Monoclonal antibodies inhibitory to human plasmin. Definitive demonstration of a role for plasmin in activating the proenzyme of urokinase-type plasminogen activator.

Four monoclonal antibodies raised against purified human plasminogen were characterized for their effects on the activation of plasminogen and on three enzymic properties of plasmin: (a) thioesterolysis, (b) fibrinolysis, (c) conversion of high-Mr urokinase to its low-Mr form. None of the monoclonal antibodies inhibited plasminogen (plg) activation by urokinase. The monoclonal antibodies characterized in this study fell into three groups. Anti-plg 1 inhibited (a), (b) and (c), while anti-plg 2 inhibited activities (a), (b) and (c) to varying degrees but also formed complexes with plasmin that were stable to sodium dodecyl sulphate. Anti-plg 3 and anti-plg 4 inhibited only activity (c). Selective use of these monoclonal antibodies demonstrated unequivocally that plasmin mediates the activation of the proenzyme form of urokinase-type plasminogen activator. Besides their use in affinity chromatography, therefore, these antibodies are valuable for defining the role of plasmin in the mechanisms of extracellular matrix degradation.