The role of a contaminant in thrombin in the human plasmin assay system.

Many of the anomalous results obtained in the fibrinolytic assay of human plasmin systems were shown to be simply explained if bovine plasminogen had been introduced into the assay system on the addition of thrombin. Experimental investigation of the proteolytic and fibrinolytic activity of systems containing plasmin and thrombin showed that enzyme activity was influenced by the presence and quantity of thrombin. The quantity of bovine plasminogen present as a contaminant in bovine fibrinogen was shown to be responsible for only 1/25th of the observed enhanced activity. Thrombin in the amounts commonly used for clotting contained sufficient proenzyme to account for all this activity. A highly purified thrombin preparation obtained from another laboratory, and thrombin purified in this laboratory by starch electrophoresis brought about no enhancement of activity. The material separated from thrombin by starch electrophoresis was shown to be enzymatically identical with bovine plasminogen and, on labelling with radioactive iodine, was shown to behave physically like bovine plasminogen. Several experiments reported in the literature were reinterpreted in the light of this observation.