Structure-Activity Relationships of a Synthetic Pentapeptide that Specifically Releases Growth Hormonein Vitro*

Tyr-dTrp-Gly-Phe-Met-NH2 is a unique synthetic Met5-enkephalin peptide analog that at concentrations of 1–100 μg/ml incubation medium acts directly on the pituitary of rats in vitro to release GH specifically but not LH, FSH, TSH, PRL, or ACTH. Structure-activity studies of this pentapeptide emphasize the prime importance of an aromatic amino acid residue at the 2 position for in vitro GH-releasing activity. Both Tyr-dTrp-Gly-Phe-Met-NH2, and Tyr-dPhe-Gly-Phe-Met-NH2 released GH in vitro, while large concentrations of seven enkephalin analogs, which did not have an aromatic amino acid residue in the 2 position, had essentially no GH-releasing activity. In addition, the importance of a dTrp or dPhe rather than an lTrp or lPhe residue in the 2 position was apparent, since neither Tyr-lTrp-Gly-Phe-Met-NH2 nor Tyr-lPhe-Gly-Phe- Met-NH2 were active. Although the GH-releasing activity of the C-terminal nonamidated Tyr-dTrp-Gly-Phe-Met peptide was not determined, the nonamidated Tyr-dPhe-Gly-Phe-Met peptide was ...