Biosynthesis and processing of the N-terminal part of proopiomelanocortin in Xenopus laevis: characterization of gamma-MSH peptides.

The aim of this study was to determine the terminal products of processing of the N-terminal part of proopiomelanocortin (POMC) in pituitary melanotrope cells of Xenopus laevis. Biosynthetic in vitro labelling studies showed that POMC is rapidly processed to form N-terminal peptides with an estimated molecular mass of 18 kDa, 9 kDa and 4 kDa. All peptides were released into the medium, indicating that they are processing end products. An antiserum was raised against the synthetic N-terminal eight amino acids of the putative Xenopus gamma-MSH which is present in the N-terminal part of POMC. With immunocytochemistry we demonstrated that gamma-MSH-immunoreactive material in the pituitary gland is restricted to the pars intermedia. A radioimmunoassay in combination with reversed-phase HPLC revealed the presence of at least two gamma-MSH-like peptides. Complete purification followed by electrospray ionization mass spectrometry and amino acid sequence determination showed that these peptides are gamma 1-MSH and glycosylated gamma 3-MSH. The amounts of these gamma-MSH peptides were low compared to the other POMC-derived peptides, alpha-MSH and beta-endorphin. Only 10% of POMC is processed into gamma-MSH peptides and the 4 kDa peptide, leaving the 18 kDa and 9 kDa peptides as the major end products.