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Polymerized fibrin strongly enhances tissue plasminogen activator (tPA)-mediated plasminogen activation, concomitant with exposure of 'fibrin-specific' epitopes at 'Aalpha148-160' and 'gamma312-324'. To investigate which aspects of polymerization are involved in these activities, we explored the fibrin polymerization process by evaluating the ability of factor XIIIa-crosslinked fibrinogen polymers to expose 'fibrin-specific' epitopes and enhance plasminogen activation. Crosslinked normal fibrinogen, fibrinogen with deficient [des Bbeta1-42] or defective [Birmingham (AalphaR16H)] fibrin 'D:E' assembly sites ('E(A)'), or with defective end-to-end self-association sites ('D:D') [Cedar Rapids (gammaR275C)], exposed both 'fibrin-specific' epitopes and enhanced tPA-dependent plasminogen activation, whereas non-crosslinked fibrinogens showed minimal or no such activities. Epitope expression in crosslinked fibrinogen was retained in the presence of the fibrin E(A) site peptide homolog, gly-pro-arg-pro (GPRP), which inhibits fibrin D:E association, except for the Aalpha148-160 epitope in des Bbeta1-42 fibrinogen, which was not expressed. Fibrin prepared from crosslinked normal or abnormal fibrinogen, except for the des Bbeta1-42 fibrin epitopes, which were reduced or absent, expressed 'fibrin-specific' epitopes even in the presence of GPRP, which otherwise impairs such expression in non-crosslinked fibrin. Epitope exposure in fibrin prepared from non-crosslinked fibrinogen was nearly normal in Cedar Rapids fibrin (heterozygous D:D defect), but reduced in Birmingham fibrin (heterozygous E(A) defect), nil in des Bbeta1-42 fibrin (E(A) deficient), and absent in all cases in the presence of GPRP. In contrast, plasminogen activation stimulatory activity that had been exposed in crosslinked normal fibrinogen or in crosslinked des Bbeta1-42 or Cedar Rapids fibrin, was preserved to a large extent in the presence of GPRP, suggesting that once enhanced stimulatory activity and epitopes are exposed, they are not completely reversible. The findings indicate that end-to-end intermolecular associations (D:D) are not critical for 'fibrin-specific' epitope exposure, but that polymerization brought about in fibrinogen through factor XIIIa crosslinking, or in fibrin through 'D:E' interactions, is necessary for 'fibrin-specific' (more correctly, 'polymerization-specific') epitope exposure and enhancement of plasminogen activation.

In previous publications [e.g. Voskuilen, Vermond, Veeneman, Van Boom, Klasen, Zegers & Nieuwenhuizen (1987) J. Biol. Chem. 262, 5944-5946] we have shown that fibrin(ogen) chain fragment A alpha-(148-160) contains a site that contributes to the acceleration of Glu-plasminogen activation by tissue-type plasminogen activator (t-PA). In contrast with fibrin, this peptide, however, does not enhance the rate of mini-plasminogen activation. Therefore, possibly more stimulatory sites than A alpha-(148-160) are present in fibrin. In the present investigation we have localized a possible second type of stimulatory site in the fibrin(ogen) molecule. A whole CNBr digest of fibrinogen was applied to a Bio-Gel P-2 column run in water, pH 4. Two peaks with stimulatory activity were observed, one at the void volume and one between the void volume and the total volume. The former contained the previously described stimulating fragment FCB-2 [which comprises A alpha-(148-160)]; the latter had not been observed before and was characterized further. The stimulating material in the low-M(r) fraction of the Bio-Gel P-2 column was precipitated at pH 8.3 in a virtually pure form. It has a high tryptophan content, and an M(r) of 6500 as assessed by SDS/PAGE. On reduction, a main band of M(r) 2500 is seen, plus a weakly staining band of M(r) 4000. These properties plus the amino acid sequence data identify the fragment as FCB-5. FCB-5 consists of two chains, i.e. gamma-(311-336) and gamma-(337-379), linked by a single disulphide bond between Cys-gamma-326 and Cys-gamma-339. Both these chains and the disulphide bond appear to be essential for rate enhancement. FCB-5 enhances the activation rates of Glu-, mini- and micro-plasminogen, with all five kringles, only kringle V and without kringles respectively. FCB-5 binds t-PA, but none of the plasminogen forms binds to FCB-5. This indicates that the rate enhancements induced by FCB-5 are due to an effect on t-PA.

The sequence fibrinogen-A alpha-(148-160) can mimic part of the fibrin-induced rate enhancement of the activation of plasminogen by tissue-type plasminogen activator. Previously we have reported that the lysine residue at position A alpha-157 is crucial. During our further investigations on A alpha-157 we found that lysine at position A alpha-157 may be replaced by glutamic acid. This unexpected finding prompted us to re-investigate the requirements of this position. We prepared analogues of A alpha-(148-160) in which the lysine residue at position A alpha-157 was replaced by lysine derivatives (acetyl-lysine, benzyloxycarbonyl-lysine and methanesulphonylethyloxycarbonyl-lysine), acidic residues (aspartic acid and glutamic acid), basic residues (arginine and ornithine), polar residues (glutamine and methanesulphonylethyloxycarbonylornithine), apolar residues (alanine, valine, norleucine and glutamic acid 4-nitrobenzyl ester) and glycine. These analogues were tested for their stimulatory activity. When aspartic acid, glutamic acid 4-nitrobenzyl ester or norleucine is present at position A alpha-157 in A alpha-(148-160) virtually all stimulatory capacity is lost. With valine at position A alpha-157 the stimulatory activity is marginal. None of the other replacements at position A alpha-157 caused loss of rate-enhancing properties. From these results we conclude that for the rate-enhancing effect of A alpha-(148-160) the side chain of the amino acid residue at position A alpha-157 must fulfill certain requirements: there must be one (as in alanine) or no (as in glycine) carbon atom in the side chain, or at least two carbon atoms and a polar group (charged or uncharged) to which a rather bulky group (such as the benzyloxycarbonyl group) or a polar group (such as the methanesulphonylethyloxycarbonyl group) may be attached. The highest activity [even higher than native A alpha-(148-160)] was obtained with ornithine, methanesulphonylethyloxycarbonylornithine or methanesulphonylethyloxycarbonyl-lysine at position A alpha-157.

The rate of activation of plasminogen by tissue-type plasminogen activator (t-PA) is greatly increased by fibrin, but much less by fibrinogen. Fibrin(ogen) fragments such as the fibrin(ogen) cyanogen bromide fragment FCB-2 and FCB-5, and a synthetic peptide with the sequence of fibrinogen A alpha-(148-160), a constituent of FCB-2, also have rate-enhancing properties. In order to find a possibly smaller, still stimulating site within A alpha-(148-160) we synthesized successive linear amino-terminally acylated hexapeptides [i.e. A alpha-(148-153), A alpha-(149-154)'d, .... A alpha-(155-160)] from the sequence A alpha-(148-160). The only hexapeptide within the sequence A alpha-(148-160) capable of enhancing the rate of plasminogen-to-plasmin conversion by t-PA appears to be the amino-terminally acylated peptide comprising the sequence A alpha-(154-159). This peptide enhances the plasminogen activation rate six-fold; half-maximal activation rate is reached at a peptide concentration of 56 microM.

Fibrin, not fibrinogen, enhances the rate of tPA catalysed plasminogen activation. In earlier studies we have shown that a site involved in this rate enhancement is located in a tridecapeptide, i.e. fibrinogen A alpha-(148-160). This sequence comprises a special charge distribution in which a stretch with alternating neutral and acidic amino acids is embraced by basic amino acids. In this study we found that the disruption of charge distribution as caused by replacing valine 152 by other (charged and/or polar) amino acids leads to loss of rate-enhancing capacity. Also lysine at position A alpha-157 was replaced by lysine derivatives and other amino acids. We found that the side chain of the amino acid at position A alpha-157 must contain no (as in glycine) or one carbon atom without substitution (alanine). When the side chain contains two or more carbon atoms, there should also be a polar group in the side chain. We also synthesized a series of hexapeptides covering the sequence of A alpha-(148-160), and found that only A alpha-(154-159) is stimulatory, notwithstanding the fact that the peptides A alpha-(152-157), A alpha-(153-158) and A alpha-(155-160) also contain lysine A alpha-157. We conclude that the shortest peptide with stimulation activity is A alpha-(154-159); that the charge distribution in A alpha-(148-160) is important; that it is not lysine A alpha-157 per se that is crucial, but rather the properties and orientation of the side chain of A alpha-157.

In the present work we have determined Km,app and kcat,app values for tissue-type plasminogen-activator-catalyzed activation of Glu-plasminogen, Lys-plasminogen and mini-plasminogen in the absence and in the presence of fibrinogen-derived fragments. These were CNBr fragment 2, the A alpha chain remnant of CNBr fragment 2 (A alpha 148-207) and plasmin-generated fragment D-EGTA. The time course of plasmin formation from the various types of plasminogen (plg) was measured spectrophotometrically in a coupled assay system where D-valyl-L-leucyl-L-lysine p-nitroanilide served as a plasmin substrate. The kinetic constants are summarized as follows. (Values in parentheses are concentrations at which the minimum Km,app and maximum kcat,app value is reached.) (Table: see text). In conclusion our results show that CNBr fragment 2, A alpha 148-207 and to some extent D-EGTA mimic the accelerating effect of fibrin. The first two of these fragments did not accelerate activation of mini-plasminogen, lacking the kringle structures I-IV. This suggests that the stimulating effects of these two fragments were dependent on the presence of kringles I-IV of the plasminogen molecule.