TRIABIN, A HIGHLY POTENT EXOSITE INHIBITOR OF THROMBIN

Triabin, a new thrombin inhibitor, has been purified from the saliva o f Triatoma pallidipennis, a blood-sucking triatomine bug, It forms a n oncovalent complex with thrombin at a molar ratio of 1:1, inhibits thr ombin-induced platelet aggregation, and prolongs thrombin clotting tim e and activated partial thromboplastin time, However, it only minimall y suppresses the amidolytic activity of thrombin, as measured by a chr omogenic peptide substrate assay. It completely blocks trypsin-catalyz ed cleavage of thrombin, probably via protection of the anion-binding exosite and inhibits the effect of thrombomodulin on thrombin in a dos e-dependent fashion, These results indicate that the inhibitor is dire cted toward the anion-binding exosite of thrombin, The protein was par tially sequenced and the information used to isolate cDNA clones from a T. pallidipennis salivary gland library. Four slightly polymorphic v ariants coding for mature proteins of 142 amino acids preceded by a pu tative leader sequence were obtained. The recombinant protein expresse d in the periplasmic space of Escherichia coli has a biological activi ty similar to that of salivary triabin, as tested in a thrombin-induce d platelet aggregation assay, In addition, recombinant triabin inhibit s thrombin catalyzed hydrolysis of fibrinogen with a K-i of about 3 pM .