N-acetylation and C-terminal proteolysis of β-endorphin in the anterior lobe of the horse pituitary

Abstract

β-Endorphin is post-translationally processed to both N-acetylated and C-terminally shortened derivatives in the anterior lobe of the horse pituitary, a processing pattern qualitatively different from that of the rat and virtually every other mammalian species. Thus, separation of the molecular forms of β-endorphin using gel filtration and ion exchange chromatography showed that the horse anterior lobe primarily contains β-endorphin-1-31 and N-acetyl-β-endorphin-1-27 along with smaller amounts of β-lipotropin, β-endorphin-1-27, and N-acetyl-β-endorphin-1-31 and -1-26, in contrast to the rat anterior lobe, which contains approximately equal amounts of β-lipotropin and β-endorphin-1-31. Immunohistochemical experiments using an antiserum which specifically recognizes N-acetylated β-endorphin peptides confirmed that N-acetyl-β-endorphin immunoreactivity is present in the anterior lobe of the horse, but not the rat. The intermediate lobe of both species primarily synthesizes N-acetylated, C-terminally shortened β-endorphin peptides, and while distinct species differences do occur, they were relatively minor, consisting of quantitative differences in the relative proportion of each peptide. These results are consistent with earlier reports that β-endorphin processing in the rat pituitary is tissue specific; the anterior and intermediate lobes produce entirely different sets of β-endorphin peptides. In the equine pituitary, however, both pituitary lobes produce the same multiple β-endorphin forms, possessing both opioid and nonopioid properties, although their relative amounts differ.