Vipoxin. A protein from Russell's viper venom with high affinity for biogenic amine receptors

We have purified a small, basic protein with high affinity and selectivity for biogenic amine receptors to apparent homogeneity from the venom of Russell's viper (Vipera russelli). This protein, which we designate "vipoxin," has Mr = 13,000, and appears to exist in solution as a single polypeptide chain. It may contain 2 atypical amino acids. Vipoxin inhibits in a dose-dependent manner the binding of 3H-ligands to biogenic amine receptors, with apparent Ki values of 3 nM at alpha 1-adrenergic receptors, 5 nM at alpha 2-adrenergic receptors, 15 nM at dopamine receptors, and 32 nM at serotonin receptors. At concentrations up to 1 microM, vipoxin is inactive at beta-adrenergic, histamine, nicotinic cholinergic, muscarinic cholinergic, adenosine, gamma-aminobutyric acid, benzodiazepine, or opiate receptor binding sites. The effect of vipoxin is essentially irreversible over 20 h at alpha 1- and alpha 2-adrenergic receptors and serotonin receptors and is only slightly reversible at dopamine receptors. Norepinephrine protects alpha-adrenergic receptors from inhibition by vipoxin, while dopamine does not. Vipoxin has no protease activity but does have phospholipase A2 activity, which cannot account for its action on receptors, since receptor binding is assayed in the presence of 1 mM CoSO4 which completely and selectively inhibits the phospholipase activity. Other phospholipases A2 in the same venom lack vipoxin's action on receptors. In physiologic experiments, vipoxin behaves as an agonist at alpha 2-adrenergic receptors in the rat vas deferens and is over an order of magnitude more potent than norepinephrine itself. At alpha 1-adrenergic receptors, it is neither a simple agonist nor an antagonist, but selectively potentiates norepinephrine. Vipoxin may be a useful tool for biogenic amine receptor characterization.