Abstract

A glycine-leucine-rich peptide was isolated from norepinephrine-stimulated skin secretions of the Sante Fe frog Leptodactylus laticeps (Leptodactylidae) whose primary structure (Gly-Leu-Val-Asn-Gly-Leu-Leu-Ser-Ser-Val-Leu-Gly-Gly-Gly-Gln-Gly-Gly-Gly -Gly-Leu-Leu-Gly-Gly-Ile-Leu) contains the (GXXXG)(3) motif found in the plasticins, previously identified only in phyllomedusid frogs (Hylidae). Circular dichroism studies showed that the secondary structure of the peptide, termed plasticin-L1, was markedly solvent-dependent displaying a random coil conformation in water, a beta-sheet structure in methanol, and an alpha-helical conformation in 50% trifluoroethanol-water. A synthetic replicate of the peptide did not inhibit the growth of Escherichia coli or Staphylococcus aureus or lyse human erythrocytes at concentrations up to 500 mu M. At relatively high concentrations (> 1 microM), the peptide produced a significant (P < 0.05), although modest (139% of basal rate at 3 mu M), increase in the rate of glucose-induced release of insulin from rat clonal BRIN-BD11 beta cells without increasing the rate of release of lactate dehydrogenase. A peptide, termed ocellatin-L2 was also identified in the skin secretion that was identical to the previously described ocellatin-L1 except for the substitution Asn(23) to Asp. Ocellatin-L2 was devoid of antimicrobial and hemolytic activity but also showed significant activity in stimulating insulin release from BRIN-BD11 cells (181% of basal rate at 3 microM). (C) 2009 Elsevier Inc. All rights reserved.