Structural characteristics of the saxitoxin receptor on nerve.

The effects of uranyl ion (UO22+; at low concentrations binds specifically to phosphate groups) and the cationic dye methylene blue (MB+; binds strongly to carboxyl groups) on saxitoxin (STX) potency in crayfish axon has been studied by means of intracellular microelectrodes. At pH 6.00 +/- 0.05 and 13.5 mM Ca2+, addition of 10.0 muM UO22+ + 5.0 nM STX had only slightly, if any, less effect on the spike's maximum rate of rise [0.79 +/- 0.04 (viz., mean +/- SEM) of control value] than did addition of 5.0 nM STX alone (0.72 +/- 0.05). Under the same conditions of pH and Ca2+ concentration, 1.0 mM MB+ had approximately the same effect: 1.0 mM MB+ + 5.0 nM STX, 0.76 +/- 0.03; 5.0 nM STX alone, 0.70 +/- 0.04. However, at pH 7.00 +/- 0.05 and lower Ca2+ concentrations, 1.0 mM MB+ significantly reduced STX potency. Using 6.0 mM Ca2+: 1.0 mM MB+ + 5.0 nM STX, 0.92 +/- 0.01; 5.0 nM STX alone, 0.68 +/- 0.08. Using 3.0 mM Ca2+, the corresponding values were 0.94 +/- 0.03 and 0.67 +/- 0.04. It is concluded that: (1) In accord with previous suggestions, the ionized acidic group known to exist in the Na channel (and to which a guanidinium group of STX appears to bind) is very likely a carboxyl group and not a phosphate group. (2) The accessible part of the Na channel mouth serving as the saxitoxin receptor probably does not include phospholipid in its structure proper.