Purification and characterization of a γ-melanotropin precursor from frozen human pituitary glands

Abstract

A new melanocyte-stimulating peptide has been isolated from acid extracts of frozen human pituitary glands by salt/ethanol fractionation, Sephadex G-75 gel filtration and DEAE- and CM-cellulose ion-exchange chromatography. The peptide is glycosylated has an N-terminal tryptophan residue and an apparent mol.wt. of 16000 as estimated by sodium docecyl sulphate/polyacrylamide-gel electrophoresis. Its amino acid analysis closely resembles residues Trp(-105) to Gln(-29) predicted for the common precursor protein of bovine corticotropin and β-lipotropin by Nakanishi, Inoue, Kita, Nakamura, Chang, Cohen & Numa. This fragment is expected to have melanotropin activity due to the tetrapeptide -His-Phe-Arg-Trp (residues -51 to -48) of the predicted sequence of the common precursor. It was found to have a molar potency of 1x10-5 relative to α-melanotropin in the frog skin bioassay. These characteristics are consistent with the isolated melanotropin peptide being a noncorticotropin, non-lipotropin peptide of the human common precursor protein of corticotropin and lipotropin. The peptide neither potentiates the adrenal weight-maintenance activity of corticotropin-(1-24)-tetracosapeptide when administered to hypophysectomized rats, nor stimulates release of non-esterified fatty acids from isolated rat epididymal cells. A second N-terminal-tryptophan glycopeptide was also isolated, which had an amino-acid composition similar to that predicted for the bovine common precursor protein, residues Trp(-105) to Gly(-35).