Interaction of Presynaptically Toxic Phospholipases A₂ with Membrane Receptors and Other Binding Sites

Abstract

Certain phospholipases A₂ isolated from snake venoms are highly potent neurotoxins acting primarily at the presynatic sites of some peripheral and central synapses Among the most extensively studied are β-bungarotoxin. crotoxin and taipoxin. At the neuromuscular junction. the early effects vary with the toxins and experimental conditions. an initial decrease in neurotransmission followed by an increase being common to most of the presynaptically toxic phospholipases A₂. However the ultimate blockade of transmission is the principal cause of neurotoxicity for these toxins. The phospholipase activity has been generally regarded to be related to, but not by itself sufficient for. the neuromuscular blockade by these toxins in physiological solution. Direct evidence for binding to a receptor is provided by reversible radioligand binding assays coupled with chemical cross-linking and photoaffinity labeling techniques. Specific binding of the presynaptic toxin with high to very high affinity to neuron-specific proteins in the plasma membranes of synaptosomes from the brain is sharply distinguished from the poor binding of the nontoxic phospholipase A₂. It is concluded that strong binding to presynaptic membrane receptors is also necessary for the neurotoxic actions of these neurotoxins in physiological solutions. The two parameters combined are sufficient to explain the presynaptic effects of the toxins. Electrophysiological experiments have indicated that a subtype of voltage-dependent K⁺ channels that is sensitive to dendrotoxins, mast cell degranulating peptide and charybdotoxin also serves as the membrane receptor for β-bungarotoxin. Biochemical studies have confirmed the existence of receptors for the four kinds of toxins. Identificacion, solubilization, and complete purification of the receptor proteins from the brain have been accomplisbed, although microheterogeneity exists The purified receptor proteins have been reconstituted into functional K⁺ channels Crotoxin. taipoxin and several other related phospholipases A₂ bind to more than one membrane sites At least some of them can also bind to the β-bungarotoxin receptor with moderate affinities, and to other sites with higher affinities and with partial overlapping. A small number of additional polypeptides has been identified as possible receptors, with each being shared by certain toxins. Some negatively charged phospholipids may also be involved in the binding of the toxins, at least in the case of crotoxin The norepinephrine uptake system has been implicated as one of the possible receptors for taipoxin and some other toxins.