Inhibition of phosphorylation of rat synaptosomal proteins by snake venom phospholipase A 2 neurotoxins (β-bungarotoxin, notexin) and enzymes ( Naja naja atra, Naja nigricollis)

Abstract

E. Ueno and P. Rosenberg. Inhibition of phosphorylation of rat synaptosomal proteins by snake venom phospholipase A 2 neurotoxins (β-bungarotoxin, notexin) and enzymes ( Naja naja atra, Naja nigricollis). Toxicon 28, 1423–1437, 1990.—Some snake venom neurotoxins, such as β-bungarotoxin (β-BuTX) and notexin, which inhibit the release of neurotransmitter at both peripheral and central presynaptic terminals possess phospholipase A 2 activity. In contrast, most snake venom phospholipase A 2 enzymes such as those isolated from Naja naja atra and Naja nigricollis are structurally homologous to these neutrotoxins but do not have any specific or potent presynaptic action although they have higher enzymatic activities than the neurotoxins. In order to investigate the mechanisms of presynaptic action of the snake venom neurotoxins, we studied their effects on phosphorylation of rat brain synaptosomal proteins. It is known that phosphorylation of synapsin I, a neuron specific and synaptic vesicle associated phosphoprotein, increases neurotransmitter release. Incubation of cerebral cortical synaptosomes with 32P- orthophosphate at 37°C for 30 min, caused significant phosphorylation of a wide mol. wt range of proteins including most markedly those proteins in the mol. wt range (81,000–86,000) of synapsin I. Both snake venom phospholipase A 2 neurotoxins and enzymes (5, 15 and 50 nM) inhibited phosphorylation in a Ca 2+-dependent manner with the following order of potencies: β- BuTX > N. n. atra phospholipase A 2 ≥ notexin > N. nigricollis phospholipase A 2. Five nanomoles of β-BuTX, which has the lowest phospholipase A 2 activity, inhibited phosphorylation of a wide range of mol. wt proteins (51,000–188,000) by 42–58%. At the same concentration, N.n. atra phospholipase A 2 (which possesses the highest enzymatic activity), notexin and N. nigricollis phospholipase A 2 caused less inhibition than β-BuTX, ranging from 0–40% depending on the agent used. These results indicate that there is no correlation between their potencies in inhibiting phosphorylation and the levels of their phospholipase A 2 activities. An inhibitory activity on phosphorylation may be at least partially responsible for a presynaptically-induced block of neurotransmitter release.