Differential Mechanisms for the N-Acetylation of Alpha-Melanocyte-Stimulating Hormone and Beta-Endorphin in the Intermediate Pituitary of the Frog, Xenopus laevi s

Abstract

Immunohistochemical analysis of the pituitary of Xenopus laevis revealed the colocalization of α-melanocyte-stimulating-hormone (MSH)-related immunoreactivity and N-acetyl-β-endorphin-related immunoreactivity in the cells of the intermediate pituitary. In order to determine whether the immunoreactive N-acetylated β-endorphin is released in parallel with the immunoreactive α-MSH, intermediate pituitaries were incubated in L-15 medium for 24 h. The medium and an acid extract of the intermediate pituitaries from each incubation were separately fractionated by a combination of gel filtration chromatography, reverse-phase high-performance liquid chromatography, and cation exchange chromatography. In the intermediate pituitary extract, the major form of α-MSH had chromatographic properties which corresponded to nonacetylated α-MSH (ACTH)(1–13)amide; whereas the major form of β-endorphin had an apparent molecular weight of 1.2 kDa and was N-acetylated. The 1.2-kDa form of β-endorphin and ACTH(1–13)amide were present in equimolar amounts. Analysis of the medium indicated that both end products were released in parallel. However, as reported in the literature, there was a significant increase in the N-acetylation of ACTH(1–13)amide during secretion. There was no further processing of β-endorphin during secretion. Collectively, these observations indicate that in the intermediate pituitary of X. laevis there are separate mechanisms for the N-acetylation of α-MSH and β-endor-phin.