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 proces sing 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 pepti des with an estimated molecular mass of 18 kDa, 9 kDa and 4 kDa. All p eptides were released into the medium, indicating that they are proces sing end products. An antiserum was raised against the synthetic N-ter minal eight amino acids of the putative Xenopus gamma-MSH which is pre sent in the N-terminal part of POMC. With immunocytochemistry we demon strated that gamma-MSH-immunoreactive material in the pituitary gland is restricted to the pars intermedia. A radioimmunoassay in combinatio n with reversed-phase HPLC revealed the presence of at least two gamma -MSH-like peptides. Complete purification followed by electrospray ion ization mass spectrometry and amino acid sequence determination showed that these peptides are gamma(1)-MSH and glycosylated gamma(3)-MSH. T he 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 th e 18 kDa and 9 kDa peptides as the major end products.