Ergothioneine is a L-histidine derivative that is Nalpha,Nalpha,Nalpha-trimethyl-L-histidine in which the hydrogen at position 2 on the imdazole ring is replaced by a mercapto group. A naturally occurring metabolite of histidine synthesized by bacteria and fungi with antioxidant properties. It is found ubiquitously in plants and animals and is present in many human foodstuffs. It has a role as an antioxidant, a chelator, a xenobiotic metabolite, a plant metabolite and a fungal metabolite. It is an amino-acid betaine, a sulfur-containing amino acid and a L-histidine derivative. It is a conjugate base of an ergothioneine(1+). It is a tautomer of an ergothioneine thione form.