Purification, crystallization and preliminary X-ray diffraction analysis of an acidic phospholipase A2 with vasoconstrictor activity from Agkistrodon halys pallas venom

Effects of the Heterodimeric Neurotoxic Phospholipase A2 from the Venom of Vipera nikolskii on the Contractility of Rat Papillary Muscles and Thoracic Aortas

Phospholipases A2 (PLA2s) are a large family of snake toxins manifesting diverse biological effects, which are not always related to phospholipolytic activity. Snake venom PLA2s (svPLA2s) are extracellular proteins with a molecular mass of 13-14 kDa. They are present in venoms in the form of monomers, dimers, and larger oligomers. The cardiovascular system is one of the multiple svPLA2 targets in prey organisms. The results obtained previously on the cardiovascular effects of monomeric svPLA2s were inconsistent, while the data on the dimeric svPLA2 crotoxin from the rattlesnake Crotalus durissus terrificus showed that it significantly reduced the contractile force of guinea pig hearts. Here, we studied the effects of the heterodimeric svPLA2 HDP-1 from the viper Vipera nikolskii on papillary muscle (PM) contractility and the tension of the aortic rings (ARs). HDP-1 is structurally different from crotoxin, and over a wide range of concentrations, it produced a long-term, stable, positive inotropic effect in PMs, which did not turn into contractures at the concentrations studied. This also distinguishes HDP-1 from the monomeric svPLA2s, which at high concentrations inhibited cardiac function. HDP-1, when acting on ARs preconstricted with 10 μM phenylephrine, induced a vasorelaxant effect, similar to some other svPLA2s. These are the first indications of the cardiac and vascular effects of true vipers' heterodimeric svPLA2s.

AhV_aPA-induced vasoconstriction involves the IP₃Rs-mediated Ca²⁺ releasing

AhV_aPA, the acidic PLA₂ purified from Agkistrodon halys pallas venom, was previously reported to possess a strong enzymatic activity and can remarkably induce a further contractile response on the 60 mM K⁺-induced contraction with an EC₅₀ in 369 nM on mouse thoracic aorta rings. In the present study, we found that the p-bromo-phenacyl-bromide (pBPB), which can completely inhibit the enzymatic activity of AhV_aPA, did not significantly reduce the contractile response on vessel rings induced by AhV_aPA, indicating that the vasoconstrictor effects of AhV_aPA are independent of the enzymatic activity. The inhibitor experiments showed that the contractile response induced by AhV_aPA is mainly attributed to the Ca²⁺ releasing from Ca²⁺ store, especially sarcoplasmic reticulum (SR). Detailed studies showed that the Ca²⁺ release from SR is related to the activation of inositol trisphosphate receptors (IP₃Rs) rather than ryanodine receptors (RyRs). Furthermore, the vasoconstrictor effect could be strongly reduced by pre-incubation with heparin, indicating that the basic amino acid residues on the surface of AhV_aPA may be involved in the interaction between AhV_aPA and the molecular receptors. These findings offer new insights into the functions of snake PLA₂ and provide a novel pathogenesis of A. halys pallas venom.