A novel post translational modification involving bromination of tryptophan - Identification of the residue, L-6-bromotryptophan, in peptides from Conus imperialis and Conus radiatus venom

We report a novel post-translational modification involving halogenation of tryptophan in peptides recovered from the venom of carnivorous marine cone snails (Conus). The residue, L-6-bromotryptophan, was identified in the sequence of a heptapeptide, isolated from Conus imperialis, a worm-hunting cone, This peptide does not elicit gross behavioral symptoms when injected centrally or peripherally in mice, L-6-Bromotryptophan was also identified in a 33-amino acid peptide from Conus radiatus; this peptide has been shown to induce a sleep-like state in mice of all ages and is referred to as bromosleeper peptide, The sequences of the two peptides Pca-Cys-Gly-Gln-Ala-Trp*-Cys-NH2 [GRAPHICS] were determined using a combination of mass spectrometry, amino acid, and chemical sequence analyses, where Pca = pyroglutamic acid, Hyp = hydroxyproline, Gla = gamma-carboxyglutamate, and Trp* = L-6-bromotryptophan, The precise structure and stereo chemistry of the modified residue were determined as L-6-bromotryptophan by synthesis, co elution, and enzymatic hydrolysis experiments, To our knowledge this is the first documentation of tryptophan residues in peptides/proteins being modified in a eukaryotic system and the first report of halogenation of tryptophan in vivo.