Thrombospondin 1 is a multidomain trimeric glycoprotein from platelets and a variety of normal and transformed cells of both mesenchymal and epithelial origin, which functions in cell adhesion and cell-cell interactions. We have recently shown that human thrombospondin 1 binds and inhibits the neutrophil enzymes, neutrophil elastase [Hogg, P.J., Owensby, D.A., Mosher, D.F., Misenheimer, T.M., & Chesterman, C.N. (1993a) J. Biol. Chem. 268, 7139-7146] and cathepsin G [Hogg, P.J., Owensby, D.A., & Chesterman, C.N. (1993b) J. Biol. Chem. 268, 21811-21818]. One mole of thrombospondin 1 trimer binds 3 mol of neutrophil elastase or up to 6 mol of cathepsin G, with site-binding dissociation constants around the nanomolar range, and the enzymes have been shown to interact with thrombospondin 1 in the vicinity of the calcium-binding type 3 repeats. None of the protein modules in this region, or within the whole thrombospondin 1 molecule, have previously been implicated in the inhibition of proteinases. We noted that there are two stretches of eight amino acids each in the human thrombospondin 1 type 3 repeats, residues 735-742 and 794-801, that have striking similarity to a reactive-site consensus sequence derived from selected members of the Kazal and Streptomyces subtilisin inhibitor families. Synthetic peptides corresponding to the putative P5 through P4' residues of both proposed reactive centers interacted efficiently with the active site of cathepsin G and were competitive inhibitors of the fibronectin-degrading and platelet-activating activities of this enzyme, while only the peptide corresponding to residues 793-801 efficiently interacted with the active site of neutrophil elastase and competitively inhibited its fibronectin-degrading activity.(ABSTRACT TRUNCATED AT 250 WORDS)