Synthesis and characterization of an N‐terminal‐specific 125I‐photoaffinity derivative of μ‐Conotoxin GIIIA which binds to the voltage‐dependent sodium channel

An N-terminal, iodinatable photoaffinity derivative of mu-Conotoxin GIIIA, 4-Azido-salicylyl-mu-Conotoxin GIIIA (CTXASA), was synthesized by solid phase peptide synthesis. The binding of 125I-CTXASA to the voltage dependent sodium channel from electroplax of Electrophorus electricus was specific, as demonstrated by saturation binding experiments. Using autoradiography, 125I-CTXASA labeled a protein with a molecular mass of 260 kDa, consistent with the apparent molecular mass of the sodium channel. This labeling could be suppressed by excess of tetrodotoxin and mu-Conotoxin GIIIA.