ALPHA-MELANOCYTE-STIMULATING HORMONE AND N-ACETYL-BETA-ENDORPHIN IMMUNOREACTIVITIES ARE LOCALIZED IN THE HUMAN PITUITARY BUT ARE NOT RESTRICTED TO THE ZONA INTERMEDIA

The adult human pituitary lacks a well defined intermediate lobe, and it is uncertain whether the POMC cells that remain in the zone interme dia represent melanotropes or corticotropes. In the present study, we investigated whether the N-acetylated beta-endorphin- and alpha-MSH-re lated peptides that are characteristically produced by melanotropes in the rat and other species are localized in the human pituitary. Seque ntial gel filtration and ion exchange HPLC analysis revealed that smal l amounts of alpha-N-acetyl-beta-endorphin-(1-31), as well as beta-end orphhin-(1-27) and beta-endorphin-(1-26), were detectable in human pit uitary 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, althoug h again, the desacetyl form of alpha-MSH predominated. Immunohistochem istry revealed that N-acetyl-beta-endorphin immunoreactivity was coloc alized with ACTH and beta-endorphin in a subpopulation of zona interme dia cells. Unexpectedly, immunoreactive N-acetyl-beta-endorphin was al so observed in a comparable proportion of corticotropes dispersed thro ughout the anterior lobe. alpha-MSH immunoreactivity was similarly dis tributed. These results indicate that N-acetylation is not restricted to the zona intermedia, suggesting that the strict dichotomy between c orticotrope and melanotrope POMC processing observed in the rat and ot her species does not extend to the human pituitary.