A mass spectrometric assay for novel peptides: application to Xenopus laevis skin secretions

The peptides secreted by the South African frog Xenopus laevis were screened systematically using a strategy based on fast atom bombardment mass spectrometry (FAB-MS). High performance liquid chromatography (HPLC) of crude and Sephadex G-10 chromatographed secretions showed that many more peptides were present in these secretions than those previously identified, i.e., xenopsin, caerulein, TRH and PGLa. Fractions from the HPLC were analyzed directly by FAB-MS to determine the molecular weights of these novel peptides. Subsequent analyses, using a combination of FAB-MS, manual Edman degradation, enzymatic digestions and amino acid analyses, identified the partial and sometimes complete sequences of these peptides which had molecular weights ranging from 700-2,700. Many peptides with structural features that are often indicative of biological activity, e.g., C-terminal amides and pyroglutamic acid, were readily identified by FAB-MS. In some cases, molecular weight data combined with partial sequence data was sufficient to identify peptides as originating from PGLa and the spacer regions in the precursors to xenopsin and caerulein.