Biosynthesis, processing, and control of release of melanotropic peptides in the neurointermediate lobe of Xenopus laevis.
The neurointermediate lobes of dark-adapted toads Xenopus laevis were incubated for 30 min in [3H]arginine and then "chased" for various time periods. By use of this pulse-chase paradigm there were detected 10 trichloroacetic acid (TCA)-precipitable peptides separated on acid-urea polyacrylamide gels and one TCA-soluble peptide separated by high-voltage electrophoresis (pH 4.9) with melanotropic activity. Each of these peptides had a different degree of melanocyte stimulating hormone (MSH) activity as revealed by the Anolis skin bioassay. Three of these TCA-precipitable peptides comigrated with ACTH, beta-lipotrophin, and alpha-MSH on acid-urea gels. Evidence suggesting a precursor-product mode of biosynthesis of the melanotropic peptides is presented. 7 of the 10 TCA-precipitable peptides and the one TCA-soluble peptide with melanotropic activity were released into the medium. The half-time of release of the TCA-precipitable peptides was about 2 h, whereas the half-time of TCA-soluble peptide release was about 30 min. The release of these peptides was inhibited by 5 X 10(-5) M dopamine. Dopamine inhibition of release did not appear to affect the biosynthesis of the melanotropic peptides, but did appear to enhance the degradation of the newly synthesized TCA-soluble peptide in the tissue. White adaptation of the toads greatly decreased the biosynthesis of all of the TCA-precipitable melanotropic peptides.
D000818:Animals; D001120:Arginine / Q000378:metabolism; D004298:Dopamine / Q000378:metabolism*; D004591:Electrophoresis, Polyacrylamide Gel; D009074:Melanocyte-Stimulating Hormones / Q000096:biosynthesis* / Q000302:isolation & purification; D010904:Pituitary Gland, Posterior / Q000378:metabolism*; D014981:Xenopus / Q000378:metabolism
D009074:Melanocyte-Stimulating Hormones; D001120:Arginine; D004298:Dopamine