DIFFERENTIAL ACETYLATION OF PRO-OPIOMELANOCORTIN-DERIVED PEPTIDES IN THE PITUITARY-GLAND OF XENOPUS-LAEVIS IN RELATION TO BACKGROUND ADAPTATION

Immunocytochemical analysis revealed the presence of acetylated endorp hins in both melanotropes and corticotropes of the pituitary gland of Xenopus laevis. Chemical acetylation studies to determine the steady-s tate level of acetylated versus non-acetylated endorphins showed that virtually all endorphins are acetylated in both melanotropes and corti cotropes. Apparently Xenopus is unique among vertebrates as non-acetyl ated endorphins are major endproducts in the distal lobe of all other vertebrate species studied thus far. The dynamics of endorphin biosynt hesis in melanotrope cells using pulse-chase analysis coupled to immun oaffinity chromatography revealed that processing of pro-opiomelanocor tin to produce N-terminal-acetylated endorphins is very rapid. To dete rmine the effect of long-term background adaptation on acetylation sta tus 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 b oth physiological states the major intracellular form of alpha-MSH-rel ated peptides in melanotropes was desacetyl alpha-MSH while the major endorphin-related peptide was alpha,N-acetyl-beta-endorphin[1-8]. In t he medium of superfused neurointermediate lobes of black background-ad apted 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 acetylat ion of melanotropin and endorphins, with rapid intracellular acetylati on of endorphins while melanotropin is acetylated at the time of its e xocytosis. In the medium of superfused neurointermediate lobes of whit e background-adapted animals the amount of desacetyl alpha-MSH was muc h higher than in the medium of lobes of black-adapted animals. Therefo re, the secretory signals from melanotrope cells of black- and white-a dapted Xenopus appear to differ with respect to the degree of acetylat ion of the melanotropins. This difference may underlie the strategy of Xenopus to regulate dermal melanophore activity during physiological background adaptations.