Ergothioneine is a naturally occurringamino acid and is a thiourea derivative of histidine, containing a sulfur atom on the imidazole ring. This compound is made in relatively few organisms, notably Actinobacteria, Cyanobacteria, and certain fungi. Ergothioneine was discovered in 1909 and named after the ergot fungus from which it was first purified, with its structure being determined in 1911. In vitro assays, as well as in vivoanimal models, are used in basic research to identify its potential biological properties. In humans, ergothioneine is acquired exclusively through the diet and accumulates in erythrocytes, bone marrow, liver, kidney, seminal fluid, and eyes. Ergothioneine requires a specific transporter, ETT, also known as OCTN1, to enter cells. ETT expression has been confirmed in human and animal cell lines and its functional transport of ergothioneine has been observed in preliminary studies in vivo. Although the effect of ergothioneine in vivo is an active area of research, its physiological role in humans is undetermined.
Metabolism and sources
Ergothioneine has been found in bacteria, plants, and animals, sometimes at millimolar levels. Foods found to contain ergothioneine include liver, kidney, black beans, kidney bean, and oat bran, with the highest levels in bolete and oyster mushrooms. Levels can be variable, even within species and some tissues can contain much more than others. In the human body, the largest amounts of ergothioneine are found in erythrocytes, eye lens, semen, and skin. Although many species contain ergothioneine, only a few make it; the others absorb it from their diet or, in the case of plants, from their environment. Biosynthesis has been detected in Actinobacteria, such as Mycobacterium smegmatis and certain fungi, such as Neurospora crassa. The metabolic pathway to produce ergothioneine starts with the methylation of histidine to produce histidine betaine. The sulfuratom is then incorporated from cysteine. The biosynthetic genes of ergothioneine have been described in Mycobacterium smegmatis, Neurospora crassa, and Schizosaccharomyces pombe. Other species of bacteria, such as Bacillus subtilis, Escherichia coli, Proteus vulgaris, and Streptococcus, as well as fungi in the Saccharomycotina cannot make ergothioneine.
Structure
Ergothioneine is a thiourea derivative of the betaine of histidine and contains a sulfur atom bonded to the 2-position of the imidazole ring. Typical of thioureas, ergothioneine is less reactive than typical thiols such as glutathione towards alkylating agents like maleimides. It also resists oxidation by air. However, ergothioneine can be slowly oxidized over several days to the disulfide form in acidic solutions.
Ergothioneine derivatives
Various derivatives of ergothioneine have been reported in the literature, such as S-methyl-ergothioneine or selenium-containing selenoneine.
Preliminary research
Ergothioneine has antioxidant properties in vitro. Under laboratory conditions, it scavenges hydroxyl radicals and hypochlorous acid, inhibits production of oxidants by metal ions, and may participate in metal ion transport and regulation of metalloenzymes. Although potential effects of ergothioneine are under preliminary research, its physiological role in vivo is unknown.
Safe intake levels
The Panel on Dietetic Products for the European Food Safety Authority finds that safe daily limits of 2.82 mg/kg of body weight for infants, 3.39 mg/kg for small children, and 1.31 mg/kg for adults, including pregnant and breastfeeding women.
