INTERACTION OF SUBSTITUTED GUANIDINES WITH THE TETRODOTOXIN-BINDING COMPONENT IN ELECTROPHORUS-ELECTRICUS

In efforts to understand the molecular properties of ion channels in biomembranes, we have investigated the interaction of substituted guanidines with the Na+ channel site in membranes isolated from Electrophorus electricus. This interaction was measured by equilibrium competitive binding studies with [3H]tetrodotoxin ([3H]TTX); TTX has been shown to bind specifically to the Na+ channel in electrically excitable membranes. Although guanidine and small substituted guanidines such as methylguanidine or aminoguanidine competed with [3H]TTX for the membrane binding site, the apparent KI values for these derivatives were nearly seven orders of magnitude higher than the Kd for TTX. On the other hand, the binding of the guanidines was considerably enhanced by introducing a substituent aromatic ring or aliphatic chain. Detailed analysis of the binding of aliphatic guanidines of varying chain length clearly demonstrated the contribution made by hydrophobic interactions. These results suggest that the channel site may include a hydrophobic region in close proximity to the carboxylate previously postulated to be involved in TTX binding. © 1979.