Summary
Triabin, a 17 kDa protein from the saliva of the assassin bug Triatoma pallidipennis is a potent thrombin inhibitor interfering with the anion-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)-stimulated aggregation of washed human platelets and the rise in cytoplasmic
calcium in platelets were inhibited by triabin at nanomolar concentrations. In contrast,
the rise in calcium induced by the thrombin receptor-activating peptide (10 μM) was
not suppressed by triabin. In isolated porcine pulmonary arteries, preconstricted
with PGF2γ, thrombin (2 nM) elicited an endothelium-dependent relaxation which was inhibited
by triabin in the same concentration range as found for the inhibition of platelet
aggregation. Higher concentrations of triabin were required to diminish the contractile
response of endothelium- denuded pulmonary vessels to thrombin (10 nM). In cultured
bovine coronary smooth muscle cells, the mitogenic activity of thrombin (3 nM), measured
by [3H]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 anion-binding exosite thrombin inhibitor triabin
is one of the most potent inhibitors of thrombin-mediated cellular effects.
