Receptor-mediated endocytosis of thrombin-antithrombin III complex by the human monocytoid cell line U937

Receptor-mediated endocytosis of thrombin-antithrombin III (TAT) complex was characterized with the monocytoid cell line U937. At 4 degrees C, U937 cells bound TAT but not antithrombin III in a rapid, specific, saturable, and reversible manner. [125I]TAT binding to U937 cells was not inhibited by antithrombin III and hardly by thrombin, but 75% inhibited by a 50 molar excess of the unlabeled TAT. There were about 79,000 binding sites/cell (Kd = 68 nM). The TAT receptor on U937 cells may be distinct from the serpin-enzyme complex (SEC) receptor, since a synthetic peptide corresponding to the recognition sequence of SEC receptor did not compete with TAT binding to the cells. Receptor-bound TAT was internalized into the cells after switching the temperature from 4 to 37 degrees C and thereafter degraded in lysosomes.