Post-translational processing of pro-opiomelanocortin (POMC)-derived peptides during fetal monkey pituitary development. II. β-lipotropin (β-LPH)-related peptides

We have studied the post-translational processing of POMC-derived peptides during fetal monkey pituitary development using immunoassay and reverse-phase high-performance liquid chromatography (RP HPLC). Whole pituitary glands obtained from Day 50 and 55 fetal monkeys and separated lobes from Day 65 to 155 were extracted, fractionated, and analyzed for β-melanotropin (β-MSH), midportion β-endorphin (β-EP), and acetylated β-EP immunoactivity. Separated adult pituitary lobes were analyzed for comparison. At Day 50, POMC-containing cells were located in both the anterior and intermediate pituitary lobes by immunofluorescence staining, the majority of these cells were localized in the anterior lobe. The Day 50 and 55 whole pituitaries contained predominantly β-lipotropin (β-LPH), γ-lipotropin (γ-LPH), β-EP(1-31), and 2.2-kda β-MSH. No acetylated products were found in Day 50 whole pituitary extracts. By Day 55, carboxy-shortened and acetylated β-EPs were barely detectable in whole pituitary extracts. These forms were more apparent in the Day 65 separated neurointermediate lobe (NIL) extracts, and were similar to adult proportions by Day 80. The adult anterior lobe contained predominantly β-LPH, β-EP, and γ-LPH. Adult NILs contained almost exclusively 2.2-kda β-MSH, α-N-acetyl β-EP(1-31) and α-N-acetyl β-EP(1-27). The production of 2.2-kda β-LPH in the monkey NIL indicates that monkey β-LPH is different from rat β-LPH in that it must contain the paired-basic cleavage site required for the formation of 2.2-kda β-MSH that is known to be lacking in rat β-LPH. Another finding was that monkey β-EP contains a Tyr residue at position 27 as found in human β-EP but appears to have the rat Gln substitution at position 31. The post-translational processing patterns characteristic of each lobe were well established by midterm fetal development (Day 80).