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The fibrinolytic activity of proteases secreted by chick embryo fibroblasts infected with Rous sarcoma virus was studied by use of a procedure in which a fibrin clot was formed with highly purified fibrinogen and thrombin above the cell layer. This procedure results in the formation of fibrin that is apparently a more suitable substrate for studies on fibrinolysis than is fibrin prepared by other methods. Since neither plasminogen nor serum were included in the assay system in the present studies, the fibrinolytic activity observed cannot be ascribed to the conversion of the plasminogen in serum to plasmin by a plasminogen activator produced by transformed cells. Our procedure, therefore, measures proteolytic activities other than those reported by previous investigators. Maintenance of some of the transformed phenotypes of Rous sarcoma virus transformed chick embryo fibroblasts such as morpholigical change and increased rate of glucose uptake apparently does not depend on the presence of plasminogen in the culture medium.
Chen et al. (Sat,) studied this question.