Loren Pickart, and it can naturally be found in human saliva, plasma, and urine.12 (The GHK in skin care products is synthetic.) Experts originally believed GHK was a liver cell growth factor, but as research advanced, they learned that the peptide could alter far more cells than just those in the liver
Chang, C.-H., Tsai, W.-C., Lin, M.-S., Hsu, Y.-H., & Pang, J.-H
In addition, plasma may have a low glutathione content and may be difficult to measure
Journal of Cell Biology, 2018, 217(1): 65-77
Am J Trop Med Hyg 92(1):2227 Esperanza Espino F, Bibit JA, Beulah Sornillo J, Tan A, von Seidlein L, Ley B (2016) Comparison of three screening test kits for G6PD enzyme deficiency: implications for its use in the radical cure of vivax malaria in remote and resource-poor areas in the Philippines