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.The frog Xenopus laevis produces a skin secretion which is principally composed of peptides, a number of which are analogous, both in structure and biological activity, to certain mammalian brain and gut hormones [l -31. The amphibian peptides are released by processing of larger polypeptide precursors and some recognition sites for the proteolytic enzymes involved have been established. For example, pairs of basic amino acids [4, 51 and single arginine residues [6] are often, though not always, sites of precursor cleavage. A cysteine protease, which cleaves specifically after hydrophobic-basic amino acid pairs has recently been identified in the secretion [7]. In this laboratory, systematic screening of the Xenopus laevis skin exudate utilizing partial purification by HPLC and characterisation by FAB/MS has shown that, in addition to the hormone-like peptides themselves, other fragments of the precursors remain intact and form major components of the secretion [8,9]. The largest of these peptides have lytic activity and a-helical projections of their sequences give them amphiphilic character [lo, 111, properties shared by the bee venom peptide, melittin [12]. Two highly similar 23-aminoacid peptides, which have been named PGS by this group [13], or the magainins in work subsequently published by Zasloff [lo], were also isolated. Antimicrobial as well as lytic activity has been demonstrated for these amphiphilic species [lo]. It has been speculated that this group of peptides might find applications in human therapy.Convential cDNA-cloning methods have provided the sequence of a common precursor for this pair of peptides [Ill, which is longer than that originally reported by Zasloff [lo]. Within this precursor copies of the active peptides are separated by spacers of more hydrophilic character, also 23 amino acids in length; a portion of these spacers is very rich in the acidic residues, glutamic and aspartic acids. The other published precursors of the Xenopus luevis skin peptides also contain spacers with similar highly acidic regions [14 -181.The earli...
