Pro-opiomelanocortin-derived peptides in the pig pituitary: alpha- and gamma 1-melanocyte-stimulating hormones and their glycine-extended forms.

Pro-opiomelanocortin (POMC)-related peptides in extracts of anterior and neurointermediate pituitary lobes from pigs were characterized by gel chromatography, reversed-phase chromatography and radioimmunoassays. The peptide content was ca. 3-fold greater in the anterior lobe compared to the neurointermediate lobe (19.8 nmol POMC/anterior lobe vs 7.0 nmol/neurointermediate lobe). In the neurointermediate lobe 93% of POMC was processed to alpha-melanocyte-stimulating hormone (alpha-MSH) and analogs exclusively of low molecular weight. Most of the remaining adrenocorticotropic hormone (ACTH)-related material consisted of the glycine-extended intermediate ACTH-(1-14) and analogs. In contrast only one fourth to one third of the N-terminal part of POMC (N-POMC) was processed to amidated gamma-MSH and its C-terminal glycine-extended precursor. The relative amount of amidated gamma-MSH was the same as alpha-MSH and analogs (94%). However, more than 95% of these peptides were of high molecular weight. In the anterior lobe 2.3% of N-POMC was processed and 94% was amidated gamma-MSH of only high molecular weight. These results show that gamma-MSH and alpha-MSH are amidated to the same extent and that gamma 1-MSH and gamma 2-MSH immunoreactivity are present in both the anterior lobe and the neurointermediate lobe. The results suggest that the production of amidated peptides is not regulated by the amidation process itself but at an earlier step (e.g. at the proteolytic cleavage).