AMMODYTIN-L, AN INACTIVE PHOSPHOLIPASE-A(2) HOMOLOG WITH MYOTOXICITY IN MICE, BINDS TO THE PRESYNAPTIC ACCEPTOR OF THE BETA-NEUROTOXIC AMMODYTOXIN-C IN TORPEDO - AN INDICATION FOR A PHOSPHOLIPASE A(2) ACTIVITY-INDEPENDENT MECHANISM OF ACTION OF BETA-NEUROTOXINS IN FISH

A Ser48 phospholipase A(2)-homologue, ammodytin L, which is myotoxic i n mammals and devoid of any phospholipase A(2) activity, completely in hibits the specific binding of the neurotoxic phospholipase A(2), ammo dytoxin C, to fish presynaptic membranes from Torpedo marmorata electr ic organ. In cross-linking experiments, I-125-ammodytin L labels the s ame membrane proteins as I-125-ammodytoxin C (70, 38.5-57.4 and 19.7 k Da). The formation of these adducts is completely prevented by the pre sence of ammodytoxin C but not of a non-toxic phospholipase A(2), ammo dytin I-2. A chimeric phospholipase A(2), constructed by associating t he N-terminal half of ammodytoxin to the C-terminal half of ammodytin L, possesses a low, but significant phospholipase A(2) activity, howev er it is not toxic to mice, probably due to abolition of the specific neuronal acceptor binding in mammals. Nevertheless, the chimeric phosp holipase A(2) is able to interact with the ammodytoxin acceptor in Tor pedo marmorata electric organ. The existence of neuronal accepters for ammodytin L and for the chimeric phospholipase A(2) suggests that the y may act as neurotoxins in fish. As ammodytin L does not posses any e nzymatic activity it, therefore, appears to be an excellent tool to in vestigate the mechanism of action of beta-neurotoxins independently of their phospholipase A(2) activity. (C) 1998 Academic Press.