Maleimide Conjugates of Saxitoxin as Covalent Inhibitors of Voltage-Gated Sodium Channels

(+)-Saxitoxin, a naturally occurring guanidinium poison, functions as a potent, selective, and reversible inhibitor of voltage-gated sodium ion channels (Na(V)s). Modified forms of this toxin bearing cysteine-reactive maleimide groups are available through total synthesis and are found to irreversibly inhibit sodium ion conductance in recombinantly expressed wild-type sodium channels and in hippocampal nerve cells. Our findings support a mechanism for covalent protein modification in which toxin binding to the channel pore precedes maleimide alkylation of a nucleophilic amino acid. Second-generation maleimide-toxin conjugates, which include bioorthogonal reactive groups, are also found to block channel function irreversibly; such compounds have potential as reagents for selective labeling of Na(V)s for live cell imaging and/or proteomics experiments.