In vivo biosynthesis of melanotropins and related peptides in the pars intermedia of Xenopus laevis.

To study in vivo biosynthesis of pars intermedia peptides in Xenopus laevis, [3H]lysine was administered by an osmotic minipump via a cannula inserted near the pituitary gland. Following extraction of the neurointermediate lobe, high-performance liquid chromatography was used to separate the newly synthesized peptides. In black-background adapted animals, [3H]lysine was incorporated into a number of peptides. The elution characteristics of these peptides corresponded exactly with those of peptides synthesized during in vitro incubation of neurointermediate lobes, and which were identified as des-N alpha-acetyl-alpha-MSH, a gamma-MSH-like peptide, two corticotropin-like intermediate lobe peptides, and two forms of endorphin. In white-background adapted Xenopus, practically no synthesis of pars intermedia peptides occurred. Transfer of black-adapted toads to a white background at the beginning of infusion led to storage of newly synthesized peptides. When such animals were maintained on a white background for 10 days, des-N alpha-acetyl-alpha-MSH, but not alpha-MSH, was present in the pars intermedia; this supports the notion that des-N alpha-acetyl-alpha-MSH constitutes the "storage form" of alpha-MSH.