Biosynthesis of Pairs of Peptides Related to Melanotropin, Corticotropin and Endorphin in the Pars Intermedia of the Amphibian Pituitary Gland

Abstract

This study concerns the biosynthesis of a number of peptides in the neurointermediate lobe of the pituitary gland of the aquatic toad, Xenopus laevis. Using pulse-chase incubations in virro and high-performknce liquid chromatographic analysis, it could be shown that these peptides are synthesized through processing of a prohormone, pro-opiomelanocortin; all peptides were released into the incubation medium. On the basis of electrophoretic analysis, selective amino acid incorporation and immunoprecipitation, as well as peptide mapping by high-performance liquid chromatography, the peptides were classified into three distinct groups : two related to melanocyte-stimulating hormone (melanotropin), two related to adrenocorticotropic hormone (corticotropin) and two endorphin-like peptides. Using tryptic and chymotryptic maps of synthetic cr-melanotropin and des-AcaNa-melanotropin as references, one of the melanotropin-like peptides was identified as des-AcaN-a-melanotropin; the other one represents neither a-melanotropin nor any other known melanotropic peptide. The two peptides that were immunologically related to corticotropin had characteristics consistent with a structures resembling a peptide previously named 'corticotropin-like intermediate lobe peptide', corticotropin-(18 -39). The two endorphin-like peptides, although highly related, do not have the same primary structure. In view of the apparent structural differences between the two peptides in each group, the possible occurrence of two prohormones is discussed.Many amphibians, when placed on a black background, respond by dispersing the melanin granules in the dermal melanophores of their skin, thus causing the animal to darken. This phenomenon, known as background adaptation, is mediated by melanotropin from the pars intermedia of the pituitary gland. It has been demonstrated by a number of investigators that the amphibian pars intermedia produces not one, but several melanotropic peptides [l -S ] . The pars intermedia from black-background-adapted South African clawed toads, Xenopus luevis, when incubated in vitro, synthesizes a precursor; this precursor is subsequently processed to a number of smaller peptides, including some melanotropic ones [4,5]. Such a precursor-product mode of peptide biosynthesis is also found in mammalian pituitary glands. A common prohormone to corticotropin and P-endorphin has been established in a mouse pituitary tumor cell line [6][7][8], as well as in the pars intermedia and the anterior lobe of the rat pituitary gland [9,10]. The circumstance whereby several products, each with its own specific bioactivity, are derived from a common prohormone raises important questions concerning the regulation of both the biosynthesis and the release of such products. For X . luevis, aspects of the control of product release from the neurointermediate lobe, particularly the effect of dopamine and the possible involveAbbreviations and Trivial Names. Melanotropin, melanocyte-stimulating hormone; corticotropin, adrenocorticotropic hormone; H...

Adrenocorticotropic Hormone in Relation to Interrenal Function during Stress in Tilapia (Oreochromis mossambicus)

“…The fact that both tilapia forms are released into the medium might indicate differential processing of POMC in the corticotropes of this species, possibly leading to phosphorylated and nonphosphorylated forms (Ben nett et al, 1983) or to molecules of varying size (e.g., by carboxypeptidase activity; Friedman et al, 1993). The possibility that one of the forms represents POMC can be excluded, since the prohormone elutes at 41 min in the HPLC system used (Martens et al, 1982). Different degrees of glycosylation appear unlikely on the basis of the data by Iturriza and Estivariz (1986).…”

“…Reverse-phase high-performance liquid chromatog raphy was performed as described by Martens et al (1982). To the fractions collected, 10 |xg BSA was added, and the samples were dried in a Speedvac (Sa vant).…”

The development of the pars intermedia and its role in the regulation of dermal melanophores in the larvae of the amphibian Xenopus laevis

“…Analysis of biosynthetic events in neurointermediate lobes of X. laevis showed that the production of melanotropins has the characteristics of a precursor-product relationship (Loh and Gainer, 1977;Jenks and van Overbeeke, 1980). In vitro and in vivo studies by Martens et al (1980Martens et al ( , 1982aMartens et al ( -c, 1983 revealed that in Xenopus the biosynthesis commences with the production of a prohormone, pro-opiomelanocortin (POMC) which is subsequently processed to yield three melanotropic peptides, namely, a-MSH, des-N-a-acetyl-ol-MSH, and a y,-MSH-like peptide, as well as several other peptides, including two forms of a corticotropin-like intermediate peptide (CLIP) and two peptides structurally related to the endogenous opiate B-endorphin. Analysis of release showed spontaneous secretion of all these peptides with the exception of des-N-a-acetyl-a-MSH which was acetylated to form a-MSH during the release process (Martens et al, 1981a).…”

Evidence for a proenkephalin-like precursor in amphibian brain.

“…Evidence already has been provided for two distinct proopiomelanocortin-related precursors in the pituitary of fish and amphibians (31,32 [Met]enkephalinArg6-Gly7-Leu8 was detected, significant octapeptide immunoreactivity (>10 pmol/g) was present in higher molecular weight peptide(s) (Mr 2,000-3,000). Thus, avian proenkephalin is also closely related to the mammalian precursor.…”