The pathway of plasminogen transformation was studied in plasma, particularly in relation to fibrin formation and the subsequent stimulation of plasminogen activation. Plasminogen was activated by urokinase (low fibrin-affinity) or tissue-type plasminogen activator (high fibrin-affinity). Formation of 125I-labelled free and inhibitor-bound plasminogen derivatives was quantified after their separation by acetic acid/urea/polyacrylamide-gel electrophoresis. In plasma activator converted Glu-plasminogen (residues 1-790) into Glu-plasmin, which was complexed to alpha 2-plasmin inhibitor. When this inhibitor was saturated, Glu-plasmin was autocatalytically converted into Lys-plasmin (residues 77-790). No plasmin-catalysed Lys-plasminogen formation was observed. Upon fibrin formation, activation initially followed the same Glu-plasminogen-into-Glu-plasmin conversion pathway, and stimulation of plasminogen activation was only observed with tissue-type plasminogen activator. In agreement with the emergence of novel effector function, on early plasmin cleavage of fibrin [Suenson, Lützen & Thorsen (1984) Eur. J. Biochem. 140, 513-522] the fibrin-binding of Glu-plasminogen increased when solid-phase fibrin showed evident signs of degradation. This was associated with the formation of considerable amounts of the more easily activatable Lys-plasminogen, most of which was fibrin-bound. At the same time the rate of plasmin formation with urokinase increased over that in unclotted plasma and the rate of plasmin formation with tissue-type plasminogen activator accelerated. Altogether these processes favoured enhanced fibrin degradation. The rates of Lys-plasminogen and plasmin formation abruptly decreased after lysis of fibrin, probably owing to a compromised effector function on further fibrin degradation.

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SummaryIncreasing concentrations of EACA produce a biphasic pattern of inhibition and enhancement of urokinase-induced lysis of bovine fibrin containing bovine plasminogen, while the inhibition of fibrinolysis induced by a porcine tissue plasminogen activator increases uniformly. The biphasic EACA pattern is also observed with human plasminogen in fibrinolytic and caseinolytic assays of urokinase. The biphasic EACA pattern produced with urokinase is related to the presence of a genuine form of plasminogen. The enhancement phase is caused by an increased rate of plasminogen activation in the presence of EACA. A brief treatment of genuine plasminogen with acid at ionic strength 0.15 results in an enhanced susceptibility to plasminogen activators and in a partial abolishment of the biphasic response. These acid-induced alterations of plasminogen seem to be reversed by acid dialysis at low ionic strength. Other preparations of plasminogen with enhanced susceptibility to activators have lost the ability to produce a biphasic pattern of inhibition and enhancement of urokinase-induced plasminogen activation in the presence of EACA and this ability does not return after acid dialysis at low ionic strength. EACA inhibits all plasmin preparations, whether prepared from genuine or altered forms of plasminogen, in the same uniform manner.Our results show that different forms of plasminogen can be identified by differences in the susceptibilities to activators, by their response to EACA, and by the reversibility or irreversibility of the alterations.

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