A synthetic human Agouti-related protein-(83-132)-NH2 fragment is a potent inhibitor of melanocortin receptor function. 
Chemical synthesis of Agouti proteins - Agouti and Agouti-related proteins - is complicated by their large size and by multiple cysteine residues located in the carboxyl terminal regions.  Three human Agouti-related protein (AGRP) fragments, two of which correspond to a proposed endoprotease cleavage site between amino acids 82 and 83, were synthesized and tested for anti-melanotropic activity using Xenopus laevis dermal melanophores.  Amino-terminal fragments AGRP(25-51) and (54-82) were devoid of significant antagonist activity, whereas the amidated carboxyl-terminal AGRP fragment (83-132)-NH2 was potently active with an inhibitory equilibrium dissociation constant (Ki) of 0.7 nM. The ability to synthesize functionally active AGRP should help unravel its role in the central nervous system and its unusual properties with respect to interaction with the melanocortin family of G-protein coupled receptors.