MS approaches to select peptides with post-translational modifications from amphibian defense secretions prior to full sequence elucidation

Peptide families are characterized by structural motifs, which often comprise specific post-translational modifications (PTMs) required for biological activity. In conventional bioactivity-based peptidomics studies natural peptide mixtures are chromatographically separated and the bioactive fractions purified to homogeneity, prior to structural characterization. In this paper we illustrate the reverse methodology, in which the primary structures of peptides with presumed bioactivity are first determined before investigating functions/bioactivities. We exemplify mass spectrometry (MS)-based strategies (employing, in particular, high resolution MS) to specifically select peptides – from complex mixtures such as frog defensive secretions – by virtue of the occurrence of particular PTMs, including amidation, disulfide-bonding, l- to d-amino acid isomerization, tyrosine-sulfation, proline-hydroxylation, and aminoterminal pyroglutamate formation.