Photoaffinity labeling of the electroplax sodium channel with tetrodotoxin derivatives. II. Comparison of the photoreactivity of different photoactivable groups in the tetrodotoxin binding site.

Four photoactivable tetrodotoxin (TTX) derivatives and the corresponding tritiated compounds were synthesized and purified. The photoactivable groups introduced were a nitro-azidophenyl (NAP) group and a trifluoromethyl diazirinobenzoyl (TDB) group, as nitrene and carbene precursors, respectively, as well as two isomers of the fluoro-nitrophenoxy group, a new photoreactive group selective for nucleophiles. The binding affinities of the four derivatives to the sodium channel were similar to that of TTX, but the values of specific photoincorporation were unexpectedly low (up to 2.5%), suggesting that the photoactivable groups are likely to be oriented unfavorably for labeling reactions in the TTX binding site. Two of the labeled sodium channel proteins were purified and digested. Peptides labeled with TTXenNAP lost most of the label during high performance liquid chromatography in 0.1% trifluoroacetic acid-acetonitrile, while peptides labeled with TTXenTDB retained the labels during the procedures. The TTXenTDB-labeled peptides were eluted in four major fractions with 80% recovery of the amount applied on a reverse-phased C4 column. However, further purification is necessary to identify the labeled sites of the peptides.