Binding to saxitoxin to electrically excitable neuroblastoma cells.

Saxitoxin inhibits the action potential Na+ ionophore of electrically excitable neuroblastoma cells with a KI of 3.7 nM. Binding experiments detect a single class of saturable binding sites with KD = 3.9 nM and a binding capacity of 156 fmol/mg of cell protein (78 sites per micrometer2 of cell surface). Saturable binding is completely inhibited by tetrodotoxin but is unaffected by scorpion toxin or batrachotoxin. No saturable binding is observed in cultures of clone N103, a variant neuroblastoma clone lacking the action potential Na+ response. Thus, saxitoxin binds specifically to the action potential Na+ ionophore in neuroblastoma cells. Comparison of saxitoxin and scorpion toxin binding reveals that there are three saxitoxin receptor sites for each scorpion toxin receptors site. The implications of this stoichiometry are considered.