Bothrojaracin, a new thrombin inhibitor isolated from Bothrops jararaca venom: characterization and mechanism of thrombin inhibition.

A new thrombin inhibitor, bothrojaracin, has been identified and purified to homogeneity from the venom of Bothrops jararaca, the most common venomous snake of South America. Bothrojaracin has an isoelectric point of 4.2 and a molecular mass of 27 kDa and is made of two distinct polypeptide chains of 15 and 13 kDa, linked by disulfide bridges. Purified bothrojaracin is devoid of phospholipase A2, amidolytic, or fibrino (geno)lytic activity. Bothrojaracin forms a noncovalent complex with alpha-thrombin, without changing its catalytic activity on small peptide substrates. Bothrojaracin behaves as a potent and specific antagonist of thrombin-induced platelet aggregation and secretion, characterized by an IC50 ranging from 1 to 20 nM depending on the alpha-thrombin concentration. Bothrojaracin prolongs fibrinogen clotting time, and this effect is related to a competitive inhibition of the binding of alpha-thrombin to fibrin(ogen) (Ki 15 nM). Binding of alpha-thrombin to thrombomodulin is inhibited up to 87% by bothrojaracin, and the rate of protein C activation by alpha-thrombin is also decreased. Bothrojaracin antagonizes the inhibition of thrombin amidolytic activity by hirudin. These results indicate that bothrojaracin acts as a very potent ligand of the exosite of alpha-thrombin.