Editing Antioxidant (section)

==== Thioredoxin and glutathione systems ====
The [[thioredoxin]] system contains the 12-k[[atomic mass unit|Da]] protein thioredoxin and its companion [[thioredoxin reductase]].<ref>{{Cite journal |vauthors=Nordberg J, Arnér ES |date=December 2001 |title=Reactive oxygen species, antioxidants, and the mammalian thioredoxin system |journal=Free Radical Biology & Medicine |volume=31 |issue=11 |pages=1287–312 |doi=10.1016/S0891-5849(01)00724-9 |pmid=11728801}}</ref> Proteins related to thioredoxin are present in all sequenced organisms. Plants, such as ''[[Arabidopsis thaliana]],'' have a particularly great diversity of isoforms.<ref>{{Cite journal |vauthors=Vieira Dos Santos C, Rey P |date=July 2006 |title=Plant thioredoxins are key actors in the oxidative stress response |journal=Trends in Plant Science |volume=11 |issue=7 |pages=329–34 |bibcode=2006TPS....11..329V |doi=10.1016/j.tplants.2006.05.005 |pmid=16782394}}</ref> The active site of thioredoxin consists of two [[vicinal (chemistry)|neighboring]] cysteines, as part of a highly conserved CXXC [[sequence motif|motif]], that can cycle between an active dithiol form (reduced) and an oxidized [[disulfide]] form. In its active state, thioredoxin acts as an efficient reducing agent, scavenging reactive oxygen species and maintaining other proteins in their reduced state.<ref>{{Cite journal |vauthors=Arnér ES, Holmgren A |date=October 2000 |title=Physiological functions of thioredoxin and thioredoxin reductase |journal=European Journal of Biochemistry |volume=267 |issue=20 |pages=6102–9 |doi=10.1046/j.1432-1327.2000.01701.x |pmid=11012661 |doi-access=free}}</ref> After being oxidized, the active thioredoxin is regenerated by the action of thioredoxin reductase, using [[NADPH]] as an [[electron donor]].<ref>{{Cite journal |vauthors=Mustacich D, Powis G |date=February 2000 |title=Thioredoxin reductase |journal=The Biochemical Journal |volume=346 |issue=1 |pages=1–8 |doi=10.1042/0264-6021:3460001 |pmc=1220815 |pmid=10657232}}</ref>

The [[glutathione]] system includes glutathione, [[glutathione reductase]], [[glutathione peroxidase]]s, and [[glutathione S-transferase|glutathione ''S''-transferases]].<ref name="MeisterB" /> This system is found in animals, plants and microorganisms.<ref name="MeisterB" /><ref>{{Cite journal |vauthors=Creissen G, Broadbent P, Stevens R, Wellburn AR, Mullineaux P |date=May 1996 |title=Manipulation of glutathione metabolism in transgenic plants |journal=Biochemical Society Transactions |volume=24 |issue=2 |pages=465–9 |doi=10.1042/bst0240465 |pmid=8736785}}</ref> Glutathione peroxidase is an enzyme containing four [[selenium]]-[[cofactor (biochemistry)|cofactors]] that catalyzes the breakdown of hydrogen peroxide and organic hydroperoxides. There are at least four different glutathione peroxidase [[isozyme]]s in animals.<ref>{{Cite journal |vauthors=Brigelius-Flohé R |date=November 1999 |title=Tissue-specific functions of individual glutathione peroxidases |journal=Free Radical Biology & Medicine |volume=27 |issue=9–10 |pages=951–65 |doi=10.1016/S0891-5849(99)00173-2 |pmid=10569628}}</ref> Glutathione peroxidase 1 is the most abundant and is a very efficient scavenger of hydrogen peroxide, while glutathione peroxidase 4 is most active with lipid hydroperoxides. Surprisingly, glutathione peroxidase 1 is dispensable, as mice lacking this enzyme have normal lifespans,<ref>{{Cite journal |vauthors=Ho YS, Magnenat JL, Bronson RT, Cao J, Gargano M, Sugawara M, Funk CD |date=June 1997 |title=Mice deficient in cellular glutathione peroxidase develop normally and show no increased sensitivity to hyperoxia |journal=The Journal of Biological Chemistry |volume=272 |issue=26 |pages=16644–51 |doi=10.1074/jbc.272.26.16644 |pmid=9195979 |doi-access=free}}</ref> but they are hypersensitive to induced oxidative stress.<ref>{{Cite journal |vauthors=de Haan JB, Bladier C, Griffiths P, Kelner M, O'Shea RD, Cheung NS, Bronson RT, Silvestro MJ, Wild S, Zheng SS, Beart PM, Hertzog PJ, Kola I |date=August 1998 |title=Mice with a homozygous null mutation for the most abundant glutathione peroxidase, Gpx1, show increased susceptibility to the oxidative stress-inducing agents paraquat and hydrogen peroxide |journal=The Journal of Biological Chemistry |volume=273 |issue=35 |pages=22528–36 |doi=10.1074/jbc.273.35.22528 |pmid=9712879 |doi-access=free |hdl-access=free |hdl=10536/DRO/DU:30101410}}</ref> In addition, the glutathione ''S''-transferases show high activity with lipid peroxides.<ref>{{Cite journal |vauthors=Sharma R, Yang Y, Sharma A, Awasthi S, Awasthi YC |date=April 2004 |title=Antioxidant role of glutathione S-transferases: protection against oxidant toxicity and regulation of stress-mediated apoptosis |journal=Antioxidants & Redox Signaling |volume=6 |issue=2 |pages=289–300 |doi=10.1089/152308604322899350 |pmid=15025930}}</ref> These enzymes are at particularly high levels in the liver and also serve in [[detoxification]] metabolism.<ref>{{Cite journal |vauthors=Hayes JD, Flanagan JU, Jowsey IR |year=2005 |title=Glutathione transferases |journal=Annual Review of Pharmacology and Toxicology |volume=45 |pages=51–88 |doi=10.1146/annurev.pharmtox.45.120403.095857 |pmid=15822171}}</ref>