Ammodytin L, an Inactive Phospholipase A2Homologue with Myotoxicity in Mice, Binds to the Presynaptic Acceptor of the β-Neurotoxic Ammodytoxin C inTorpedo:An Indication for a Phospholipase A2Activity-Independent Mechanism of Action of β-Neurotoxins in Fish?

A Ser48 phospholipase A2-homologue, ammodytin L, which is myotoxic in mammals and devoid of any phospholipase A2activity, completely inhibits the specific binding of the neurotoxic phospholipase A2, ammodytoxin C, to fish presynaptic membranes fromTorpedo marmorataelectric organ. In cross-linking experiments,125I-ammodytin L labels the same membrane proteins as125I-ammodytoxin C (70, 38.5-57.4 and 19.7 kDa). The formation of these adducts is completely prevented by the presence of ammodytoxin C but not of a non-toxic phospholipase A2, ammodytin I2. A chimeric phospholipase A2, constructed by associating the N-terminal half of ammodytoxin to the C-terminal half of ammodytin L, possesses a low, but significant phospholipase A2activity, however it is not toxic to mice, probably due to abolition of the specific neuronal acceptor binding in mammals. Nevertheless, the chimeric phospholipase A2is able to interact with the ammodytoxin acceptor inTorpedo marmorataelectric organ. The existence of neuronal acceptors for ammodytin L and for the chimeric phospholipase A2suggests that they may act as neurotoxins in fish. As ammodytin L does not posses any enzymatic activity it, therefore, appears to be an excellent tool to investigate the mechanism of action of β-neurotoxins independently of their phospholipase A2activity.