Different molecular forms of plasminogen and plasmin produced by urokinase in human plasma and their relation to protease inhibitors and lysis of fibrinogen and fibrin

Abstract

Urokinase-activated human plasma was studied by gel electrophoresis, gel filtration, crossed immunoelectrophoresis and electroimmunoassay with specific antibodies and by assay of esterase and protease activity of isolated fractions. Urokinase induced the formation of different components with plasminogen+plasmin antigenicity. At low concentrations of urokinase, a component with a K_D value of 0.18 by gel filtration and post β₁ mobility by gel electrophoresis was detected. The isolated component had no enzyme or plasminogen activity. In this plasma sample fibrinogen was not degraded for 10h, but when fibrin was formed, by addition of thrombin, fibrin was quickly lysed, and simultaneously a component with a K_D value of 0 and α₂ mobility appeared, which was probably plasmin in a complex with α₂ macroglobulin. This complex showed both esterase and protease activity. After gel filtration with lysine buffer of the clotted and lysed plasma another two components were observed with about the same K_D value by gel filtration as plasminogen (0.35), but β₁ and γ mobilities by gel electrophoresis. They appeared to be modified plasminogen molecules, and possibly plasmin with γ mobility. Similar processes occurred without fibrin at higher urokinase concentrations. Here a relatively slow degradation of fibrinogen was correlated to the appearance of the plasmin–α₂ macroglobulin complex. The fibrin surface appeared to catalyse the ultimate production of active plasmin with a subsequent preferential degradation of fibrin and the formation of a plasmin–α₂ macroglobulin complex. The gel filtration and electrophoresis of the plasma protease inhibitors, α₁ antitrypsin, inter-α-inhibitor, antithrombin III, and C₁-esterase inhibitor indicated that any complex between plasmin and these inhibitors was completely dissociated. The β₁ and post β₁ components appear to lack correlates among components occurring in purified preparations of plasminogen and plasmin.