Functional and sequence characterization of coagulation factor IX/factor X-binding protein from the venom of Echis carinatus leucogaster

A new coagulation factor IX/factor X-binding protein (IX/X-bp) from Echis carinatus leucogaster venom has been purified and designated ECLV IX/X-bp. ECLV IX/X-bp binds factor IX and X in a Ca²⁺-dependent manner and is devoid of thrombin-inhibitory and platelet-aggreagating activities. The apparent dissociation constants (K(d)) for binding of ECLV IX/X-bp to factor IX and factor X are 6.6 and 125 nM, respectively. Upon the addition of Mg²⁺, the required Ca²⁺ concentration for optimal binding of ECLV IX/X-bp to factor IX and factor X was prominently reduced. Mg²⁺ also increases the affinity of factor X for the venom protein. Direct binding of IX/X-bp to factor IX and X could also be detected by far-Western blotting, and results of the experiment ruled out the lectin-like mechanism of ECLV IX/X-bp. The complete amino acid sequence and the disulfide pattern of ECLV IX/X-bp was deduced by enzymatic hydrolysis and automated sequencing of the S-pyridylethylated protein. The venom protein is a heterodimer with one subunit of 131 amino acid residues and another of 125 residues. Both subunits are homologous to each other and to other snake venom proteins of the C-type lectin superfamily.