The shifting sands of venom: Divergent blood clotting factor activation pathways and differential Factor Va co-factor dependence for the venoms of Middle Eastern desert vipers (Eristicophis and Pseudocerastes species)

Desert vipers (Eristicophis and Pseudocerastes) employ diverse procoagulant strategies despite their close phylogenetic relationships, yet the biochemical mechanisms underlying the variations in their coagulotoxic activities remain poorly understood. We conducted comprehensive coagulation factor activation studies across all four species in the clade: Eristicophis macmahoni (Ema), Pseudocerastes fieldi (Pfi), P. persicus (Ppe), and P. urarachnoides (Pur). Plasma clotting assays revealed extreme variation in coagulation speed (17.3-376.1 s), with E. macmahoni and P. urarachnoides emerging as rapid coagulators. Fluorometric zymogen activation studies demonstrated that all species require Factor Va as a cofactor for optimal activity, with species-specific preferences for either the endogenous form of Factor Va produced by circulating thrombin or the FVa produced by the venom directly cleaving Factor V. This suggests that the venoms cleave FV at sites distinct from the cleavage site utilised by endogenous thrombin. The species had dramatically distinct patterns of factor activation: Factor XII was most strongly activated factor, and had a venom rank order of Pur ≫ Ema > Pfi = Ppe; Factor X was the next most potently activated factor, with a venom rank order of Ema = Pfi > Pur > Ppe; followed by Factor VII Pfi > Ppe > Ema = Pur; and prothrombin (Factor II) Ema > Pur, with activation by Pfi or Ppe not detectable. None of the venoms activated Factor XI. Notably, despite being suggested as a bird-specialist, P. urarachnoides exhibiting the highest Factor XII activation capacity is significant as avians have secondarily lost FXII as a component in their coagulation cascade. As such, the potent activation of FXII suggest non-avian prey species such as lizards or small rodents make up a significant proportion of this species diet. Antivenom efficacy studies revealed species-specific patterns, with the VINS Jorven antivenom showing superior neutralization against P. urarachnoides and Eristicophis macmahoni relative to the Inosan Inoserp-MENA antivenom and National Antivenom and Vaccine Production Centre of Saudi Arabia antivenom. These toxicology findings reveal that despite morphological similarity and shared ecological niches, desert vipers have evolved fundamentally distinct coagulotoxicity approaches to prey immobilization. These species-specific pathophysiological profiles have important clinical implications for antivenom development and bite treatment protocols in endemic regions.