A group of
peptides from the salivary gland of the tick Hyalomma marginatum rufipes, a vector of
Crimean Congo hemorrhagic fever show weak similarity to the madanins, a group of
thrombin-inhibitory
peptides from a second tick species, Haemaphysalis longicornis. We have evaluated the anti-
serine protease activity of one of these H. marginatum
peptides that has been given the name hyalomin-1. Hyalomin-1 was found to be a selective inhibitor of
thrombin, blocking coagulation of plasma and inhibiting S2238 hydrolysis in a competitive manner with an inhibition constant (Ki) of 12 nM at an ionic strength of 150 mM. It also blocks the
thrombin-mediated activation of
coagulation factor XI,
thrombin-mediated platelet aggregation, and the activation of
coagulation factor V by
thrombin. Hyalomin-1 is cleaved at a canonical
thrombin cleavage site but the cleaved products do not inhibit coagulation. However, the C-terminal cleavage product showed non-competitive inhibition of S2238 hydrolysis. A
peptide combining the N-terminal parts of the molecule with the cleavage region did not interact strongly with
thrombin, but a 24-residue fragment containing the cleavage region and the C-terminal fragment inhibited the
enzyme in a competitive manner and also inhibited coagulation of plasma. These results suggest that the
peptide acts by binding to the active site as well as exosite I or the
autolysis loop of
thrombin. Injection of 2.5 mg/kg of hyalomin-1 increased
arterial occlusion time in a mouse model of
thrombosis, suggesting this
peptide could be a candidate for clinical use as an antithrombotic.