Many lower vertebrates exhibit colour change in response to the background. A dual hormonal control of colour change by two antagonistic pituitary melanophorotropic hormones was first postulated in amphibia by Hogben and Slome. It is well established that the melanotropins alpha- and beta-MSH are responsible for pigment dispersion in the integumentary melanophore of lower vertebrates and that these molecules are derived from a common precursor protein, proopiocortin, by specific processing within the intermediate lobe. No evidence has been found for an antagonistic hormone in amphibia, although the existence of such a molecule in the pituitary gland of teleost fishes has long been recognized and was termed the melanophore-concentrating hormone by Enami. Early attempts to separate the two hormones proved unsuccessful. Recently, Baker and Ball re-invoked the dual hormone concept, and it has been suggested that a melanin-concentrating hormone (MCH) is synthesized in the hypothalamus of teleosts and stored and released by the neurohyphophysis. We have now isolated a novel peptide from the pituitary of the salmon (Oncorhynchus keta) possessing an antagonistic function to MSH, and we describe here its chemical and biological characteristics.

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Two gonadotropins, GTH I and GTH II, were isolated from pituitaries of spawning coho salmon (Oncorhynchus kisutch) using sequential extractions with ammonium acetate (pH 9.0) and 40% ethanol, precipitation with 80% ethanol, gel filtration chromatography (Sephadex G-100), anion-exchange chromatography (Mono-Q Sepharose), and gel filtration chromatography (Sephadex G-75). Coho salmon GTH I and GTH II stimulated steroidogenesis in vitro in a similar dose-dependent manner when incubated with either ovaries or testes of prepubertal coho salmon. An in vivo bioassay using coho salmon parr demonstrated that coho salmon GTH I and GTH II did not contain thyrotropic activity. Molecular weights were estimated by gel filtration chromatography to be 43,000 and 39,000 for GTH I and GTH II, respectively. Analysis of coho salmon GTH I and GTH II on reversed-phase high-performance liquid chromatography (rpHPLC) revealed that they consist of alpha and beta subunits with N-terminal amino acid residues of Tyr, Gly (alpha, beta of GTH I) and Tyr,Ser (alpha, beta of GTH II). Coho salmon GTH I-beta and GTH II-beta differed from each other in amino acid composition, N-terminal amino acids (Gly vs. Ser), and molecular weights in SDS-PAGE (19,000 vs. 20,000) and had a high degree of chemical similarity to chum salmon GTH I-beta and GTH II-beta, respectively. Specific rabbit antisera to the beta subunits of coho salmon GTH I and GTH II were generated. The observation of two GTHs with distinctly different chemical characteristics in coho salmon is similar to what has previously been found in chum salmon.

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