The adult human pituitary lacks a well defined intermediate lobe, and it is uncertain whether the
POMC cells that remain in the
zona intermedia represent melanotropes or corticotropes. In the present study, we investigated whether the N-acetylated
beta-endorphin- and
alpha-MSH-related
peptides that are characteristically produced by melanotropes in the rat and other species are localized in the human pituitary. Sequential gel filtration and ion exchange HPLC analysis revealed that small amounts of alpha-N-acetyl-beta-endorphin-(1-31), as well as
beta-endorphin-(1-27) and beta-endorphin-(1-26), were detectable in human pituitary extracts, although beta-endorphin-(1-31) was clearly the major form. Consistent with this analysis, low levels of
alpha-MSH, but not N,O-
diacetyl-
alpha-MSH, were identified by reverse-phase HPLC, although again, the desacetyl form of
alpha-MSH predominated. Immunohistochemistry revealed that
N-acetyl-beta-endorphin immunoreactivity was colocalized with
ACTH and
beta-endorphin in a subpopulation of
zona intermedia cells. Unexpectedly, immunoreactive
N-acetyl-beta-endorphin was also observed in a comparable proportion of corticotropes dispersed throughout the anterior lobe.
alpha-MSH immunoreactivity was similarly distributed. These results indicate that N-acetylation is not restricted to the
zona intermedia, suggesting that the strict dichotomy between corticotrope and melanotrope
POMC processing observed in the rat and other species does not extend to the human pituitary.