Isolation of an inhibitor selective for collagen-stimulated platelet aggregation from the soft tick Ornithodoros moubata.

Soluble extracts from the soft tick Ornithodoros moubata were found to inhibit collagen-, ADP-, and thrombin-stimulated platelet aggregation. One inhibitory component was purified to homogeneity by a combination of gel filtration, ion-exchange, and reverse phase high pressure liquid chromatography. The purified activity, named moubatin, is a protein of molecular weight 17,000 and it inhibits the aggregation of washed human platelets stimulated by collagen with an IC50 of approximately 50 nM in the standard assay. At a concentration of moubatin that maximally inhibited collagen-stimulated platelet aggregation, no inhibition of aggregation initiated by other effectors, including arachidonic acid, thrombin, ristocetin, and the calcium ionophore A23187, was observed. Moubatin also inhibits collagen-dependent aggregation in plasma. At a higher concentration of moubatin (> 1 microM) it was also possible to demonstrate an inhibitory effect on the final extent of aggregation induced by a low concentration of ADP. Although moubatin selectively inhibits platelet activation by collagen, it has only a minimal effect on the adhesion of platelets to collagen. The amino acid sequences of peptides derived from proteolytic cleavage of moubatin suggest that moubatin is a unique protein, consistent with its novel functional activity.