ABSTRACT

β-Endorphin-and α-melanotrophin (α-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, α,N-acetyl β-endorphin and α-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 α-MSH-related peptides were resolved by gel exclusion and ion exchange chromatography. The purified β-endorphin- and α-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 α,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 α,N-acetyl β-endorphin (1–8) and the larger forms of β-endorphin. The higher molecular weight forms were identified as the α,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 α-MSH were also observed. In the frogs adapted to a black background des-acetyl α-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 α-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

Yapsin 1 is an aspartic protease from Saccharomyces cerevisiae and belongs to a class of aspartic proteases that demonstrate specificity for basic amino acids. It is capable of processing prohormone substrates at specific basic residue cleavage sites, similar to that of the prohormone convertases, to generate bioactive peptide hormones. An antibody raised against yapsin 1 was previously shown to immunostain endocrine cells of rat pituitary and brain as well as lysates from bovine pituitary secretory granules demonstrating the existence of yapsin 1-like aspartic proteases in mammalian endocrine tissues, potentially involved in peptide hormone production. Here, we show the specific staining of yapsin 1 immunoreactivity in the α-cells of human pancreatic islets. No staining was observed in the β- or δ-cells, indicating a specificity of the staining for glucagon-producing and not insulin- or somatostatin-producing cells. Purified yapsin 1 was also shown to process proglucagon into glucagon in vitro, demonstrating that the prototypical enzyme of this subclass of enzymes can correctly process proglucagon to glucagon. These findings suggest the existence of a yapsin 1-like enzyme exclusively in the α-cells of the islets of Langerhans in humans, which may play a role in the production of glucagon in that tissue.