title: Metorphamide: isolation, structure, and biologic activity of an amidated opioid octapeptide from bovine brain.
abstract: Acid acetone extracts of caudate nucleus from bovine brain were found to contain an amidated opioid octapeptide with the following structure: Tyr-Gly-Gly-Phe-Met-Arg-Arg-Val-NH2. The peptide has been named metorphamide. Bovine metorphamide appears to be derived by proteolytic cleavage from proenkephalin, the common precursor to [Met5]enkephalin and [Leu5]enkephalin. The cleavage within the precursor giving rise to the carboxyl terminus of metorphamide occurs at a single arginine residue and is followed by transformation of a carboxyl-terminal glycine into an amide group. Metorphamide was detected in bovine caudate nucleus extracts by radioimmunoassay, and it was purified to homogeneity by gel filtration and reversed-phase high performance liquid chromatography. Amino acid composition analysis and automated Edman degradation in the gas-phase sequencer confirmed the postulated amino acid sequence. Carboxyl-terminal amidation of bovine metorphamide was shown by stability to carboxypeptidase A digestion and full crossreactivity in a radioimmunoassay that required the carboxyl-terminal amide as part of the recognition site. A synthetic replicate of metorphamide as well as several synthetic analogs were tested for opioid activity in several bioassays and binding assays, and metorphamide was found to have a high mu-binding activity. Metorphamide is the only known naturally occurring opioid peptide that has a high mu-binding activity. The kappa-binding activity is approximately equal to 50% that of the mu-binding activity, but delta-binding activity is negligible.
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001667:Binding, Competitive; D001681:Biological Assay; D001921:Brain / Q000378:metabolism; D002268:Carboxypeptidases; D043422:Carboxypeptidases A; D002417:Cattle; D002421:Caudate Nucleus / Q000032:analysis*; D002851:Chromatography, High Pressure Liquid / Q000379:methods; D004744:Enkephalin, Methionine / Q000031:analogs & derivatives* / Q000302:isolation & purification / Q000494:pharmacology; D006168:Guinea Pigs; D008297:Male; D051379:Mice; D009119:Muscle Contraction / Q000187:drug effects; D009197:Myenteric Plexus / Q000187:drug effects; D011817:Rabbits; D051381:Rats; D011957:Receptors, Opioid / Q000378:metabolism; D013329:Structure-Activity Relationship; D014649:Vas Deferens / Q000187:drug effects
chemical_list: D011957:Receptors, Opioid; D004744:Enkephalin, Methionine; C039700:adrenorphin; D002268:Carboxypeptidases; D043422:Carboxypeptidases A