Differential acetylation of pro-opiomelanocortin-derived peptides in the pituitary gland of Xenopus laevis in relation to background adaptation

Abstract

Abstract Immunocytochemical analysis revealed the presence of acetylated endorphins in both melanotropes and corticotropes of the pituitary gland of Xenopus laevis. Chemical acetylation studies to determine the steady-state level of acetylated versus non-acetylated endorphins showed that virtually all endorphins are acetylated in both melanotropes and corticotropes. Apparently Xenopus is unique among vertebrates as non-acetylated endorphins are major endproducts in the distal lobe of all other vertebrate species studied thus far. The dynamics of endorphin biosynthesis in melanotrope cells using pulse-chase analysis coupled to immunoaffinity chromatography revealed that processing of pro-opiomelanocortin to produce N-terminalacetylated endorphins is very rapid. To determine the effect of long-term background adaptation on acetylation status of endorphins and α-MSH-related peptides, Xenopus laevis were adapted for 3 or 6 weeks to either a black or a white background. In both physiological states the major intracellular form of α-MSH-related peptides in melanotropes was desacetyl α-MSH while the major endorphin-related peptide was α,N-acetyl-β-endorphin[1–8]. In the medium of superfused neurointermediate lobes of black background-adapted animals the major form of secreted melanotropins and endorphins was α-MSH and α,N-acetyl-β-endorphin[1–8] respectively. We conclude that there is a marked spatio-temporal difference in acetylation of melanotropin and endorphins, with rapid intracellular acetylation of endorphins while melanotropin is acetylated at the time of its exocytosis. In the medium of superfused neurointermediate lobes of white background-adapted animals the amount of desacetyl α-MSH was much higher than in the medium of lobes of black-adapted animals. Therefore, the secretory signals from melanotrope cells of black- and white-adapted Xenopus appear to differ with respect to the degree of acetylation of the melanotropins. This difference may underlie the strategy of Xenopus to regulate dermal melanophore activity during physiological background adaptations. Journal of Endocrinology (1995) 146, 159–167