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Table 4 Antioxidants: enzymatic – nonenzymatic inactivation of free radicals.

From: Uric acid: A new look at an old risk marker for cardiovascular disease, metabolic syndrome, and type 2 diabetes mellitus: The urate redox shuttle

ENZYMATIC ANTIOXIDANTS
SUPER OXIDE DISMUTASE (SOD)
Reactions catalyzed: [O2- + SOD → H2O2 + O2]
Various isoforms: ecSOD (extracellular); Mn-SOD (mitochondrial); Cu/Zn-SOD (intracellular)
CATALASE – Location: peroxisome.
Reaction catalyzed: [2 H2O2 + catalase → 2 H2O + O2]
GLUTATHIONE PEROXIDASE – Location: mitochondrion, cytosol, and systemic circulation.
Glutathione (GSH or glutamyl-cysteinyl-glycine tripeptide): the reduced -SH of GSH is oxidized to disulfide GSSG.
Glutathione peroxidase-catalyzed reation: [GSH + 2 H2O2 → GSSG + H2O + O2]
Glutathione reductase-catalyzed reaction: [GSSG → GSH] at the expense of [NADH → NAD+] and/or [NAD(P)H → NAD(P)+]
ENZYMATIC – NONENZYMATIC INACTIVATION OF FREE RADICALS. NITRIC OXIDE SYNTHASE Location: membrane.
Isoforms:
eNOS (endothelial): good
nNOS (neuronal): good
iNOS (inducible-inflammatory): bad
O2- and nitric oxide (NO) are consumed in this process with the creation of reactive nitrogen species (RNS).
O2- + NO → ONOO-(peroxynitrite) + tyrosine → nitrotyrosine.
Nitrotyrosine reflects redox stress and leaves a measurable footprint.
NO the good; O2 the bad; ONOO- the ugly *
NONENZYMATIC ANTIOXIDANTS
Vitamins (A, C, and E):
Thiols: Sulfhydryl (-SH)-containing molecules.
Albumin: Is an antioxidant because of it is a thiol-containing macromolecule.
Apoproteins: Ceruloplasmin and transferrin. Bind copper and iron in forms, which cannot participate in the Fenton reaction.
Uric acid: Early on in the atherosclerotic process in physiologic ranges: antioxidant.
PARADOX: Late in elevated range prooxidant with loss of supporting antioxidants above and in a milieu of oxidative – redox stress within the atherosclerotic intima. In MS, T2DM and advanced vulnerable atherosclerotic plaques SOD, Catalase, and GPX are depleted. The Urate Redox Shuttle.
PARADOX: antioxidants may become prooxidant in a certain milieu.
  1. * Beckman JS and Koppenol WH [1996] Nitric oxide, superoxide, and peroxynitrite: the good, the bad, and ugly. Am J Physiol 271(5 Part 1): C1424–C1437