Inhibition of trypsin-like serine proteinases by tripeptide arginyl and lysyl chloromethylketones

Monitoring of plasmin and plasminogen activator activity in blood of patients under fibrinolytic treatment by reteplase

There are no reliable data on plasmin or plasminogen activator (PA) activities in blood of patients receiving fibrinolytic treatment. This is due to continuing in vitro action of PA after blood withdrawal. These artefactual changes of PA or plasmin activities have been prevented by arginine stabilization of blood samples of myocardial infarction patients treated with plasminogen activators. Twelve patients with myocardial infarction were treated with reteplase 2 x 10,000,000 units in bolus application; one patient was treated with 100 mg t-PA in continuous infusion. Blood was immediately stabilized with EDTA and arginine. The plasma was analyzed with newly developed assays for plasmin and PA. Maximal plasmin activities in blood were obtained at 40 to 60 minutes reteplase treatment time (0.1-0.6 U/mL = approximately 0.05-0.3 micromol/L plasmin). The 50% clearance rate for plasmatic Pli was greater than 30 minutes. The plasmatic reteplase concentration peaked at approximately 2,000 U/mL after the first bolus infusion and at approximately 1,500-3,500 U/mL after the second bolus infusion. Reteplase was cleared to 50% within less than 30 minutes, also with great inter-individual variation. Arginine stabilization of blood allows reliable determinations of activities of plasmin and PA in blood of patients under fibrinolytic treatment: substantial plasmin activities occur in patients treated by reteplase. Therapeutic thrombolysis might be improved, imitating the physiologic cellular thrombolysis; i.e., polymorphonuclear phagocytes (PMN) that can be activated by singlet oxygen ((1)O(2)). PMN might be superior to PA in selective lysis of pathologic thrombi.

Tissue plasminogen activator (tPA) inhibits human neutrophil superoxide anion production in vitro

Because neutrophils contribute to reperfusion injury associated with acute myocardial infarction (MI), and because tissue plasminogen activator (tPA) is often used in the management of MI, we evaluated the effect of tPA on superoxide (O2.-) production by human neutrophils in vitro. We found that adding increasing amounts of tPA significantly (r = 0.89, P < 0.025) and progressively reduced O2.- generation by neutrophils treated with phorbol myristate acetate (PMA) in vitro. Furthermore, adding tPA that had been previously treated with the protease inhibitor, D-Phe-Pro-Arg-chloromethyl ketone HCl (PPACK), also decreased neutrophil O2.- generation in vitro (P < 0.05). In contrast, adding L-arginine, a component of the tPA preparation and a precursor of nitric oxide (NO), did not inhibit PMA-induced neutrophil O2.- production. Also, adding increasing concentrations of tPA did not reduce (P > 0.05) the concentrations of O2.- produced by xanthine oxidase (XO) in vitro. Our findings suggest that tPA reduces neutrophil O2.- generation by a mechanism that is not related to L-arginine, is not dependent on tPA proteolytic activity, and is not a function of direct scavenging. This property may account for some of the effectiveness of tPA in the treatment of MI and/or make tPA valuable for treating acute respiratory distress syndrome (ARDS) or other inflammatory disorders involving neutrophil O2.- production.

In vitro studies on a novel micelle-forming peptide with anticoagulant activity

A novel peptide, tert-butyloxycarbonyl-L-arginine-L-proline lauryl ester laurate, synthesised by a solution-phase method, formed micelles in aqueous solutions and was observed to exhibit anticoagulant activity as shown by clotting assays such as the thrombin time (TT) test, the prothrombin time (PT) test and the activated partial thromboplastin time (APTT) test. TT and PT were found to be normal up to a particular concentration of peptide, and above that level they increased with increasing concentration of peptide, while APTT did not exhibit much significance. Conductometric and potentiometric studies showed that the peptide formed a stable micelle, and the anticoagulant activity of this peptide was also compared with a non-arginine-containing peptide (control) known to form micelles. The anticoagulant action in the micellar form could be due to the inhibition of thrombin, as seen from the decrease in amidolytic activity. The inhibitory activity of the peptide was explained in the light of micelle formation.

Radioiodination of the active site of tissue plasminogen activator: a method for radiolabeling serine proteases with tyrosylprolylarginyl chloromethyl ketone

Tyrosylprolylarginyl chloromethyl ketone (YPRck) is a radioiodinatable inhibitor that irreversibly binds the active site of tissue plasminogen activator (tPA). A two-step reaction is employed where (1) the YPRck reagent is iodinated and (2) the 125I-YPRck is reacted with the tPA sample; therefore the oxidative effects of conventional protein iodination are avoided. Using fibrin binding as a probe of native tPA binding function, YPRck labeling was shown to be superior to other types of surface iodination. 125I-YPRck was prepared at a high specific radioactivity; i.e., one 125I per 3.5 molecules of peptidyl chloromethyl ketone. Labeled YPRck formed a one to one covalent, sodium dodecyl sulfate stable, complex with tPA resulting in a preparation of 10 mCi per milligram protein, which corresponded to an incorporation ratio of 1:3.5 (125I-YPRck:tPA). Both one-chain and two-chain forms of tPA reacted with YPRck. Radiolabeling tPA with 125I-YPRck occurred in a time-dependent manner with half-maximal incorporation at approximately 30 min under the conditions employed in these studies. The pH optimum for the reaction of tPA with 125I-YPRck was 7.4. Solutions of tPA at less than 1 microgram/ml were efficiently labeled with 125I-YPRck, thus allowing the quantitation of functional protease by incorporation of radiolabel. Significantly, 125I-YPRck specifically labeled tPA in cell culture supernatants after transient transfection of cells with plasmid DNA containing the gene for tPA. Other serine proteases were tested for their relative reactivity with 125I-YPRck. Thrombin and Factor Xa incorporated 125I-YPRck to higher levels than two-chain tPA; whereas plasmin, urokinase, and other plasma proteases were not as efficiently radiolabeled. The use of 125I-YPRck allows rapid and specific radiolabeling of a large number of tPA samples in a nondenaturing environment with a known localization of the radiolabeling reagent.

The effect of tissue type plasminogen activator (tPA) on osteoclastic resorption in embryonic mouse long bone explants: a possible role for the growth factor domain of tPA

Osteoblasts produce proteolytic enzymes and their production is regulated by osteotropic agents. It has been suggested that these proteases play a role in bone resorption by removing the superficial collagenous layer from the bone matrix and indirectly inducing migration of osteoclast precursors towards the bone matrix. We examined the effect of the plasminogen activator tPA on osteoclastic resorption using 17-day-old mouse embryonic long bone explants representing different stages of osteoclast development, that is, radii containing already mature osteoclasts and metacarpals containing no mature osteoclasts but only osteoclast precursors/progenitors which are still confined to the periosteum. Tissue type PA stimulated osteoclastic resorption (measured as 45Ca-release) in 17-day-old fetal metacarpals but not in radii of the same animal. Blocking the enzymatic activity of tPA did not inhibit its effect on osteoclastic resorption. Plasmin, the direct product of PA enzymatic activity, did not induce osteoclastic resorption. However, a tPA-mutant missing the growth-factor-like domain of the molecule, failed to stimulate 45Ca-release from the metacarpals. In addition, in both systems tPA and transforming growth factor alpha had similar effects on osteoclastic resorption. The finding that tPA stimulated 45Ca-release only in the metacarpals suggests that tPA has an effect on osteoclast formation rather than on the activity of already mature osteoclasts. Under the experimental conditions used this effect seems to be mediated by the growth factor domain of tPA rather than by the enzymatic activity of the molecule.

Possible role of vascular intima for generation of coagulant activity in patients undergoing coronary thrombolysis with recombinant tissue-type plasminogen activator. A randomized, placebo-controlled study

In our present placebo-controlled study on recombinant tissue-type plasminogen activator (rt-PA) and heparin treatment of patients with acute ischaemic heart disease (IHD), we studied the extent of fibrin resolution and generation of coagulant activity. In rt-PA treated patients the lysis of fibrin in vivo (median 60 nmol of fibrin--estimated as fibrinogen equivalents) was significantly higher (p less than 0.02) than can be accounted for solely by lysis of a coronary thrombus (approximately 2 nmol) and circulating soluble fibrin (median 15 nmol). We observed a 200% increase of plasma concentrations of both prothrombin fragment 1 + 2 (p less than 0.001) and thrombin-antithrombin III complexes (p less than 0.001) as a consequence of rt-PA treatment, indicating that the coagulant activity is primarily caused by a physiological activation of the coagulation system. We conclude that an important contribution to the activation of coagulation in patients undergoing coronary thrombolysis is lysis of fibrin deposited widespread on the vascular intima, and that this process causes an intimal-dependent activation of the coagulation system.

Depression of factor XII-dependent fibrinolytic activity characterizes patients with early myocardial reinfarction after recombinant tissue-type plasminogen activator therapy

Twenty patients with acute myocardial infarction treated with recombinant tissue-type plasminogen activator (rt-PA) had endogenous factor XII-dependent fibrinolytic activity levels measured throughout the hospital period and those levels were prospectively correlated with the incidence of recurrent myocardial infarction until 8 weeks after hospital discharge. Within the follow-up period, recurrent myocardial infarction was observed in 8 patients, whereas the remaining 12 patients showed no clinical evidence of recurrence. The patients in the reinfarction group were characterized by a more pronounced depletion of and sustained lower levels of factor XII-dependent fibrinolytic activity than were the patients with no reinfarction (p less than 0.05). The decrease in fibrinolytic activity during rt-PA therapy was significantly associated with a depletion of functional alpha 2-antiplasmin, the primary plasmin inhibitor. These results indicate that, paradoxically, coronary thrombolysis with rt-PA involves depletion of endogenous factor XII-dependent fibrinolytic activity levels, which constitutes a risk for early myocardial reinfarction.

A cyclopeptidic suicide substrate preferentially inactivates urokinase-type plasminogen activator

c[Arg-aB-(CH2+SCH3 phi)-Gly4] was designed and studied as a mechanism-based inactivator (suicide substrate) for plasminogen activators (u-PA and t-PA) and plasmin. This compound inhibited u-PA and fulfills criteria expected for the involvement of an enzyme-activated inhibitor: first-order and irreversible process, saturation kinetics, protection by substrate. The limiting first-order rate constant kinact and the apparent enzyme-inhibitor dissociation constant KI were 0.021 s-1 and 9 microM, respectively at pH 7.5 and 25 degrees C. The activation of plasminogen by u-PA is compromised after this enzyme has been treated by the reagent. Plasmin and t-PA were inactivated 40- and 2330-fold less efficiently than u-PA, respectively.

Thrombin immobilized to extracellular matrix is a potent mitogen for vascular smooth muscle cells: nonenzymatic mode of action

Esterolytically inactive diisopropyl fluorophosphate-conjugated thrombin (DIP-alpha-thrombin) stimulated 3H-thymidine incorporation and proliferation of growth-arrested vascular smooth muscle cells (SMCs), similar to native alpha-thrombin. Half-maximal mitogenic response of SMCs was obtained at 1 nM thrombin and was specifically blocked by the leech-derived, high-affinity thrombin inhibitor, hirudin. Native thrombin and a variety of thrombin species that were chemically modified to alter thrombin procoagulant or esterolytic functions were found to induce 3H-thymidine incorporation to a similar extent. Exposure of SMCs to DIP-alpha-thrombin caused a rapid and transient expression of the c-fos protooncogene, determined by Northern blot analysis. These results indicate that thrombin is a potent mitogen for SMCs through a distinct non-enzymatic domain. Binding of 125I-alpha-thrombin to SMC cultures revealed an apparent dissociation constant of 6 nM and an estimated 5.4 x 10(5) binding sites per cell. This binding was inhibited to the same extent by native thrombin and by its nonenzymatic form, DIP-alpha-thrombin. Moreover, the chemotactic fragment of thrombin (CB67-129), which failed to elicit a mitogenic response, competed for 125I-alpha-thrombin binding to SMCs. Cross-linking analysis of 125I-alpha-thrombin to SMCs revealed a specific cell-surface binding site 55 kDa in size. Finally, thrombin immobilized to a naturally produced extracellular matrix retained potent mitogenic activity toward SMCs. These observations lend support to the possibility that in vivo, subendothelial basement membranes sequester thrombin (as well as other bioactive molecules), which may stimulate localized and persistent growth of arterial SMCs. Thrombin may thus be involved directly in progression of atherosclerotic plaque formation.

Reaction of thrombin and proteinases of the fibrinolytic system with a mechanism-based inhibitor, 3,4-dihydro-3-benzyl-6-chloromethylcoumarin

Thrombin, plasmin and tissue plasminogen activator (one- and two-chain forms) were examined with respect to their reaction with the suicide substrate, 3,4-dihydro-3-benzyl-6-chloromethylcoumarin, at 4 degrees C. The enzymes were irreversibly inhibited and the apparent second-order rate constants ki/Ki were 31,000, 316, 187 and 250 M-1.s-1, respectively. The extent of fibrin clot lysis induced by urokinase and two-chain tissue plasminogen activator was considerably decreased after treatment of these enzymes with the dihydrocoumarin derivative (molar excess of inhibitor over enzyme ranging from 6 to 21 for urokinase and 50 to 1500 for tissue plasminogen activator). This inhibitor has been tested as anticoagulant in human plasma and was effective at prolonging the prothrombin time from 12 to 40 s.

Effect of fibrin-targeting on clot lysis with urokinase-type plasminogen activator

The effect of fibrin-targeting of urokinase-type plasminogen activator (u-PA) on its fibrinolytic potency was studied using recombinant fusion proteins of u-PA with the NH2-terminal region of tissue-type plasminogen activator (t-PA/u-PA) and chemical complexes of u-PA with F(ab')2 fragments of a fibrin specific monoclonal antibody (u-PA/MA-15C5-F(ab')2). Two chain derivatives of a low Mr variant of u-PA comprising amino acids Leu144-Leu411 (tcu-PA-32k), obtained by cleavage of recombinant single-chain u-PA (rscu-PA-32k) with thrombin (rtcu-PA-32k/T) or plasmin (rtcu-PA-32k/P) were investigated. The plasmin-derived two chain u-PA moieties, rtcu-PA-32k/P, rt-PA/tcu-PA-32k/P and rtcu-PA-32k/MA-15C5-F(ab')2/P had high specific activities in amidolytic and fibrin plate assays (130,000 and 150,000 IU/mg u-PA, 43,000 and 71,000 IU/mg u-PA and 32,000 and 56,000 IU/mg u-PA respectively). The thrombin-derived two chain u-PA moieties had a very low amidolytic activity, corresponding to less than or equal to 1 percent of that of their plasmin-derived counterparts. On fibrin plates, however, rtcu-PA-32k/T had a negligible activity, whereas rt-PA/tcu-PA-32k/T and rtcu-PA-32k/MA-15C5-F(ab')2/T had specific activities of 12,000 and 25,000 IU/mg u-PA respectively. The catalytic efficiency for plasminogen activation of rtcu-PA-32k/MA-15C5-F(ab')2/T is 4,000-fold lower than that of rtcu-PA-32k/MA-15C5-F(ab')2/P, but its concentration required for 50 percent lysis in 2 hours of a 125I-fibrin labeled plasma clot in human plasma (C50) is only 25-fold higher. The catalytic efficiency of rt-PA/tcu-PA-32k/T is 1,600-fold lower and the C50 100-fold higher than that of rt-PA/tcu-PA-32k/P. The catalytic efficiency and the fibrinolytic potential of rtcu-PA-32k/T are negligible as compared to that of rtcu-PA-32k/P. These observations may be explained by conversion of the thrombin derived two chain u-PA moieties to their plasmin-derived analogues at the fibrin surface. This conversion appears to be most efficient for the antibody conjugate which has a high fibrin-affinity, less efficient for the t-PA/u-PA chimera which has only moderate fibrin-affinity, and negligible for the unconjugated u-PA moiety which has no fibrin-affinity. These findings illustrate the importance of plasmin-mediated positive feedback mechanisms in u-PA mediated clot lysis.

Biochemical properties of conjugates of urokinase-type plasminogen activator with a monoclonal antibody specific for cross-linked fibrin

Equimolar mixtures of recombinant single chain urokinase-type plasminogen activator (rscu-PA) and a murine monoclonal antibody (MA-15C5) directed against fragment-D dimer of human cross-linked fibrin were conjugated, using the cross-linking agent N-succinimidyl 3-(2-pyridyldithio)propionate (PySSProSu). The conjugate (rscu-PA/MA-15C5), purified by immunoadsorption on a urokinase antibody and affinity chromatography on fibrin fragment-D dimer with a yield of 42 +/- 15% (mean +/- SD, n = 3), contained an average of 1.2 +/- 0.3 IgG molecules/rscu-PA molecule. On non-reduced SDS/PAGE it migrated as a main band with apparent Mr of 200,000. Specific amidolytic activities expressed/mass of u-PA were less than 250 IU/mg for rscu-PA/MA-15C5 and rscu-PA, 140,000 +/- 13,000 IU/mg and 100,000 +/- 17,000 IU/mg for their plasmin-generated two chain derivatives rtcu-PA/MA-15C5 and rtcu-PA respectively. Specific activities on fibrin plates were 100,000 +/- 24,000 IU/mg and 130,000 +/- 49,000 IU/mg for rscu-PA/MA-15C5 and rtcu-PA/MA-15C5 respectively, as compared to 180,000 +/- 15,000 IU/mg for both rscu-PA and rtcu-PA. Activation of plasminogen with rscu-PA/MA-15C5 (Km = 0.37 +/- 0.16 microM, k2 = 0.0063 +/- 0.0030 s-1 or rtcu-PA/MA-15C5 (Km = 19 +/- 3.0 microM, k2 = 2.0 +/- 0.10 s-1) in purified systems followed Michaelis-Menten kinetics with Km and k2 values comparable to those of rscu-PA and rtcu-PA. In an in vitro system composed of a 125I-fibrin-labeled whole human plasma clot immersed in citrated human plasma, dose- and time-dependent lysis was obtained; 50% lysis in 2 h required 1.4 microgram/ml of rscu-PA or 0.33 microgram/ml of rtcu-PA, but only 0.22 microgram u-PA/ml of rscu-PA/MA-15C5 or 0.15 microgram u-PA/ml of rtcu-PA/MA-15C5. Addition of purified fragment-D dimer reversed the increased fibrinolytic potency of rscu-PA/MA-15C5 in a concentration-dependent way (50% inhibition at 7.2 micrograms fragment-D dimer/ml). Thus, conjugation of u-PA moieties with the fibrin-specific antibody MA-15C5 targets the plasminogen activator to the clot, resulting in a significant increase of their fibrinolytic potencies as compared to their unconjugated counterparts: 6.4-fold for rscu-PA and 2.2-fold for rtcu-PA.

Tissue plasminogen activator-induced secretion of type-1 plasminogen activator inhibitor in cultured human fibroblasts

Tissue plasminogen activator (t-PA) regulation of type-1 plasminogen activator inhibitor (PAI-1) secretion by WI-38 human fibroblasts was investigated utilizing a specific ELISA to measure PAI-1 antigen levels. Incubation of confluent fibroblasts with t-PA in serum-free medium resulted in a concentration and time-dependent increase in the secretion of PAI-1, with the maximum response (a two-fold increase) observed following a 4 hour incubation with 0.1 microM t-PA. This t-PA effect was reduced by cycloheximide, suggesting that the involvement of specific protein synthesis, and by inhibitors of t-PA's enzymatic activity, suggesting a requirement for a free catalytic site. The t-PA effect could not be attributed to endotoxin contamination. Increased levels in cell-associated PAI-1 were also detected following incubation of fibroblasts with t-PA. This previously undescribed effect, although seen at relatively high concentrations of t-PA, may have important biological implications.

Active site selective labeling of serine proteases with spectroscopic probes using thioester peptide chloromethyl ketones: demonstration of thrombin labeling using N alpha-[(acetylthio)acetyl]-D-Phe-Pro-Arg-CH2Cl

The feasibility of a new approach to incorporation of spectroscopic probes into the active sites of certain serine proteases has been demonstrated. The method is based on inactivation of a serine protease with a thioester derivative of a peptide chloromethyl ketone. The thiol group generated by reaction of the covalent enzyme-inhibitor complex with NH2OH provides a unique site for subsequent labeling with thiol-reactive probes. To evaluate the method, N alpha-[(acetylthio)acetyl]-D-Phe-Pro-Arg-CH2Cl was synthesized by reaction of the thrombin-specific tripeptide chloromethyl ketone with succinimidyl (acetylthio)acetate and purified by sulfopropyl-Sephadex and Sephadex G-10 chromatography. Reverse-phase high-performance liquid chromatography indicated that the product was 90 +/- 3% pure. The compound was quantitated by using 5,5'-dithiobis(2-nitrobenzoic acid) to measure the concentration of thiol produced in the presence of NH2OH. On this basis, titrations of the irreversible loss of human alpha-thrombin activity had end points of 1.1 +/- 0.1 mol of inhibitor/mol of active sites, indicating a 1:1 stoichiometry for inactivation. Incubation of N alpha-[(acetylthio)acetyl]-D-Phe-Pro-Arg-thrombin with 5-(iodoacetamido)fluorescein in the presence of NH2OH resulted in incorporation of 0.96 mol of the fluorescence probe/mol of active sites and the appearance of fluorescein fluorescence associated with the active site containing B-chain on sodium dodecyl sulfate-polyacrylamide gels. Fluorescence labeling of thrombin required reaction of the inhibitor at the active site as well as subsequent generation of the thiol group with NH2OH. It is concluded that active site selective labeling can be achieved by using this approach, which is likely to be applicable to other proteases, peptide chloromethyl ketones, and a wide variety of probes.

Indirect active-site titration of plasminogen activators

A method for the determination of the molar concentration of active tissue plasminogen activator (TPA) and urokinase (UK) has been developed. The method employs the principle of back-titration of a calibrated trypsin standard with a calibrated standard solution of a chloromethyl ketone inhibitor reactive with both trypsin and activator. Both trypsin and chloromethyl ketone standards are calibrated using a guanidinobenzoyl ester active-site titrant. Less than 20 ng of activator can be accurately determined by this method. The method is used to assay commercial samples of TPA and UK, to calculate the specific activity of such samples, and to determine kinetic constants of plasma activator inhibitor.

Highly sensitive peptide-4-methylcoumaryl-7-amide substrates for blood-clotting proteases and trypsin

Seventy-four peptide amides of 7-amino-4-methylcoumarin (Mec) of the type Boc-Xaa-Yaa-Arg-NH-Mec were newly synthesized and tested to find specific substrates for blood-clotting proteases and trypsin. The Xaa and Yaa residues of these substrates have been replaced by 12 and 15 different amino acids, respectively. Among these peptides, the followings were found to be most sensitive substrates for individual enzymes: Boc-Asp(OBzl)-Pro-Arg-NH-Mec (kcat = 160 s-1, Km = 11 microM, kcat/Km = 15,000,000 M-1 s-1) for human alpha-thrombin, Z-less than Glu-Gly-Arg-NH-Mec (kcat = 19 s-1, Km = 59 microM, kcat/Km = 320,000 M-1 s-1) for bovine factor Xa, Boc-Gln-Gly-Arg-NH-Mec (kcat = 5.8 s-1, Km = 140 microM, kcat/Km = 42,000) for bovine factor XIIa, Boc-Asp(OBzl)-Ala-Arg-NH-Mec (kcat = 9.2 s-1, Km = 120 microM, kcat/Km = 77,000 M-1 s-1) for bovine activated protein C, and Boc-Gly-Phe-Arg-NH-Mec (kcat = 29 s-1, Km = 230 microM, kcat/Km = 130,000 M-1 s-1) for bovine plasma kallikrein. Moreover, Boc-Glu(OBzl)-Ala-Arg-NH-Mec (kcat = 46 s-1, Km = 370 microM, kcat/Km = 120,000 M-1 s-1) was newly found as a good substrate for human factor XIa. Bovine trypsin effectively hydrolyzed peptide-NH-Mec substrates containing Ala and Pro at the P2 site. The most reactive substrate was Boc-Gln-Ala-Arg-NH-Mec (kcat = 120 s-1, Km = 6.0 microM, kcat/Km = 20,000,000 M-1 s-1).

Susceptibility of plasminogen activators to suicide inactivation

Halomethylated derivatives of dihydrocoumarins are efficient enzyme-activated inhibitors ("suicide" substrates) of plasminogen activators. Kinetic analysis indicate that the one-chain and two-chain forms of the human plasminogen activator are inhibited by 3,4-dihydro-3-benzyl-6-chloromethylcoumarin through a mechanism-based inactivation characterized by the following kinetic parameters (4 degrees C, pH 6.8) : k2 equal to 0.02 s-1 and 0.03 s-1 (for one- and two-chain tissue plasminogen activators, respectively) and Ki equal to 0.16 mM for both forms. Human urokinase and human tissue-type plasminogen activator can be discriminated on the basis of their inhibition by this suicide substrate. The design of a new series of suicide substrates of serine proteases (functionalized cyclopeptides possessing a potential alkylating function closely related to that found in halomethylated derivatives of dihydrocoumarins) is described.

Activation with plasmin of tow-chain urokinase-type plasminogen activator derived from single-chain urokinase-type plasminogen activator by treatment with thrombin

Thrombin converts single-chain urokinase-type plasminogen activator (scu-PA) to an inactive two-chain derivative (thrombin-derived tcu-PA) by hydrolysis of the Arg-156--Phe-157 peptide bond. In the present study, we show that inactive thrombin-derived tcu-PA (specific activity 1000 IU/mg) can be converted with plasmin to active two-chain urokinase-type plasminogen activator (specific activity 43,000 IU/mg) by hydrolysis of the Lys-158--Ile-159 peptide bond. This conversion follows Michaelis-Menten kinetics with a Michaelis constant Km of 37 microM and a catalytic rate constant k2 of 0.013 s-1. The catalytic efficiency (k2/Km) for the activation of thrombin-derived tcu-PA by plasmin is about 500-fold lower than that for the conversion of intact scu-PA to tcu-PA. tcu-PA, generated by plasmin treatment of thrombin-derived tcu-PA, has similar properties to tcu-PA obtained by digestion of intact scu-PA with plasmin (plasmin-derived tcu-PA); its plasminogen activating potential and fibrinolytic activity in an in vitro plasma clot lysis system appear to be unaltered. These observations confirm that the structure of the NH2-terminal region of the B chain of u-PA is an important determinant for its enzymatic activity, whereas that of the COOH-terminal region of the A chain is not.

Differential reactivity of Glu-Gly-Arg-CH2Cl, a synthetic urokinase inhibitor, with single-chain and two-chain forms of urokinase-type plasminogen activator

Glu-Gly-Arg-CH2Cl, a synthetic inhibitor which alkylates the active-site histidine of urokinase with an apparent second-order rate constant of approximately 10(4) M-1 s-1, reacts differently with single-chain and two-chain forms of urokinase-type plasminogen activators (scu-PA and tcu-PA). Kinetic analysis indicated that the plasminogen activating potential of tcu-PA is irreversibly inhibited in a two-step reaction according to (formula; see text) with Ki = 5.0 X 10(-6) M and k2 = 0.05 s-1. The plasminogen-activating potential of scu-PA, however, is competitively inhibited by Glu-Gly-Arg-CH2Cl with Ki = 1.3 X 10(-6) M. Reversibility of the interaction of Glu-Gly-Arg-CH2Cl with scu-PA was confirmed by the restoration of full enzymatic activity after removal of inhibitor. The differential interaction of Glu-Gly-Arg-CH2Cl with scu-PA and tcu-PA supports the hypothesis that these molecular forms of urokinase-type PA have intrinsically different enzymatic properties.

Comparative kinetic analysis of the activation of human plasminogen by natural and recombinant single-chain urokinase-type plasminogen activator

Single-chain urokinase-type plasminogen activator (scu-PA) may be obtained from conditioned cell culture media (natural scu-PA) or by expression of the cDNA encoding human scu-PA in Escherichia coli (recombinant scu-PA). The activation of Glu-plasminogen by natural and recombinant scu-PA can be described by a sequence of three reactions, each of which obeys Michaelis-Menten kinetics. Initial activation of plasminogen to plasmin by scu-PA (reaction I) occurs with a high affinity (Km below 0.8 microM) for both scu-PAs, while the catalytic rate constant (k2) is 0.017 s-1 for recombinant scu-PA but only 0.0009 s-1 for natural scu-PA. Subsequent conversion of scu-PA to urokinase (two-chain urokinase-type plasminogen activator, tcu-PA) by generated plasmin (reaction II) occurs with a comparable affinity (Km about 5 microM) for natural and recombinant scu-PA and with a k2 of 0.23 s-1 for natural and 1.2 s-1 for recombinant scu-PA. Finally, activation of plasminogen by tcu-PA (reaction III) occurs with low affinity (Km 30-50 microM) but with a high catalytic rate constant (k2 about 5 s-1) for both natural and recombinant tcu-PA. The differences in the kinetic parameters of the activation of plasminogen by natural or recombinant scu-PA are thus mainly due to differences in turnover rate in the first reaction. Indeed, the catalytic rate constant of the first reaction is about 20-times higher for recombinant scu-PA than for natural scu-PA. Thus, surprisingly, the artificial, unglycosylated recombinant scu-PA molecule has a better catalytic efficiency than its natural glycosylated counterpart.

The inactivation of one- and two-chain forms of tissue plasminogen activator by a series of peptidyl chloromethyl ketones

The inhibition of one- and two-chain forms of tissue-type plasminogen activator by a series of peptide derivatives of arginine chloromethyl ketone was studied. Both forms of the enzyme were inhibited by such reagents at varying rates, dependent on the peptide sequence of the inhibitor. The order of relative effectiveness of the inhibitors was the same for both one- and two-chain tissue plasminogen activator. However, an approximately ten to twenty-fold higher concentration of reagent was required with the one-chain activator to produce similar rates of inactivation. It appears that the specificity of the active sites of the one and two chain forms of tissue plasminogen activator are very similar. It is suggested that the differences in enzymatic properties of the two forms are due mainly to effects on binding (Ki) rather than on the rate of alkylation and inhibition (k2).

The incorporation of a fluorescent probe into the active sites of one- and two-chain tissue-type plasminogen activator

Dansyl-glutamyl-glycyl-arginyl chloromethyl ketone (DEGR-CK) was shown to inactivate both one- and two-chain human, recombinant tissue-type plasminogen activator (t-PA). The interaction of DEGR-CK with both forms of t-PA was accompanied by an identical increase in the fluorescence intensity and a blue shift in the wavelength of maximum emission, which suggests that the environment of the incorporated DEGR is similar in both one- and two-chain t-PA. The kinetics of the interaction of t-PA with DEGR-CK could be followed by both loss of activity and increase in fluorescence. The second order rate constants (k2/Ki) obtained with these two methods agreed quite well. With two-chain t-PA the values were 42 X 10(4) M-1 min-1 and 46 X 10(4) M-1 min-1 by the activity loss and fluorescence methods, respectively. With one-chain t-PA the results were 2.5 X 10(4) M-1 min-1 and 3.1 X 10(4) M-1 min-1. The rate at which one-chain t-PA is inactivated by DEGR-CK is 15 times lower than the rate with two-chain t-PA. The results demonstrated, however, that the cleavage of the one-chain protein to the two-chain form is not required for reactivity with DEGR-CK. This fluorescently labeled t-PA should be useful in probing the interactions of one- and two-chain t-PA with other proteins.

Influence of the fast-acting inhibitor of plasminogen activator on in vivo thrombolysis induced by tissue-type plasminogen activator in rabbits. Interference of tissue-derived components

The influence of endotoxin-induced elevated plasma levels of the fast-acting inhibitor of plasminogen activator (PA-inhibitor) on thrombolysis was investigated in rabbits with a jugular vein thrombus. Infusion of human tissue-type plasminogen activator (t-PA) produced similar degrees of thrombolysis in control and endotoxin-treated rabbits, although no free t-PA could be demonstrated in plasma of endotoxin-treated animals. Infusion of t-PA in an extracorporeal arteriovenous shunt resulted in loss of thrombolytic activity in endotoxin-treated animals but not in control animals. Blood clots superfused in vitro with mixtures of t-PA and normal plasma lysed in contrast to clots superfused with t-PA and PA-inhibitor-rich plasma. However, addition of rabbit lung slices to the plasma surrounding the blood clot, reversed the inhibition of thrombolysis by PA-inhibitor-rich plasma. This indicates that tissue-derived factor(s) are involved in the regulation of in vivo thrombolysis. These hypothetical factor(s) are, however, very unstable in plasma, which has thus far precluded their further characterization.

Characterization of functional domains in human tissue-type plasminogen activator with the use of monoclonal antibodies

Two murine monoclonal antibodies (MA-2G6 and MA-1C8), secreted by hybridomas obtained by fusion of myeloma cells with spleen cells from mice immunized with human tissue-type plasminogen activator (t-PA), inhibited the activity of t-PA on fibrin plates. MA-2G6 inhibited the amidolytic activity of t-PA and did not react with t-PA in which the active-site serine was blocked with diisopropylfluorophosphate nor with t-PA in which the active-site histidine was alkylated by reaction with D-Ile-Pro-Arg-CH2Cl. This indicated that MA-2G6 is directed against an epitope covering the active site of t-PA. MA-1C8 did not inhibit the amidolytic activity of t-PA, but abolished both the binding of t-PA to fibrin and the stimulatory effect of fibrin on the activation of plasminogen by t-PA. Thus MA-1C8 is directed against an epitope which covers the fibrin-binding site of t-PA. The A and B chains of partially reduced two-chain t-PA were separated by immunoadsorption on immobilized MA-1C8 and MA-2G6. The purified B chain reacted with MA-2G6 but not with MA-1C8 and activated plasminogen following Michaelis-Menten kinetics with kinetic constants similar to those of intact t-PA (Km = 100 microM and kcat = 0.02 s-1). However, fibrin or CNBr-digested fibrinogen did not stimulate the activation of plasminogen by the B chain. The purified A chain reacted with MA-1C8 but not with MA-2G6. It bound to fibrin with an affinity similar to that of intact t-PA but did not activate plasminogen. It is concluded that the active center of t-PA is located in the B chain and the fibrin-binding site in the A-chain. Both functional domains are required for the regulation by fibrin of the t-PA-mediated activation of plasminogen.

A monoclonal antibody directed against the high-affinity lysine-binding site (LBS) of human plasminogen. Role of LBS in the regulation of fibrinolysis

One of thirty murine monoclonal antibodies, raised by immunization with human plasmin-alpha 2-antiplasmin complex, was found to be directed against the high-affinity lysine-binding site in plasminogen. Indeed, this antibody (MA-HAL) reacted with plasminogen and with a fragment of plasminogen composed of the first three triple-loop structures (LBS I) and was displaced by 6-aminohexanoic acid (50% displacement at 25 microM). In competitive radioimmunoassays the binding of radiolabeled plasminogen to MA-HAL was reduced to 50% with 2.3 microM alpha 2-antiplasmin or 1.3 microM histidine-rich glycoprotein, which corresponds to the known dissociation constants between these ligands and the high-affinity lysine-binding site of plasminogen. MA-HAL did not influence the activation of plasminogen by tissue-type plasminogen activator in the absence of CNBr-digested fibrinogen, but abolished the effect of CNBr-digested fibrinogen on the Michaelis constant of the reaction. MA-HAL reduced the reaction rate between plasmin and alpha 2-antiplasmin by a factor 20 and abolished the binding of plasminogen to fibrin. These results indicate that MA-HAL specifically binds to and masks the high-affinity lysine-binding site of plasminogen. It therefore is a useful tool for the investigation of the role of this structure in the regulation of fibrinolysis, both at the level of fibrin-stimulated activation of plasminogen and of the inhibition of generated plasmin.

Evaluation of inhibitor constants and alkylation rates for a series of thrombin affinity labels

The kinetics for the inactivation of thrombin (EC 3.4.21.5) by a series of peptides containing C-terminal arginyl chloromethane in the presence of substrate were determined. The inhibitor effectiveness was analysed so as to allow for both the evaluation of the affinity with which the enzyme binds the inhibitor before irreversible modification and also the rate of covalent-bond formation between enzyme and inhibitor. The results obtained show that the observed large range in inhibitor effectiveness can be accounted for almost entirely by marked differences in affinity, with only small variations in rates of formation of covalent complex.

Specificity of activated human protein C

Peptide p-nitroanilide substrates and peptidylchloromethane inhibitors were used to examine the specificity of activated human Protein C. Substrates with arginine in the P1 position had the highest activity. The best substrates and inhibitors, as judged by the second-order rate constant for their interaction with the enzyme, had an apolar residue in the P2 position. In contrast with thrombin [Kettner & Shaw (1981) Methods Enzymol. 80, 826-842], activated Protein C was able to accommodate large hydrophobic residues such as phenylalanine and leucine in the P2 position. In the P3 position, the enzyme preferred an apolar D-amino acid residue. The results of the present study have also indicated a suitable substrate and inhibitor to be used in the assay of functional protein C and of thrombomodulin.