A THROMBIN RECEPTOR IN RESIDENT RAT PERITONEAL-MACROPHAGES

Resident rat peritoneal macrophages possess 6 × 102 high-affinity binding sites per cell for bovine thrombin with a Kd of 11 pM, and 7.5 × 104 low-affinity sites with a Kd of 5.8 nM. These binding sites are highly specific for thrombin. Half-maximal binding of 125I-labeled bovine thrombin is achieved after 1 min at 37 °C, and after 12 min at 4 °C. The reversibly bound fraction of the ligand dissociates according to a biexponential time course with the rate constants 0.27 and 0.06 min-1 at 4 °C. Part of the tracer remains cell-associated even after prolonged incubation, but all cell-associated radioactivity migrates as intact thrombin upon sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The bound thrombin is minimally endocytosed as judged by the resistance to pH 3 treatment, and the receptor does not mediate a quantitatively important degradation of the ligand. The binding is not dependent on the catalytic site of thrombin, since irreversibly inactivated thrombin also binds to the receptor. 125I-labeled thrombin covalently cross-linked to its receptor migrates in sodium dodecyl sulfate-polyacrylamide gel electrophoresis with a Mr 160,000, corresponding to an approximate receptor Size of Mr 120,000. © 1991.