Isolation and structural characterisation of a novel 13-amino acid insulin-releasing peptide from the skin secretion of Agalychnis calcarifer

We describe the isolation and characterisation of an insulinotropic pepticle from the skin secretions of Agalychnis calcarifer frogs. Peptides in crude secretions obtained by mild electrical stimulation from the dorsal skin surface were purified by reversed-phase HPLC, yielding fractions in two zones with insulin-releasing activity (p < 0.001). The peaks showing greatest in vitro insulin-releasing activity were subsequently purified to homogeneity, revealing a novel insulinotropic 13-amino-acid (1653.2 Da) peptide with the primary structure RRKPLFPLIPRPK (RK-13). A database search for RK-13 showed 53.8% similarity with the N-terminal region of proline-arginine-rich antimicrobial pepticle (PR-39). Synthetic RK-13 stimulated insulin release in a dose-dependent, glucose-sensitive manner, exerting its effects through a cyclic AMP-protein kinase A pathway independent of pertussis toxin-sensitive G proteins. Unlike PR-39, RK-13 lacks antimicrobial effects on the growth of yeast, and Gram-positive and Gramnegative bacteria. Our data indicate that skin secretions of Agalychnis calcarifer frogs contain insulin-releasing peptides, including RK-13, which merit further investigation as insulin secretagogues.