Properties of Three Different Molecular Forms of the α2Plasmin Inhibitor

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Abstract
The α2plasmin inhibitor prepared by affinity chromatography on plasminogen‐Sepharose 4B appears mainly as a molecular form with Mr67000 but also as forms with Mr65000 and 60000. The 67000‐Mr and 60000. The 67000‐Mrform spontaneously converts to the lower‐molecular‐mass forms. The rate of conversion is temperature‐dependent. All three molecules react with specific immuno globulin against α2plasmin inhibitor. The plasminogen‐binding capacity of the 67000‐Mr form is lost upon its conversion into the 65000‐Mrform, but it has retained its capacity to inhibit plasmin. Upon conversion to the 60000‐Mr form the capacity to inhibit plasmin is also lost. The presumed catalytic effect of a copurified serine proteinase on these reactions could not be substantiated since the addition of a number of known serine proteinases and serine proteinase inhibitors did not influence the rate of conversion of the 67000‐Mrform to the modified forms. The functional properties of the 65000‐Mrnon‐plasminogen‐binding α2plasmin inhibitor form were compared to those of the 67000‐Mrplasminogen‐binding form. In a clot lysis assay the plasminogen‐binding inhibitor was a more efficient inhibitor of fibrinolysis than the non‐plasminogen‐binding inhibitor. Plasminogen was displaced from fibrin more readily by the plasminogen‐binding inhibitor than by the non‐plasminogen‐binding form, and activation of Glu‐plasminogen by urokinase and tissue activator was more rapid with the plasminogen‐binding inhibitor than with the non‐plasminogen‐binding form.Kinetic studies showed that the plasminogen‐binding α2plasmin inhibitor reacted more readily with plasmin than the non‐plasminogen‐binding form and that the inactivation of plasmin by the former was a biphasic process in the absence of 6‐aminohexanoic acid, but monophasic in its presence. Inactivation of plasmin by the non‐plasminogen‐binding α2plasmin inhibitor was a monophasic reaction both in the absence and the presence of 6‐aminohexanoic acid and identical to the reaction with the plasminogen‐binding form in the presence of 6‐aminohexanoic acid.