Structural characteristics of the saxitoxin receptor on nerve

Summary The effects of uranyl ion (UO ₂ ²⁺ ; 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 13.5mm Ca²⁺, addition of 10.0 m UO ₂ ²⁺ +5.0nm 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 5.0nm STX alone (0.72±0.05). Under the same conditions of pH and Ca²⁺ concentration, 1.0mm MB⁺ had approximately the same effect: 1.0mm MB⁺+5.0nm STX, 0.76±0.03; 5.0nm STX alone, 0.70±0.04. However, at pH 7.00±0.05 and lower Ca²⁺ concentrations, 1.0mm MB⁺ significantly reduced STX potency. Using 6.0mm Ca²⁺: 1.0mm MB⁺+5.0nm STX, 0.92±0.01; 5.0nm STX alone, 0.68±0.08. Using 3.0mm 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.