L-(+)-Ergothioneine (ET) is a sulfur-containing amino acid, which is only produced by Actinomycetales bacteria and non-yeast like fungi belonging to the division Basidiomycota and Ascomycota. It was originally isolated from Claviceps purpurea or rye ergot. It is obtained from L-histidine, which is converted into betaine form called hercynine. It is found in both animals and plants, and mammals usually obtain it from their diet, e.g. through mushrooms or oats. It is tautomeric in nature, and in neutral aqueous solution exists in thione form.
L-(+)-Ergothioneine (ET) has the maximum concentrations in tissues subjected to oxidative stress, with the highest being in blood, eye lens, bone marrow, semen and liver. It acts as an anti-oxidant and prevents apoptosis, by scavenging reactive oxygen and nitrogen species. The anti-oxidant activity is attributable to sulfhydryl groups. It acts as a substrate for SLC22A4 (solute carrier family 22, member 4) transporter. In alveolar macrophages, it prevents the release of interleukin-8 (IL-8) by tumor necrosis factor (TNF)α. IL-8 is an inflammatory cytokine. It also regulates the oxidative damage in liver and kidneys, and has a protective action against lipid peroxidation. It is also responsible for the conservation of endogenous glutathione and α-tocopherol. ET being an antioxidant, protects against γ and UV radiation. In UV-irradiated human dermal fibroblasts, it scavenges reactive oxygen species (ROS), and suppresses matrix metalloproteinases 1 (MMP1) expression. It might also have anti-ageing effects on skin caused by UV-radiation.