Enzymological studies of melanin concentrating hormone (MCH) and related analogues

1. 1. Salmon melanin concentrating hormone (MCH) is a cyclic heptadecapeptide possessing the following primary structure: Asp-Thr-Met-Arg-Cys-Met-Val-Gly-Arg-Val-Tyr-Arg-Pro-Cys-Trp-Glu-Val. 2. 2. In the fish, Synbranchus marmoratus, skin bioassay MCH_5-15 is equipotent to MCH whereas MCH_5-14, which comprises only the ring structure, is about 100-fold less active. 3. 3. MCH and two fragment analogues, MCH_5-15 and MCH_5-14, were studied to determine their relative stability in the presence of fish serum and purified proteolytic enzymes, trypsin and alpha-chymotrypsin. 4. 4. After 4 hr incubation in fish serum, MCH_5-15 retained 1/100, MCH_5-14 1/1000 and MCH only 6/1000 of the potency of the native hormone. 5. 5. The three peptides were also very resistant to degradation by purified proteolytic enzymes involving the following relative order of resistance: MCH_5-14 > MCH_5-15 > MCH. MCH_5-14 potency was not altered after a 1 hr incubation in either enzyme whereas MCH retained 1/10 and 4/100 of its original potency, and MCH_5-15 retained 1/10 and 8/10 of its original potency, after 1 hr in trypsin and alpha-chymotrypsin, respectively.