive loop insertion in PAI-1, we determined the inactivation rate of the inhibitor in the pH range 4.5-13. Below pH 9, inactivation led primarily to latent PAI-1, and one predominant effect of pH on the corresponding rate constant could be observed. Protonation of a group exhibiting a pKa of 7.6 (25 degrees C, ionic strength = 0.15 M) reduced the rate of formation of latent PAI-1 by a factor of 35, from 0.17 h-1 at pH 9 to about 0.005 h-1 below pH 6. The ionization with a pKa 7.6 was found to have no effect on the rate by which PAI-1 inhibits trypsin and is therefore unlikely to change the flexibility of the loop or the orientation of the reactive center. The peptides Ac-TEASSSTA and Ac-TVASSSTA (cf. P14-P7 in the reactive loop of PAI-1) formed stable complexes with PAI-1 and converted the inhibitor to a substrate for tissue type plasminogen activator. We found that peptide binding and formation of latent PAI-1 are mutually exclusive events, similarly affected by the pKa 7.6 ionization. This is direct evidence that external peptides can substitute for strand 4 in beta-sheet A of PAI-1 and that the pKa 7.6 ionization regulates insertion of complementary, internal or external, strands into this position. A model that accounts for the observed pH effects is presented, and the identity of the ionizing group is discussed based on the structure of latent PAI-1. The group is tentatively identified as His-143 in helix F, located on top of sheet A.

Quenching of protein fluorescence by transient intermediates in the liver alcohol dehydrogenase reaction.

The addition of saturating concentrations of NAD-+ and alcohol to liver alcohol dehydrogenase in a stopped flow fluorimeter results in a triphasic quenching of enzyme fluorescence. A rapid quenching occurs with a rate constant of 300 to 500 s-minus 1, followed by a slower reaction at 50 to 100 s-minus 1, and ultimately followed by a very slow reaction. The addition of NAD-+ to enzyme in the absence of substrate causes a rapid quenching of enzyme fluorescence at 300 to 500 s-minus 1, with the same amplitude as the rapid phase in the presence of substrate. These studies demonstrate that NAD-+ binding to liver alcohol dehydrogenase causes a conformational change at a rate compatible with the previously reported rate constant for proton release, indicating that proton release is probably coupled to the conformational change.

p-Aminobenzamidine as a fluorescent probe for the active site of serine proteases.

p-Aminobenzamidine is weakly fluorescent in neutral aqueous buffer, with excitation and emission maxima at 293 and 376 nm, respectively. Binding to trypsin results in a blue shift of the emission peak to 362 nm, and 50-fold fluorescence enhancement, while binding to thrombin causes a shift to 368 nm and a 230-fold fluorescence enhancement. This phenomenon is due to hydrophobic interactions, as evidenced by the similar properties observed when p-aminobenzamidine is dissolved in solvents of decreasing polarity. The absorbance spectrum of p-aminobenzamidine is red-shifted by formation of a complex with proteases, with the major difference peak appearing at 317 nm and 323 nm for trypsin and thrombin, respectively. The difference extinction coefficients were 6000 M-1 cm-1 for trypsin complex and 13,300 M-1 cm-1 for thrombin complex at the peak wavelengths. Titration of trypsin and thrombin with the probe indicated one binding site per molecule, with dissociation constants equal to the kinetically determined inhibition constants. The KD values for trypsin and thrombin were 6.1 and 65 microM, respectively. An important potential use of this probe is in studies of inhibitor and substrate binding by rapid reaction kinetic techniques. Using this probe to study the interaction of thrombin with antithrombin III yielded a bimolecular rate constant of 8.0 x 10(3) M-1 s-1, which compares favorably with the value of 8.7 x 10(3) M-1 s-1 obtained from discontinuous assays of the rate of thrombin neutralization.

Inhibitory mechanism of serpins: loop insertion forces acylation of plasminogen activator by plasminogen activator inhibitor-1.

Serpin inhibitors are believed to form an acyl enzyme intermediate with their target proteinases which is stabilized through insertion of the enzyme-linked part of the reactive center loop (RCL) as strand 4 in beta-sheet A of the inhibitor. To test critically the role and timing of these steps in the reaction of the plasminogen activator inhibitor PAI-1, we blocked the vacant position 4 in beta-sheet A of this serpin with an octapeptide. The peptide-blocked PAI-1 was a substrate for both tissue-type plasminogen activator (tPA) and trypsin and was hydrolyzed at the scissile bond. The reactivity of the peptide-blocked substrate PAI-1 was compared to that of the unmodified inhibitor by rapid acid quenching as well as photometric techniques. With trypsin as target, the limiting rate constants for enzyme acylation were essentially the same with inhibitor and substrate PAI-1 (21-23 s-1), as were also the associated apparent second-order rate constants (2.8-2.9 microM-1 s-1). With tPA, inhibitor and substrate PAI-1 reacted identically to form a tightly bound Michaelis complex (Kd approximately Km approximately 20 nM). The limiting rate constant for acylation of tPA, however, was 57 times faster with inhibitor PAI-1 (3.3 s-1) than with the substrate form (0.059 s-1), resulting in a 5-fold difference in the corresponding second-order rate constants (13 vs 2.5 microM-1 s-1). We attribute the ability of tPA to discriminate between the two PAI-1 forms to exosite bonds that cannot occur with trypsin. The exosite bonds retain specifically the distal part of the PAI-1 RCL in the substrate pocket, which favors a reversal of the acylation step. Acylation of tPA becomes effective only by separating the products of the acylation step. With substrate PAI-1, this depends on passive displacement of bonds, whereas with inhibitor PAI-1, separation is accomplished by loop insertion that pulls tPA from its docking site on PAI-1, resulting in faster acylation than with substrate PAI-1.

Transient kinetics of heparin-catalyzed protease inactivation by antithrombin III. Characterization of assembly, product formation, and heparin dissociation steps in the factor Xa reaction.

The kinetics of alpha-factor Xa inhibition by antithrombin III (AT) were studied in the absence and presence of heparin (H) with high affinity for antithrombin by stopped-flow fluorometry at I 0.3, pH 7.4 and 25 degrees C, using the fluorescence probe p-aminobenzamidine (P) and intrinsic protein fluorescence to monitor the reactions. Active site binding of p-aminobenzamidine to factor Xa was characterized by a 200-fold enhancement and 4-nm blue shift of the probe fluorescence emission spectrum (lambda max 372 nm), 29-nm red shift of the excitation spectrum (lambda max 322 nm), and dissociation constant (KD) of about 80 microM. Under pseudo-first order conditions [( AT]0, [H]0, [P]0 much greater than [Xa]0), the observed factor Xa inactivation rate constant (kobs) measured by p-aminobenzamidine displacement or residual enzymatic activity increased linearly with the "effective" antithrombin concentration (i.e. corrected for probe competition) up to 300 microM in the absence of heparin, indicating a simple bimolecular process with a rate constant of 2.1 x 10(3) M-1 s-1. In the presence of heparin, a similar linear dependence of kobs on effective AT.H complex concentration was found up to 25 microM whether the reaction was followed by probe displacement or the quenching of AT.H complex protein fluorescence due to heparin dissociation, consistent with a bimolecular reaction between AT.H complex and free factor Xa with a 300-fold enhanced rate constant of 7 x 10(5) M-1 s-1. Above 25 microM AT.H complex, an increasing dead time displacement of p-aminobenzamidine and a downward deviation of kobs from the initial linear dependence on AT.H complex concentration were found, reflecting the saturation of an intermediate Xa.AT.H complex with a KD of 200 microM and a limiting rate of Xa-AT product complex formation of 140 s-1. Kinetic studies at catalytic heparin concentrations yielded a kcat/Km for factor Xa at saturating antithrombin of 7 x 10(5) M-1 s-1 in agreement with the bimolecular rate constant obtained in single heparin turnover experiments. These results demonstrate that 1) the accelerating effect of heparin on the AT/Xa reaction is at least partly due to heparin promoting the ordered assembly of antithrombin and factor Xa in an intermediate ternary complex and that 2) heparin catalytic turnover is limited by the rate of conversion of the ternary complex intermediate to the product Xa-AT complex with heparin dissociation occurring either concomitant with this step or in a subsequent faster step.

Surface-induced alterations in the kinetic pathway for cleavage of human high molecular weight kininogen by plasma kallikrein.

We have studied the cleavage of human high molecular weight kininogen (HK) by plasma kallikrein in the absence and presence of the surfaces, dextran sulfate (DxSO4) and sulfatides. Using a combination of SDS-polyacrylamide gel electrophoresis, Western blotting with polyclonal antibodies that specifically recognize the COOH terminus of the bradykinin moiety, and high pressure liquid chromatography analyses of the cleavage reaction, we have identified two intermediates in the formation of bradykinin from intact kininogen and demonstrated that alternative cleavage pathways are followed in the absence and presence of surfaces. The COOH-terminal bradykinin cleavage occurred first both in the absence and presence of DxSO4, producing a 103-kDa HK intermediate consisting of disulfide-linked heavy and light chains that retained the kinin moiety. In the presence of DxSO4, this was followed exclusively by the NH2-terminal bradykinin cleavage and release of kinin with no apparent change in molecular mass. Subsequently, a slower cleavage of an 8-kDa peptide from the amino terminus of the HK light chain occurred to form a 95-kDa end product. In contrast to this sequential cleavage pattern, NH2-terminal bradykinin and light chain cleavages occurred randomly in the absence of DxSO4, resulting in the production of an additional 95-kDa intermediate that retained bradykinin but had lost the 8-kDa peptide from the HK light chain. Comparison of the relative rates of the three kallikrein cleavages in the absence and presence of DxSO4 indicated that the surface enhanced the rates of both bradykinin cleavages 2-4-fold, but inhibited the light chain cleavage rate approximately 10-fold, thereby accounting for the change from a partially random to a sequential cleavage pattern in the presence of the surface. Steady-state kinetic analysis revealed that DxSO4 enhanced the kcat/KM for bradykinin release by the rate-limiting NH2-terminal bradykinin cleavage by approximately 2-fold due exclusively to an increase in kcat. Sulfatides appeared to produce the same effects on the pattern of HK cleavages as DxSO4. Blocking of the nonactive site, i.e. exosite, interaction between kallikrein and HK with excess prekallikrein or a synthetic peptide containing the region of HK that interacts with the kallikrein exosite significantly reduced the rate of bradykinin release as well as HK cleavages detected by SDS-polyacrylamide gel electrophoresis either in the absence or presence of DxSO4, indicating that the exosite interaction facilitates bradykinin cleavage.