Biosynthesis, processing, and control of release of melanotropic peptides in the neurointermediate lobe of Xenopus laevis.

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Abstract
The neurointermediate lobes of dark-adapted toads, X. 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 1 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 carolinensis skin bioassay. Three of these TCA-precipitable peptides comigrated with ACTH, .beta.-lipotropin and .alpha.-MSH on acid-urea gels. Evidence suggesting a precursor-product mode of biosynthesis of the melanotropic peptides is presented. Seven of the 10 TCA-precipitable peptides and the 1 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 .times. 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.

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