TRIABIN, A HIGHLY POTENT EXOSITE INHIBITOR OF THROMBIN

Triabin, a new thrombin inhibitor, has been purified from the saliva of Triatoma pallidipennis, a blood-sucking triatomine bug, It forms a noncovalent complex with thrombin at a molar ratio of 1:1, inhibits thrombin-induced platelet aggregation, and prolongs thrombin clotting time and activated partial thromboplastin time, However, it only minimally suppresses the amidolytic activity of thrombin, as measured by a chromogenic peptide substrate assay. It completely blocks trypsin-catalyzed cleavage of thrombin, probably via protection of the anion-binding exosite and inhibits the effect of thrombomodulin on thrombin in a dose-dependent fashion, These results indicate that the inhibitor is directed toward the anion-binding exosite of thrombin, The protein was partially sequenced and the information used to isolate cDNA clones from a T. pallidipennis salivary gland library. Four slightly polymorphic variants coding for mature proteins of 142 amino acids preceded by a putative leader sequence were obtained. The recombinant protein expressed in the periplasmic space of Escherichia coli has a biological activity similar to that of salivary triabin, as tested in a thrombin-induced platelet aggregation assay, In addition, recombinant triabin inhibits thrombin catalyzed hydrolysis of fibrinogen with a K-i of about 3 pM.