The binding of plasminogen to fibrin: evidence for plasminogen-bridging.

The ability of plasminogen to cause precipitation of soluble fibrin oligomers has been observed and certain features of the phenomenon investigated. The process is mediated by the lysine-binding sites and it appears that at least two such sites are required. Studies using radiolabelled plasminogen revealed that the precipitated material contained fibrin and plasminogen in a 2:1 molar ratio. Further plasminogen molecules are able to bind to the aggregate. The clotting of fibrinogen in the presence of plasminogen was studied using nephelometry. An enhancement by plasminogen of both the rate of clotting and the opacity of the clot was demonstrated. It is proposed that these effects are explicable in terms of a plasminogen-bridging model, in which the zymogen binds divalently between two monomer units of forming polymeric fibrin.