Comparative biological activities of potent active‐site analogues of α‐melanotropin. Effect of tyrosine substitution at position‐4

We have prepared several α‐melanotropin (α‐MSH) analogues with tyrosine substituted for methionine at the 4‐position and determined their melanotropic activities on the frog (Rana pipiens), lizard (Anolis carolinensis) and S‐91 (Cloudman) mouse melanoma adenylate cyclase bioassays. The potencies of Ac‐[Tyr⁴]‐α‐MSH_4–10‐NH₂ and Ac‐[Tyr⁴]‐α‐MSH_4–11‐NH₂ were compared with α‐MSH and with their corresponding methionine and norleucine substituted analogues. The Tyr‐4 analogues were found to be less active than the Nle‐4 analogues on both the frog and lizard assays. Ac‐[Tyr⁴]‐α‐MSH_4–10‐NH₂ was found to be less active than Ac‐[Tyr⁴]‐α‐MSH_4–11‐NH₂ on the lizard bioassay, but more active than the longer fragment on the frog skin assay. Ac‐[Tyr⁴]‐α‐MSH_4–10‐NH₂ exhibited extremely prolonged biological activity on frog skin, but not on lizard skin, while the melanotropic activity of Ac‐[Tyr⁴]‐α‐MSH_4–11‐NH₂ was rapidly reversed on both assay systems. The increased potency of Ac‐[Tyr⁴]‐α‐MSH_4–10‐NH₂ over Ac‐[Tyr⁴]‐α‐MSH_4–11‐NH₂ on frog melanocytes may be related to the fact that the shorter 4–10 analogue exhibits prolonged biological activity. Interestingly, it was found that both Tyr‐4 analogues were partial agonists on the mouse melanoma adenylate cyclase bioassay, and stimulated the enzyme to only about 50% of the maximal activity of α‐MSH. We reported previously that replacement of L‐Phe‐7 by its D‐enantiomer in [Nle⁴]‐α‐MSH and its Nle‐4 containing analogues resulted in peptides with increased potency and in some instances prolonged activity. Similarly, incorporation of D‐Phe‐7 into Tyr‐4 containing melanotropin fragments produced analogues Ac‐[Tyr⁴, D‐Phe⁷]‐αMSH_4–10‐NH₂ and Ac‐[Tyr⁴, D‐Phe⁷]‐α‐MSH_4–11‐NH₂, which also exhibited greatly increased biological activity in all three assay systems. Both of these analogues were also found to have prolonged activity in the frog skin bioassay but little or no prolonged activity in the lizard skin bioassay. These two analogues turned out to be full agonists in the mouse melanoma adenylate cyclase bioassay and were equipotent to α‐MSH. These results demonstrate that substitution of tyrosine for methionine at position‐4 dramatically affects the potency and prolonged activity of these melanotropin analogues and the melanotropic activities observed as a result of such substitutions are themselves affected by concomitant substitutions at the 7(Phe) and 11 (Lys) positions of the analogues.