An Unusual Combination of Peptides from the Skin Glands of Ewing's Tree Frog, Litoria ewingi. Sequence Determination and Antimicrobial Activity

Abstract

The dorsal glandular skin extract of the brown tree frog Litoria ewingi contains six major peptides. The amino acid sequences of the peptides were determined by using a combination of mass spectrometry and automated Edman sequencing. One of the peptides, caerin 1.1 [Gly Leu Leu Ser Val Leu Gly Ser Val Ala Lys His Val Leu Pro His Val Val Pro Val Ile Ala Glu His Leu (NH₂)], has been isolated previously from Australian tree frogs of the genus Litoria, and is a wide-spectrum antimicrobial agent. The roles of the other five peptides in the amphibian skin have not, as yet, been determined. The most abundant peptide, uperin 7.1 [Gly Trp Phe Asp Val Val Lys His Ile Ala Ser Ala Val (NH₂)], shows structural similarity to the uperin group of antimicrobial peptides isolated from toadlets of the genus Uperoleia (except that uperin 7.1 lacks the final four amino acid residues of uperins from the genus Uperoleia). The significance of this finding hinges upon the distant evolutionary relationship of Litoria and Uperoleia. If the structural similarity implies a single evolutionary origin, it also implies a conservation of structural origin for at least 100 million years. Evolutionary convergence is a more likely explanation for the structural similarity of the peptides from the two genera. A related peptide, uperin 7.1.1, lacks the first two amino acid residues from the N-terminal end of uperin 7.1. Uperin 7.1 exhibits minor antimicrobial activity while uperin 7.1.1. is inactive. The three other peptides are caeridin 7.1 [Gly Leu Leu Asp Met Val Thr Gly Leu Leu Gly Asn Leu (NH₂)] and two modified peptides of the tryptophyllin group which we have named tryptophyllin 6.1 [Leu Phe Phe Trp Gly (NH₂)] and 7.1 [Ile Phe Phe Phe Pro (NH₂)]. Tryptophyllins 6.1 and 7.1 are related to similar peptides isolated from the Australian red tree frog Litoria rubella: such peptides may be neurotransmitters or neuromodulators.