Structure-activity relationships of melanin-concentrating hormone

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-O H) 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. MCH1-14 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, MCH1-14, MCH5-14 and CAM-Cys5,14-MCH showed no cross-reactivity, whereas MCH5-17 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.