Purification and characterization of joining peptide and N-terminal peptide of proopiomelanocortin from the pars distalis of the bullfrog pituitary.

The joining peptide (JP) and the N-terminal peptide of proopiomelanocortin (NPP) were isolated from an acid-acetone extract of the distal lobe of the pituitary of the bullfrog, Rana catesbeiana, and purified by gel filtration and reverse-phase high performance liquid chromatography. The amino acid sequence of the bullfrog JP resembled the sequences of the JPs of Rana ridibunda (86% similarity) and Xenopus laevis (54% similarity), as deduced from the nucleotide sequences of their cDNAs. The amino acid sequence of bullfrog NPP showed 100%, 85%, and 50% similarity with those of Rana ridibunda, Xenopus laevis, and human NPPs, respectively. Administration of bullfrog NPP (0.05-5 micrograms/ml) to perifused Rana ridibunda interrenal slices induced a dose-dependent stimulation of corticosterone and aldosterone release. The present results indicate that the primary structure of NPP has been highly conserved during evolution. These data also reveal that NPP, which has no sequence homology with ACTH, exhibits a substantial corticotropic activity.