Structure‐activity relationships of melanin‐concentrating hormone

Abstract

Melanin‐concentrating hormone (MCH) is a cyclic heptadecapeptide (H‐Asp‐Thr‐Met‐Arg‐Cys‐Met‐Val‐Gly‐Arg‐Val‐Tyr‐Arg‐Pro‐Cys‐Trp‐Glu‐Val‐OH) that induces aggregation of melanin granules within the melanophores of teleost fishes. Chemical and enzymatic modifications of MCH were conducted in order to deduce the structure‐activity relationship using an in vitro bioassay with fish scales, and a radioimmunoassay using a specific antiserum to synthetic MCH. Micro‐modification of MCH was employed with the natural peptide, and the modified form was purified by reverse‐phase HPLC. MCH_1–14 and NPS‐Trp¹⁵‐MCH were equipotent to MCH. Reduction and carboxamidomethylation of MCH caused complete loss of biological activity. Modification of the Tyr residue with tetranitromethane and Arg residues with 1,2‐cyclohexadione significantly reduced activity, while oxidation with hydrogen peroxide caused only partial loss (10%) of activity. These results suggest that the configuration of the S‐S loop is essential for activity, and Arg and Tyr may play an important role in the biological activity. In the radioimmunoassay, MCH_1–14, MCH_5–14 and CAM‐Cys^5,14‐MCH showed no cross‐reactivity, whereas MCH_5–14 and other derivatives gave inhibition slopes parallel to the MCH standard, suggesting that the antigenic determinant of the antiserum is located in the carboxy‐terminal.