Abstract
Ergothioneine (EGT) is a sulfur-containing amino acid analog that is biosynthesized in fungi and bacteria, accumulated in plants, and ingested by humans where it is concentrated in tissues under oxidative stress. While the physiological function of EGT is not yet fully understood, EGT is a potent antioxidant in vitro. Here we report that oxidized forms of EGT, EGT-disulfide (ESSE) and 5-oxo-EGT, can be reduced by the selenoenzyme mammalian thioredoxin reductase (Sec-TrxR). ESSE and 5-oxo-EGT are formed upon reaction with biologically relevant reactive oxygen species. We found that glutathione reductase (GR) can reduce ESSE, but only with the aid of glutathione (GSH). The reduction of ESSE by TrxR was found to be selenium dependent, with non-selenium-containing TrxR enzymes having little or no ability to reduce ESSE. In comparing the reduction of ESSE by Sec-TrxR in the presence of thioredoxin to that of GR/GSH, we find that the glutathione system is 10-fold more efficient, but Sec-TrxR has the advantage of being able to reduce both ESSE and 5-oxo-EGT directly. This represents the first discovered direct enzymatic recycling system for oxidized forms of EGT. Based on our in vitro results, the thioredoxin system may be important for EGT redox biology and requires further in vivo investigation.
Highlights
Ergothioneine (EGT) is a redox-active sulfur-containing amino acid derivative that is biosynthesized from the coupling of histidine (His) and cysteine (Cys) in fungi, actinobacteria, and cyanobacteria [1,2,3,4,5,6,7,8,9,10,11]
dithiobis-(2-nitrobenzoic acid) (DTNB) is a suitable model for ESSE as both disulfides are highly reactive due to the polarization of the disulfide bond caused by electron-withdrawing groups on each sulfur atom
Our computed kcat and KM values for the 5-oxo form are most there is a degree of structural similarity between 5-oxo EGT and dehydroascorbate, as assuredly greatly underestimated because the measured activity is a combination of the reduction of the 5-oxo form, 5-hydroxy form, and the disulfide form
Summary
Ergothioneine (EGT) is a redox-active sulfur-containing amino acid derivative that is biosynthesized from the coupling of histidine (His) and cysteine (Cys) in fungi, actinobacteria, and cyanobacteria [1,2,3,4,5,6,7,8,9,10,11]. Experiments have shown that OCTN1 knockout cells are more prone to DNA damage, protein oxidation, and lipid peroxidation, suggesting EGT functions as an antioxidant and cytoprotective agent in vivo [5]. Another possible function for EGT is in the detoxification of xenobiotic electrophiles [21]. This study investigated gated whether oxidized forms of EGT are substrates for Sec-TrxR. System, which is made up in part by GR, glutathione (GSH), and NADPH [35] We found that both GR and Sec-TrxR could reduce the oxidized forms of EGT, but GR.
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