Site-Specific Antagonists to Tetrodotoxin and Saxitoxin.

Tetrodotoxin TTX and saxitoxin STX are important neurobiological tools because of their unique high-specificity and high-potency blockade of voltage-gated sodium channels. Although chemically different, their actions are identical. The biophysical mechanism of their actions have been exhaustively studied, but the chemical nature of the actions remains unknown. By studying the structure-activity relations of some natural and synthetic analogues of both TTX and STX on the voltage-clamped frog skeletal muscle fiber, we have deduced that the TTXSTX binding site is a pocket 9.5 angstrom width x 6 angstrom height and 5 angstrom depth, located in a fold of the sodium-channel protein, close to the external orifice. There are 7 anchoring sites, a - g, which ion-pairs or hydrogen-bonds with surface groups in the toxinmolecules. TTX and STX share one ion-pairing and 4 H-bonding sites in common. Such a binding site also explains the actions of the TTX analogue, chiriquitoxin, and SSTX analogues, gonyautoxins and sulfocarbamoyl saxitoxins. A specifically labelled HTTX of 2900 Cimol has been synthesized for use in continuing efforts to locate the binding site.