Melanotropin, secretion

The anterior lobe of the pituitary secretes a series of ten or more peptide hormones ranging in size from the 20-residue (i-melanotropin to the 200-residue growth hormone (somatotropin). Several of these contain a common heptapeptide unit, which is marked in green in the following structure ... 

Statins, release inhibiting factors, release inhibiting hormones, neurohormones synthesized in the small-cell region of the hypothalamus and transported via the bloodstream to the anterior pituitary. Members of the statins include melanostatin, somatostatin, and prolactostatin. They inhibit the secretion of melanotropin, somatotropin, and prolactin, respectively. Together with the corresponding liberins, the statins regulate the levels of the three pituitary hormones. 

Corticotropin, adrenocorticotropln, adrenocorticotropic hormone, ACTH a polypeptide hormone secreted by the pituitary. The primary structure of the human hormone is Ser-T -Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-Gly-Lys-Lys-Arg-Arg-Pro-Val-Lys-Val-Tyr-Pro-Asn-Gly-Ala-Glu-Asp-Glu-Leu-Glu-Phe, M, 4541. Only sequence 31-33 is species-specific. Biological activity is determined by the first (hxed) 20 amino acids. Sequence 1-13 is identical to that of a-melanotropin. 

Melanotropin release inhibiting hormone (MIH), melanostatin a hormone structurally equivalent to the peptide side chain of oxytocin, Pro-Leu-Gly-NH2 (bovine, rat), or to Pro-His-Phe-Arg-Gly-NHj (bovine), MRF stimulates, whereas MIH inhibits synthesis and secretion of melanotropin by the anterior pituitary, leading respectively to darkening or lightening of amphibian skin. [Special section incorporating 4 papers on Gonadotropin-Releasing Hormone Trends Endocrinol. 7 (1996) 55-68]... 

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