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. [I-125]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. (C) 1994 Academic Inc. Press,