AbstractThe binding and processing of plasminogen by Balb/c 3T3 and SV3T3 cells was studied using125I‐labeled canine plasminogen. Throughout a 3‐day period,125I‐plasminogen in the incubation medium bound to the cells and was degraded, first to intermediate‐sized macromolecules that were the same size as the large (74,600‐dalton) and small (25,000‐dalton) chains of active plasmin, and to smaller fragments including 3‐iodo‐L‐tyrosine. Binding to SV3T3 cells was independent of the protease‐dependent morphological change (PDMC)1characteristic of these and many other transformed cells. The SV3T3, and to a somewhat lesser extent, the 3T3 cells, both accumulated and released into the incubation medium 3‐iodo‐L‐tyrosine, a terminal lysozymal digestion product.The results of a sublethal cell‐surface trypsinization assay suggest that the cell‐associated plasminogen was primarily bound to the surfaces of the 3T3 and SV3T3 cells while the macromolecular degradation products including active plasmin were inside the cells. The rate of125I plasminogen degradation exhibited by SV3T3 cells was approximately two times greater than that of 3T3 cells, which presumably reflects differences in endocytosis or lysosomal hydrolysis, or both. The rates were unaffected by addition of pancreatic or soybean trypsin inhibitor sufficient to inhibit PDMC.In the incubation medium, plasminogen was activated to plasmin by SV3T3, but not by 3T3 cells. However, 95–100 of plasmin covalently bound to a 47,000‐dalton canine serum component, which could be dissociated from plasmin by hydroxylamine: 95–100 of the plasmin was inactive to reaction with DF32P. Thus the serum component is a plasmin inhibitor. The plasmin‐containing complex in the medium had an apparent molecular weight of 212,000. Under denaturing conditions, the complex dissociated into two covalently modified plasmin‐containing species of 153,000 and 127,000 daltons. In addition to forming a complex with a serum component, the plasmin is cleaved into two small fragments (∼10,000 and 12,000 daltons
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