REEXAMINATION OF CROTOXIN-MEMBRANE INTERACTIONS

The interaction of crotoxin with synaptic membranes from Torpedo marmo rata has been re-examined, using radioiodinated toxin. In competition experiments, the 'saturable binding' is usually calculated by subtract ing the non-saturable binding, determined in the presence of an excess of unlabelled crotoxin, from total binding. Paradoxically, we observe d a notable increase of the 'saturable binding' of I-125-crotoxin, def ined in this manner in the presence of a high concentration of certain competitors (e.g. crotoxin subunit B, agkistrodotoxin, ammodytoxin an d ammodytin I-2). This potentiation effect was analysed by competition and cross-linking experiments. The dissociation of the basic crotoxin subunit CB, which differs markedly from crotoxin in its membrane- bin ding characteristics, was found to be the main reason for the observed phenomenon. I-125-CB could be released from I-125-crotoxin by exchang e with a molecule having sufficient affinity towards CA (e.g. CB or ag kistrodotoxin) or it could be dissociated from the specific crotoxin m embrane-binding site(s) by a competitor molecule. Our results, therefo re, suggest a reinterpretation of previous I-125-crotoxin binding stud ies. Copyright (C) 1996 Elsevier Science Ltd