Biosynthesis of Pairs of Peptides Related to Melanotropin, Corticotropin and Endorphin in the Pars Intermedia of the Amphibian Pituitary Gland

Abstract

This study concerns the biosynthesis of a number of peptides in the neurointermediate lobe of the pituitary gland of the aquatic toad, X. laevis. Using pulse-chase incubations in vitro and high-performance liquid chromatographic [HPLC] analysis, it was shown that these peptides are synthesized through processing of a prohormone, pro-opiomelanocortin; all peptides were released into the incubation medium. On the basis of electrophoretic analysis, selective amino acid incorporation and immunoprecipitation, as well as peptide mapping by HPLC, the peptides were classified into 3 distinct groups: 2 related to MSH, 2 related to ACTH and 2 endorphin-like peptides. Using tryptic and chymotryptic maps of synthetic .alpha.-MSH and des-Ac.alpha.N-.alpha.-MSH as references, one of the MSH-like peptides was identified as des-Ac.alpha.N-.alpha.-MSH; the other one represents neither .alpha.-MSH nor any other known melanotropic peptide. The 2 peptides that were immunologically related to ACTH had characteristics consistent with a structure resembling a peptide previously named corticotropin-like intermediate lobe peptide, corticotropin-(18-39). The 2 endorphin-like peptides, although highly related, do not have the same primary structure. In view of the apparent structural differences between the 2 peptides in each group, the possible occurrence of 2 prohormones is discussed.