Effect of tunicamycin on biosynthesis, processing and release of proopiomelanocortin-derived peptides in the intermediate lobe of the frog Rana ridibunda.

The intermediate lobe of the pituitary gland synthesizes a glycoprotein, proopiomelanocortin (POMC), which is cleaved by specific proteolytic enzymes to generate several hormonal peptides. The purpose of the present study was to examine the possible role of the carbohydrate moiety in the synthesis, intracellular processing and release of POMC-derived peptides in frog (Rana ridibunda) intermediate lobe cells. In vitro incorporation of [3H]-labelled glucosamine gave rise to three major radioactive products. Trypsin digestion of each of these glycopeptides gave a single glucosamine-labelled tryptic fragment with identical chromatographic characteristics. We conclude that Rana POMC is glycosylated in only one site (its gamma-MSH region) and that intracellular processing of this prohormone gives rise to smaller glycopeptides including glycosylated gamma-MSH. Treatment with the antibiotic tunicamycin (10 micrograms/ml, 6 hr) inhibited the glycosylation of POMC but did not significantly alter the neosynthesis of the peptide moiety of the precursor. Pulse-chase experiments combined with high-performance liquid chromatography analysis of the peptides derived from POMC revealed that inhibition of glycosylation by tunicamycin had no effect on the enzymatic cleavage of the precursor nor on the release of mature peptides. Thus, it is concluded that, in the frog, glycosylation of POMC has no influence on the biosynthesis, processing and release of intermediate lobe hormones.