Identification of a new high-affinity binding protein for neurotoxic phospholipases A2.

Ammodytoxin C is a neurotoxic phospholipase A2 which blocks the release of neurotransmitter from the nerve terminal. Using a radioiodinated derivative of the toxin, we located its specific high-affinity binding site in the demyelinated P2 fraction of porcine cerebral cortex (Kd = 15 nM; Bmax = 1.5 pmol/mg membrane protein). In cross-linking experiments on a membrane preparation, 125I-ammodytoxin C labeled a protein of 25 kDa. The formation of a specific adduct was not inhibited by nontoxic phospholipases A2 or even by neurotoxic phospholipases A2 which have practically identical pathophysiological activities to ammodytoxin C: agkistrodotoxin, Oxyuranus scutellatus 2 phospholipase A2, taipoxin, beta-bungarotoxin, notexin, and crotoxin. 125I-ammodytoxin C specific cross-linking was inhibited, however, by mannosylated BSA, suggesting the presence of a carbohydrate-recognition domain in the acceptor structure. According to the pharmacological and structural properties, the ammodytoxin acceptor from porcine cerebral cortex differs from other so far identified as phospholipase A2 acceptors and represents a new type of a high-affinity binding protein for neurotoxic phospholipases A2.