Structure of the saxitoxin binding site at sodium channels in nerve membranes. Exchange of tritium from bound toxin molecules.

The exchange of tritium into water from saxitoxin molecules that were radiolabeled at the C-11 methylene position was measured at 37 degrees in solution and in suspensions of brain membranes. High concentrations of membrane receptors were used to assure that more than 80% of the total saxitoxin (STX) present was specifically bound. The amount of back-exchanged tritium was determined either by measuring the radioactivity remaining in the STX, using a second binding assay, or by measuring the tritium in water using ion-exchange chromatography. The results show that the back-exchange is accelerated in the presence of the membranes, and that this is attributable solely to the nonspecific toxin binding. Little change in the back-exchange rate over that in solution occurs in specifically bound toxin molecules. These results place certain restrictions on the possible bonds and configurations of receptor-toxin complexes.