Thrombin is inactivated by mast cell secretory granule chymase.

In a recent paper we demonstrated that cultures of murine adherent peritoneal cells expressed cell surface-associated serine protease activity that specifically inactivated thrombin by cleaving the enzyme into defined proteolytic fragments (Pejler, G., and Seljelid, R. (1992) J. Biol. Chem. 267, 3136-3142). In the present report, the purification and further characterization of the thrombin-inactivating serine protease is described. The serine protease is shown to be expressed by mast cells. Purification of the thrombin-inactivating serine protease by a combination of anion-exchange chromatography and Superdex 75 chromatography showed that the enzyme had an apparent molecular mass of 28 kDa. N-terminal sequence analysis of the purified protein demonstrated 100% identity of the thrombin-inactivating serine protease with the secretory granule chymases: murine mast cell protease 3 and murine mast cell protease 4. The serine protease showed chymotrypsin-like substrate specificity. The thrombin-inactivating activity was markedly enhanced by optimal concentrations of heparin.