Biosynthesis of Two Structurally Different Pro‐opiomelanocortins in the Pars Intermedia of the Amphibian Pituitary Gland

Abstract

The biosynthesis of 2 forms of pro-opiomelanocortin in the pars intermedia of the pituitary gland of the African clawed toad Xenopus laevis is reported. The 2 forms could be resolved by dodecyl sulfate gel electrophoresis on a 9-16% acylamide gradient and their MW were 38,200 and 37,300. Incubation of neurointermediate lobes with [3H]glucosamine followed by tryptic digestion of the newly synthesized glycoproteins, revealed that both prohormones have only 1 glycosylated site, namely within the region corresponding to .gamma.3-melanotropin. Biosynthesis of proteins in lobes treated with tunicamycin to prevent glycosylation again resulted in the production of 2 pro-opiomelanocortins (MW 35,000 and 34,200), indicating that the 2 forms differ in their primary structure. This notion was corroborated by the results of tryptic mapping of the newly synthesized prohormones. The maps showed that the primary structures of the 2 forms of pro-opiomelanocortin differ in at least 2 parts of the molecules, 1 part concerning the endorphin region.