Differential N-acetylation of alpha-MSH and beta-endorphin in the intermediate pituitary of the turtle, Pseudemys scripta.

Steady-state analyses of the intermediate pituitary of the turtle, Pseudemys scripta, indicated that alpha-MSH-sized immunoreactive forms and beta-endorphin-sized immunoreactive forms are major end products of melanotropic cells. Three forms of alpha-MSH-related immunoreactivity were detected. The two major forms had the same reversed-phase HPLC properties as synthetic N,O-diacetyl-ACTH(1-13)-NH2 and N-acetyl-ACTH(1-13)-NH2. These forms accounted for 97% of the total alpha-MSH-related immunoreactivity detected. A minor peak of ACTH(1-13)-NH2 was also detected. Multiple forms of beta-endorphin-related immunoreactivity were detected, which varied in net positive charge (+1 to +5), apparent molecular weight (2.4 to 3.5 kDa), and degree of N-terminal acetylation. Although N-acetylated forms of beta-endorphin were detected in the turtle intermediate pituitary, the major forms of turtle beta-endorphin were nonacetylated. These features of the turtle intermediate pituitary POMC-specific N-acetylation mechanism are similar to, yet distinct from, the POMC N-acetylation mechanisms observed for mammals. These data suggest that POMC-specific N-acetylation mechanisms were present in reptiles prior to the divergence of the anapsid and synapsid lines.