Carrier Peptide
A carrier peptide is a cosmetic peptide that chelates a trace metal, almost always copper, and is marketed as delivering that metal into skin to support matrix-building enzymes.
Carrier peptides are defined by what they hold rather than what they signal, and the category is dominated by one molecule: glycyl-L-histidyl-L-lysine, or GHK, a tripeptide of about 340 daltons isolated from human plasma by Loren Pickart in the 1970s. Its histidine imidazole and terminal amine form a high-affinity site for copper at physiological pH, with an affinity between albumin and cellular uptake systems, which is the basis for calling it a carrier.
The rationale for copper is enzymatic: lysyl oxidase, which crosslinks collagen and elastin, and copper-zinc superoxide dismutase both require it as a cofactor. In fibroblast culture, GHK-Cu increases collagen and glycosaminoglycan output. Human evidence is thinner: small facial cream studies, generally a few dozen subjects over twelve weeks, reporting improvements in wrinkling and skin density, several sponsored by parties holding the ingredient. It is sold as a cosmetic ingredient and holds no drug approval.
The size argument is the strongest thing the category has. At around 340 daltons the free tripeptide sits under the usual penetration ceiling, unlike most cosmetic peptides. But chelation and transport are separate claims, and binding copper in a beaker says nothing about whether the complex crosses the stratum corneum intact or releases copper where it is needed.
The recurring error is importing culture magnitudes into skin: a stated percentage rise in collagen output reflects micromolar exposure in medium with no barrier in the way. A second error is treating copper as harmlessly beneficial, when excess copper is pro-oxidant and copper homeostasis is tightly regulated for a reason. Injectable grey-market preparations compound this, bypassing the barrier entirely with no established human safety data behind them.