GHRP-6

GHRP-6 is a hexapeptide, His-D-Trp-Ala-Trp-D-Phe-Lys, with a C-terminal primary amide. It is the earliest member of the growth hormone releasing peptide series and the compound from which the later, more selective analogs were developed, which makes it the historical reference point for the class.

Its target is the growth hormone secretagogue receptor 1a. The defining structural feature for practical purposes is that it carries two tryptophan residues, one of them D-configured, where GHRP-2 carries one and ipamorelin carries none. That doubles the oxidation-prone surface of an already short molecule.

This compound predates the identification of ghrelin itself, and a substantial part of its historical significance is that the search for its endogenous counterpart contributed to the discovery of the receptor and its natural ligand. That sequence of events is why the receptor carries the secretagogue name rather than the ghrelin name.

Alongside secretagogue activity, the animal-model literature consistently reports increased food intake, consistent with agonism at a receptor whose endogenous ligand is orexigenic. Like GHRP-2 it is reported to elevate cortisol and prolactin in those models, and it is generally described as the least selective member of the series. In-vitro work uses calcium mobilisation assays in receptor-expressing cell lines.

Purity by reversed-phase HPLC and identity by mass spectrometry are reported per lot on the Certificate of Analysis. Two tryptophan residues make oxidation the dominant analytical concern for this sequence: each oxidation event adds 16 daltons, and mono- and di-oxidised species typically resolve as separate earlier-eluting peaks.

The D-tryptophan and D-phenylalanine carry the usual epimerisation risk, producing mass-identical diastereomers detectable only by chromatography. C-terminal amidation needs separate confirmation, since the free-acid form is a common and considerably less active by-product.