Host-defence skin peptides of the Australian Streambank Froglet Crinia riparia: isolation and sequence determination by positive and negative ion electrospray mass spectrometry

A combination of positive and negative ion electrospray mass spectrometry (ES‐MS) together with automated Edman sequencing has been used to determine the amino acid sequences of the host‐defence peptides from the skin glands of the froglet Crinia riparia. The peptides are called riparins. Of the eight peptides isolated, five are neuropeptides containing intramolecular disulfide linkages; e.g. the major peptide riparin 1.4 (FFLPPCAYKGTC‐OH). Positive ion ES‐MS identifies the five residues of riparin 1.4 outside the disulfide moiety, but provides no information on the sequence within the disulfide ring. In contrast, the negative ion dissociations of the [M–H]− ion of riparin 1.4 identify the S‐S link by loss of H2S2 from the [M–H]− ion, and also provide the sequence within the disulfide unit. Other peptides are riparin 2.1 [(IIEKLVNTALGLLSGL‐NH2), a narrow‐spectrum antibiotic], signiferin 3.1 [(GIAEFLNYIKSKA‐NH2), an nNOS inhibitor] and riparin 5.1 [IVSYPDDAGEHAHKMG‐NH2], which shows no neuropeptide, antibiotic or nNOS activity. Copyright © 2006 John Wiley & Sons, Ltd.