Localization of individual lysine-binding regions in human plasminogen and investigations on their complex-forming properties

The activation of two isozymes of glu-plasminogen (I and II) by urokinase and streptokinase

Glu-plasminogen (Glu-plg) was eluted through lysine-Sepharose by using a gradient of 6 aminohexanoic acid, and two peaks corresponding to Glu-plg I and II were obtained. Glu-plg I has a molecular weight of 93,000 and Glu-plg II has a molecular weight of 89,000. When these plgs were activated by urokinase (UK) or streptokinase (SK) in the presence of S-2251 (H-D-Val-Leu-Lys-pNA0, the hydrolysis of S-2251 by Glu-plg I activated by UK or SK was larger than that by Glu-plg II activated by UK or SK. The results of SDS-PAGE indicate that the conversion of Glu-plg I to plasmin by UK was faster than that of Glu-plg II. It may be concluded that Glu-plg I is activated better to plasmin by activators than Glu-plg II.

Folding autonomy of the kringle 4 fragment of human plasminogen

Kringle 4, an 88-residue plasminogen fragment carrying a lysine-binding site, loses its affinity for lysine-Sepharose upon reductive cleavage of its disulfide bridges. Aerobic incubation of the reduced, denatured fragment results in the rapid restoration of the disulfide bonds with concomitant recovery of lysine-Sepharose affinity. The ability of the unfolded fragment to regain its native conformation suggests that the kringle structure is an autonomous folding domain. During refolding of kringle 4 the native disulfide bonds, (formula; see text) and (formula; see text), appears first. The folding intermediate possessing these two disulfide bridges already binds to lysine-Sepharose, indicating that the third native bridge, which in native kringle 4 connects residues Cys1 and Cys79, is not essential for the maintenance of the biologically active conformation of kringle 4. Comparison of the sequences of human prothrombin, urokinase, and plasminogen kringles revealed that the residues surrounding the (formula; see text) and (formula; see text) bridges constitute the most conservative segments of kringles, whereas the residues neighboring the (formula; see text) bridge are not highly conserved. We propose that conservation of various residues in the different kringles reflects their importance for the folding autonomy of kringles.

600 MHz H nuclear magnetic resonance studies of the kringle 4 fragment of human plasminogen

Kringle 4, a approximately 10,000-Da domain in the heavy chain of human plasminogen, has been isolated intact and studied by H NMR spectroscopy at 600 MHz. The spectroscopic data indicates that kringle 4 possesses a globular and flexible structure which exhibits relatively fast amide-hydrogen exchange. About 17 NH groups show retarded exchange, with half-lives of approximately 7 h in 2H2O at pH* 6.45, 25 degrees C, which indicates that regions of the kringle are buried and shielded from direct interaction with the solvent. Analysis of the methyl region spectrum accounts for all singlets and doublets in terms of the amino acid composition; resonances from the C- and N-termini residues could be identified from the magnitude of their J couplings and their response to pH titration. It is shown that elastase digestion of plasminogen generates two species of kringle 4, one that terminates with Ala85 and another that extends to Val87. The heterogeneity can be resolved by chromatography on CM-Sephadex. The interaction of kringle 4 with BASA (p-benzylaminesulfonic acid), an antifibrinolytic drug presumed to bind to the plasminogen lysine-binding sites, has been investigated through the effects of added ligand on the kringle spectrum. The kringle lysine-binding site would appear to be integrated by a cluster of interacting His and aromatic residues since many of these resonances follow a definite saturation curve pattern upon BASA titration. In contrast, only minor changes are detected in the aliphatic methyl spectra. The association constant for the BASA-kringle 4 interaction is estimated to be Ka approximately 74 mM-1, which should be compared with Ka approximately 145 mM-1 previously measured for kringle 1 under identical conditions. It is proposed that residues in the proximity of the Cys80-Cys1 disulfide bridge are proximal to, or form part of, the lysine-binding site.

The role of the lysine binding sites of human plasmin in the hydrolysis of human fibrinogen

The importance of the lysine binding sites of human plasmin for its ability to digest human fibrinogen has been assessed by analyzing the nature and rate of the products formed in the presence of epsilon-aminocaproic acid. No major differences in this regard were found when comparing Lys77-plasmin and Val442-plasmin, in the absence of epsilon-aminocaproic acid, the latter plasmin being devoid of the lysine binding sites present on residues Glu1-Val441 (K 1-4). The presence of epsilon-aminocaproic acid, at concentrations ranging from 0.5-5.0 mM, results in progressively stronger inhibition of the digestion of fibrinogen and in appearance of fibrinogen degradation products Y, D and E, for both Lys77-plasmin, and Val442-plasmin, showing the importance of lysine binding regions in this property. However, since both plasmin forms were inhibited equally well at all levels of epsilon-aminocaproic acid, these studies show that lysine binding sites other than those present on region K 1-4 of Lys77-plasmin are of primary importance to fibrinogenolysis by plasmin.

Human low-molecular-weight urinary urokinase. Partial characterization and preliminary sequence data of the two polypeptide chains

Low-molecular-weight urokinase (molecular weight 33100) was separated by analytical and preparative isoelectric focusing into five major subforms with isoelectric points between 8.7 and 9.6. These subforms are very similar in molecular weight, specific activity, amino acid composition and content of amino sugar and their N-terminal sequence constellation is identical. Low-molecular-weight urokinase consists of two polypeptide chains connected by a single disulfide bridge. The N-terminal region of the heavy chain (calculated Mr 30700) exhibits homology within the first 46 residues analyzed, with the known primary structure of other serine proteases. The mini chain (Mr 2426), whose complete sequence was determined, consists of 21 residues which show homology with the primary structure of the C-terminal region of the plasmin heavy chain. Based on sequence data and homology criteria with serine proteases a single-chain urokinase precursor is postulated having a peptide bond constellation between heavy and light chain region compatible with the requirements for serine protease activation.

Secondary-site binding of Glu-plasmin, Lys-plasmin and miniplasmin to fibrin

Active-site-inhibited plasmin was prepared by inhibition with d-valyl-l-phenylalanyl-l-lysylchloromethane or by bovine pancreatic trypsin inhibitor (Kunitz inhibitor). Active-site-inhibited Glu-plasmin binds far more strongly to fibrin than Glu-plasminogen [native human plasminogen with N-terminal glutamic acid (residues 1-790)]. This binding is decreased by alpha(2)-plasmin inhibitor and tranexamic acid, and is, in the latter case, related to saturation of a strong lysine-binding site. In contrast, alpha(2)-plasmin inhibitor and tranexamic acid have only weak effects on the binding of Glu-plasminogen to fibrin. This demonstrates that its strong lysine-binding site is of minor importance to its binding to fibrin. Active-site-inhibited Lys-plasmin and Lys-plasminogen (Glu-plasminogen lacking the N-terminal residues Glu(1)-Lys(76), Glu(1)-Arg(67) or Glu(1)-Lys(77))display binding to fibrin similar to that of active-site inhibited Glu-plasmin. In addition, alpha(2)-plasmin inhibitor or tranexamic acid similarly decrease their binding to fibrin. Glu-plasminogen and active-site-inhibited Glu-plasmin have the same gross conformation, and conversion into their respective Lys- forms produces a similar marked change in conformation [Violand, Sodetz & Castellino (1975) Arch. Biochem. Biophys.170, 300-305]. Our results indicate that this change is not essential to the degree of binding to fibrin or to the effect of alpha(2)-plasmin inhibitor and tranexamic acid on this binding. The conversion of miniplasminogen (Glu-plasminogen lacking the N-terminal residues Glu(1)-Val(441)) into active-site-inhibited miniplasmin makes no difference to the degree of binding to fibrin, which is similarly decreased by the addition of tranexamic acid and unaffected by alpha(2)-plasmin inhibitor. Active-site-inhibited Glu-plasmin, Lys-plasmin and miniplasmin have lower fibrin-binding values in a plasma system than in a purified system. Results with miniplasmin(ogen) indicate that plasma proteins other than alpha(2)-plasmin inhibitor and histidine-rich glycoprotein decrease the binding of plasmin(ogen) to fibrin.

Primary structure of porcine plasminogen. Isolation and characterization of CNBr-fragments and their alignment within the polypeptide chain

The single polypeptide chain of native plasminogen (molecular weight approx. 90000) after CNBr-cleavage and gel filtration (Sephadex G-75) yielded a high molecular weight core fraction of fragments linked by disulfide bridges and three fragments of lower molecular weight (N-terminal and C-terminal CNBr-fragments and dodecapeptide). From the reduced and S-carboxamidomethylated core fraction an additional seven fragments with molecular weights between 2000 and 38000 were obtained. The CNBr-fragments were aligned in the porcine plasminogen polypeptide chain according to sequence homologies with the known primary structure of human plasminogen. Due to the lack of two methionine residues in kringle 1 and in the N-terminal part of the light chain region and to an additional methionine residue in kringle 2 the CNBr-fragment pattern differs from that of human plasminogen. Affinity chromatography of elastase-digested, native plasminogen yielded three fragments with intact lysine binding sites, originating from the heavy chain region and a non-adsorbable fragment, corresponding to human 'mini'-plasminogen. This fragment was converted to urokinase into a proteolytically active protein which served for the isolation of the porcine plasmin light chain. With the aid of the fragments produced by the CNBr and elastase cleavage approx. 350 residues were sequenced, of which about 80% showed identity with the sequence of human plasminogen. This percentage varied depending on the region of the molecule, with the highest extent of identity (80--90%) found in the analyzed kringles 2 and 4.

Adsorption to fibrin of native fragments of known primary structure from human plasminogen

Limited proteolysis of native Glu-plasminogen with pancreatic elastase produced three major fragments, K1+2#3, K4, K5-light chain (miniplasminogen). Fibrin-binding was determined by clotting fibrinogen in the presence of 125I-labelled fragments and measuring 125I in the washed fibrin and in the supernatant. Of the fragments miniplasminogen showed the highest fibrin-binding, the strength of which was intermediate between those of Glu-plasminogen and Lys-plasminogen. The fibrin-binding of all three fragments was decreased by 6-aminohexanoic acid or tranexamic acid. This decrease was most pronounced with K1+2+3. The fibrin-binding of K1+2+3, but not that of K4 and miniplasminogen was decreased by alpha 2-antiplasmin. The fibrin-binding of K1+2+3 and mini-plasminogen was lower in a plasma clot than in a purified fibrin clot. Our results indicate that each of the three fragments can bind to fibrin. They confirm that an alpha 2-antiplasmin-binding site is located on K1+2+3. Furthermore two of the fragments, namely K4 and K1+2+3 contain lysine-binding site(s).

Comparison of the primary structure of the N-terminal CNBr fragments of human, bovine and porcine plasminogen

The primary structures of the N-terminal CNBr fragment of human, bovine and porcine plasminogen were determined by automated Edman degradation in a combination of liquid and solid-phase techniques and also by applying the carboxypeptidase method. The comparison of the fragments showed three highly homologous and two variable regions. The heptapeptide sequence responsible for intramolecular interaction is preserved in a conservative region, whereas the sequence of the acidic loop varies considerably between the species. In the bovine and porcine fragments 18 of the 57 residues are exchanged when compared with the fragment of human plasminogen, whereas only 11 exchanges occur between the two fragments of animal origin.

Studies onthe chemical nature of lysine-binding sites and on their localization in human plasminogen

The isolated 'kringle' structures 1 and 4 of human plasminogen lost lysine affinity upon photo-oxidation of histidine, but mostly retained it in the presence of 6-aminohexanoic acid. Lysine affinity was lost and could be partially restored after blocking of histidine with diethylpyrocarbonate and deblocking, or after esterification of COOH-groups and saponification. Only His-31 and most likely Asp-54 qualify as participants in a lysine binding site when the primary structures of the 'kringles' are considered.