Peptides derived from pro-opiocortin in the pituitary gland of the dogfish, Squalus acanthias

Abstract

Combined rostral, median and neurointermediate lobes from 650 pituitary glands of the dogfish Squalus acanthias were extracted in 0·1 m-HCl and subjected to filtration on Sephadex G-50. Fractions were monitored for ACTH, α-MSH, γ-MSH and endorphin by heterologous radioimmunoassay, corticotrophin-like intermediate lobe peptide (CLIP) and γ-MSH by homologous radioimmunoassay, and methionine enkephalin after enzymatic digestion. The majority (about 99%) of the immunoreactivity detected was present as small peptides, with α-MSH, CLIP and γ-MSH predominant. The single peaks of ACTH, α-MSH and CLIP contrasted with many distinct peaks of endorphin-like immunoreactivity. The γ-MSH radioimmunoassays monitored different peptides; the heterologous assay revealed a single major peak, while the homologous assay detected a number of distinct small peptides. The results suggested that there may be more than three distinct forms of MSH in the dogfish pituitary gland. Small amounts of much larger proteins were detected by a number of the radioimmunoassays, suggesting that peptides related to ACTH, γ-MSH and lipotrophin are derived from common pro-opiocortin-type precursor molecules in this phylogenetically ancient cartilaginous fish.