Identification of highly acidic peptides from processing of the skin prepropeptides of Xenopus laevis

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Abstract
The skin secretion of the frog Xenopus laevis has been fractionated by reverse‐phase HPLC and the most polar components studied by fast‐atom‐bombardment mass spectrometry (FAB/MS). Esterification of the hydrophilic peptides with methanol and ethanol was employed to improve the sensitivity of the technique. A number of small, highly acidic peptides have been identified, and alcoholysis of the peptide bonds within a number of these permitted their sequencing by FAB/MS. The sequences confirmed that they originate from acidic spacer regions found in the precursors to peptide hormones, such as caerulein, which have already been found in the secretion. In addition, acidic peptides derived from the spaces of the precursor to the antimicrobial peptides, PGS (or the magainins) have been isolated. The release of these from the preproprotein cannot be fully accounted for by documented processing mechanisms, suggesting that a novel type of cleavage site has been identified.