New insights into the functions and N ‐glycan structures of factor X activator from Russell’s viper venom

The coagulation factor X activator from Russell’s viper venom (RVV‐X) is a heterotrimeric glycoprotein. In this study, its three subunits were cloned and sequenced from the venom gland cDNAs of Daboia siamensis . The deduced heavy chain sequence contained a C‐terminal extension with four additional residues to that published previously. Both light chains showed 77–81% identity to those of a homologous factor X activator from Vipera lebetina venom. Far‐western analyses revealed that RVV‐X could strongly bind protein S, in addition to factors X and IX. This might inactivate protein S and potentiate the disseminated intravascular coagulation syndrome elicited by Russell’s viper envenomation. The N ‐glycans released from each subunit were profiled and sequenced by MALDI‐MS and MS/MS analyses of the permethyl derivatives. All the glycans, one on each light chain and four on the heavy chain, showed a heterogeneous pattern, with a combination of variable terminal fucosylation and sialylation on multiantennary complex‐type sugars. Amongst the notable features were the presence of terminal Lewis and sialyl‐Lewis epitopes, as confirmed by western blotting analyses. As these glyco‐epitopes have specific receptors in the vascular system, they possibly contribute to the rapid homing of RVV‐X to the vascular system, as supported by the observation that slower and fewer fibrinogen degradation products are released by desialylated RVV‐X than by native RVV‐X.