Human umbilical vein endothelial cells were analyzed for the presence of prothrombin. factor VII, protein C, and protein S in culture supernatants and cell extracts using specific radioimmunoassays. Only protein S was detected. Conditioned medium from 24-hour cultures and cell lysates contained 21.7 ng/mL and 88.8 ng/107 cells of protein S, respectively. Intrinsic labeling and immunoprecipitati...
