Kisspeptin

Kisspeptin-10 is the C-terminal ten residues of the KISS1 gene product, the shortest fragment that retains full reported activity at its receptor. Its C-terminus is amidated, and the final two residues are arginine and phenylalanine, which places it in the RFamide peptide family named for exactly that motif.

The arginine-phenylalanine-amide terminus is the recognition element rather than an incidental feature. Across the RFamide family, hydrolysis of the terminal amide to the free acid abolishes receptor engagement, so an apparently trivial one-dalton difference converts an active peptide into an inactive one. This governs both how the compound is analysed and how it is stored.

The receptor, KISS1R and formerly designated GPR54, is a Gq-coupled G-protein-coupled receptor signalling through phospholipase C to inositol phosphate generation and calcium mobilisation. In rodent models it is expressed on gonadotropin-releasing hormone neurons, which is the anatomical basis for the neuroendocrine literature on this peptide.

Transfected cell lines expressing the receptor provide the core in vitro dataset, with inositol phosphate accumulation, calcium flux and radioligand binding as the standard readouts, and structure-activity work across truncated and substituted analogues establishing which residues the receptor requires.

Rodent work examines gonadotropin-releasing hormone neuron electrophysiology and downstream hypothalamic-pituitary signalling, and this system is where the peptide family was first connected to reproductive neuroendocrinology in model organisms. A recurring methodological point is peptide stability in the assay medium, since the decapeptide is subject to proteolysis over the timescale of longer incubations and apparent potency depends on how much intact peptide survives.

The critical confirmation is C-terminal amidation, since the amidated peptide is one dalton lighter than the corresponding free acid and only that form is active. Mass spectrometry establishes it directly, provided the expected mass quoted on the analysis is the amidated one, and the free acid also separates chromatographically as a distinct peak.

Two residues introduce further impurity classes. Tryptophan is oxidisable, adding characteristic mass increments and producing early-eluting degradation peaks, and asparagine can deamidate to give another one-dalton shift with an accompanying isoaspartate isomer that shares its mass. A lot analysis that reports a single peak and a nominal mass has not addressed either.

The sequence carries several hydrophobic residues against a small number of charges, so it is more aggregation-prone than a comparably sized polar peptide, and reversed-phase behaviour reflects that.