Differential effects of snake venom phospholipase A 2 neurotoxin (β-bungarotoxin) and enzyme ( Naja naja atra) on protein kinases

Abstract

The phospholipase A 2 (PLA 2) neurotoxin, β-bungarotoxin (β-BuTX), presynaptically alters acetylcholine release. We previously found that β-BuTX inhibits protein phosphorylation in rat brain synaptosomes. This inhibition was not due to the inhibition of ATP synthesis, the action of arachidonic acid (AA) metabolites, or the stimulation of phosphatase activities. A typical PLA 2 enzyme from Naja naja atra ( N. n. atra) venom also inhibited phosphorylation but with lesser potency than that of β-BuTX. We now report the effects of β-BuTX and N. n. atra PLA 2 on the activities of protein kinases. Treatments of synaptic plasma membrane or cytosol with N. n. atra PLA 2 stimulated the activities of cAMP-dependent kinase, Ca 2+/calmodulindependent kinase II, and protein kinase C (PKC), whereas β-BuTX had no effect on these kinases. Calyculin A, a phosphatase-1 and -2A inhibitor, increased the stimulation of phosphorylation by N. n. atra PLA 2, indicating that the stimulation is not due to an inhibition of phosphatase activities. The stimulation of PKC by N. n. atra PLA 2 appears to be mediated by free fatty acids (FFAs) resulting from phospholipid hydrolysis by PLA 2, since (1) treatment of either synaptic plasma membrane or cytosol with N. n. atra PLA 2 produced large amounts of FFAs, and (2) AA, an exogenous FFA, stimulated PKC activity to an extent similar to that caused by N. n. atra PLA 2. Thus, the mechanisms of action of β-BuTX and N. n. atra PLA 2 appear quite different from each other although both agents inhibit phosphorylation in intact synaptosomes.