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β-Endorphin-and -melanotrophin (-MSH)-related peptides were extracted from the pars intermedia of Xenopus laevis maintained for 2, 4 or 6 weeks on a white background and for the same periods on a black background. The peptides were resolved under dissociating conditions by gel exclusion chromatography on Sephadex G-50 and they were detected by radioimmunoassay with antibodies to β-endorphin, ,N-acetyl β-endorphin and -MSH. The β-endorphin-related peptides separated into two fractions of different molecular size. Further purification of the peptides in each fraction was by ion exchange chromatography on SP-Sephadex C-25 and by high-pressure liquid chromatography. The -MSH-related peptides were resolved by gel exclusion and ion exchange chromatography. The purified β-endorphin- and -MSH-immunoreactive peptides were identified by comparison of their chromatographic properties with the corresponding peptides from porcine pituitary or by comparison with synthetic peptides.

The major form of β-endorphin in the pars intermedia of the frog adapted to a white background was identified as ,N-acetyl β-endorphin (1–8); it was accompanied by a small quantity of acetylated peptides with molecular size similar to β-endorphin. In contrast, the pars intermedia of the frogs adapted to a black background contained approximately equal amounts of ,N-acetyl β-endorphin (1–8) and the larger forms of β-endorphin. The higher molecular weight forms were identified as the ,N-acetyl derivatives of β-endorphin (1–26), (1–27) and (1–31); however after 6 weeks of white adaptation the sole remaining peptide in this group was the 26-residue peptide. An additional β-endorphin immunoreactive peptide, provisionally identified as β-endorphin (10–26), was present in both black- and white-adapted animals; the amounts of this peptide increased during white adaptation. Major differences in the processing of -MSH were also observed. In the frogs adapted to a black background des-acetyl -MSH greatly predominated over the acetyl form whereas after 6- weeks adaptation to a white background the acetylated peptide proved to be the principal component.

The results demonstrate that the proteolytic processing of β-endorphin and the acetylation of -MSH in Xenopus laevis are influenced by background adaptation. The formation of β-endorphin (1–8) appears to reflect the action of an endopeptidase that acts at the single arginine residue present at position 9. This cleavage does not appear to take place in mammalian β-endorphins where position 9 is occupied by lysine.

Journal of Endocrinology (1992) 135, 469–478