INHIBITION OF THROMBIN-MEDIATED CELLULAR EFFECTS BY TRIABIN, A HIGHLYPOTENT ANION-BINDING EXOSITE THROMBIN INHIBITOR

Triabin, a 17 kDa protein from the saliva of the assassin bug Triatoma pallidipennis is a potent thrombin inhibitor interfering with the ani on-binding exosite of the enzyme. The recombinant protein, produced by the baculovirus/insect cell system, was used to study the inhibitory effect on thrombin-mediated cellular responses. The thrombin (1 nM)-st imulated aggregation of washed human platelets and the rise in cytopla smic calcium in platelets were inhibited by triabin at nanomolar conce ntrations. In contrast, the rise in calcium induced by the thrombin re ceptor-activating peptide (10 mu M) was not suppressed by triabin. In isolated porcine pulmonary arteries, preconstricted with PGF(2 alpha), thrombin (2 nM) elicited an endothelium-dependent relaxation which wa s inhibited by triabin in the same concentration range as found for th e inhibition of platelet aggregation. Higher concentrations of triabin were required to diminish the contractile response of endothelium-den uded pulmonary vessels to thrombin (10 nM). In cultured bovine coronar y smooth muscle cells, the mitogenic activity of thrombin (3 nM), meas ured by [H-3]thymidine incorporation, was also suppressed by triabin. In all these assays, the inhibitory effect of triabin was dependent on the thrombin concentration used. These studies suggest that the new a nion-binding exosite thrombin inhibitor triabin is one of the most pot ent inhibitors of thrombin-mediated cellular effects.