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

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 end-products 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-terminal-acetylated endorphins is very rapid. To determine the effect of long-term background adaptation on acetylation status of endorphins and alpha-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 alpha-MSH-related peptides in melanotropes was desacetyl alpha-MSH while the major endorphin-related peptide was alpha, N-acetyl-beta-endorphin[1-8]. In the medium of superfused neuro-intermediate lobes of black background-adapted animals the major form of secreted melanotropins and endorphins was alpha-MSH and alpha, N-acetyl-beta-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 alpha-MSH was much higher than in the medium of lobes of black-adapted animals.(ABSTRACT TRUNCATED AT 250 WORDS)