Isolation and structural characterization of peptides related to α- and γ-melanocyte-stimulating hormone (MSH) from the frog brain

Peptides that are derived from the processing of proopiomelanocortin were isolated in pure form from the brain of the frog Rana ridibunda. The primary structure of the most abundant of those peptides was established as: Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val. This amino acid sequence is identical to that of mammalian and frog pituitary α-melanocyte-stimulating hormone (MSH) and the peptide co-eluted with synthetic desacetyl α-MSH, indicating that it is COOH-terminally α-amidated. A second component, which exhibited a shorter retention time, co-eluted with the glycine-extended form of desacetyl α-MSH [ACTH(1-14)]. The primary structure of the third peptide isolated in pure form from the brain extract was established as: Lys-Tyr-Val-Met-Ser-His-Phe-Arg-Trp-Asn-Lys-Phe-NH₂. This sequence corresponds to Lys-γ₁-MSH as predicted from the nucleotide sequence of frog proopiomelanocortin. The presence of substantial amounts of desacetyl α-MSH and Lys-γ₁-MSH in the frog brain supports the concept that, in amphibia, melanotropins may act as neurotransmitters and/or neuromodulators as well as hormonal peptides.