The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki
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The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

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38 The Mitochondrial Free Radical Theory of Aging The LECs that are discussed throughout the rest of this book, and especially in the rest of this chapter, are listed in Table 3.2. Also listed are the molecules which, though not LECs, are important in MiFRA because they react with LECs and/or are formed by reactions involving LECs. 3.5. LEC-Related Reactions Relevant to MiFRA The reactions that will concern us in the rest of this book are described in Table 3.3; several of the most important destructive ones are also depicted in Figure 3.1. The reactions fall into the following categories: a. donation of an electron by a LEC to a non-LEC — reduction b. removal of an electron by a LEC from a non-LEC — oxidation c. donation of an electron by a LEC to another LEC — usually disproportionation d. fusion of a non-LEC with a LEC forming a LEC — condensation e. fusion of a LEC with another LEC forming a non-LEC — termination f. fission of a LEC forming a LEC and a non-LEC — dissociation g. reactions not involving LECs but of relevance nevertheless Because some of the reactions involve joining or splitting of lipids to form other lipids, the equations that involve more than one lipid use subscripts a, b etc. to distinguish them. Tables 3.2 and 3.3 may seem rather intimidating, but there is no need to absorb them; they are supplied here mainly for ease of reference through the rest of the book. At this point, the most important thing to appreciate about these reactions is the reason why I defined “LEC” as I did: as noted in Table 3.1, reactions that happen in biological systems preserve “LEC parity.” That is, if you start with an odd number of LECs you end with an odd number, and if you start with an even number of LECs you end with an even number. This is not so for free radicals, nor for reactive oxygen species. It is also worth stressing at this point that not all LECs behave in the same way. For example, the reaction between two ascorbate radicals and that between two lipid radicals are completely different: the former involves the incorporation of two protons, whereas the latter uses no protons but instead makes a bond. This is a crucial difference, because the bond is permanent, whereas the oxidised ascorbate (dehydroascorbate) is still an independent molecule which can be restored to its reduced form. This property of ascorbate, together with its extreme readiness to undergo disproportionation, is the main reason why it is so valuable to us. A third crucial feature which should be observed in this list is that some pairs of reactions cause the change of one participant to a different form and then back again. For example, superoxide turns ferric iron into ferrous, and hydrogen peroxide turns it back to ferric. Putting those two reactions in sequence: O2• — + Fe 3+ H2O2 + Fe 2+ • +H + O2• — + H2O2 +H + O2 + Fe 2+ • HO• + H2O + Fe 3+ HO• + H2O + O2 gives us a chemical change which, if thought of as a single reaction, doesn’t involve iron at all. But that cumulative reaction does not appear in Table 3.3, and the reason it doesn’t appear is that it doesn’t happen. Iron (or copper, or some other ion that behaves the same way) needs to be present so that the intermediate state is available. Iron therefore acts as a catalyst, causing a normally impossible reaction to be achieved by a detour that needs less activation energy. This is the way that all catalysts work, and why a reaction needs so much

An Introduction to Free Radicals Table 3.2. Molecules of relevance to MiFRA LECs that are free radicals O2• — Superoxide HO• Hydroxyl radical HO2• Perhydroxyl radical L• Lipid radical LOO• Lipid peroxyl radical LO• Lipid alkoxyl radical Q• — Ubisemiquinone TocO• Tocopheryl radical Asc• — Ascorbate radical GS• Glutathiyl radical LECs that are not free Fe 2+ • Ferrous iron ion radicals Cu + • Cuprous copper ion cyt-c 2+ • Reduced (ferrous) cytochrome c Non-LECs: simple H + Proton, or hydrogen cation hydrogen/oxygen OH — Hydroxide anion derivatives O2 Molecular oxygen H2O Water Hydrogen peroxide H2O2 Non-LECs: respiratory Q Ubiquinone chain components QH2 Ubiquinol cyt-c 3+ Oxidized (ferric) cytochrome c Non-LECs: lipids and L, LH, LH2 Lipid peroxidation LOOH Lipid hydroperoxide derivatives LO Lipid aldehyde Non-LECs: antioxidants TocOH Tocopherol (or Vitamin E) (non-enzymatic) AscH2 Ascorbate (or ascorbic acid and derivatives or Vitamin C) Asc Dehydroascorbate GSH Glutathione (reduced) GSSG Glutathione disulphide Non-LECs: metal ions Fe 3+ Ferric iron ion Cu 2+ Cupric iron ion Non-LECs: NADH Nicotinamide adenine dinucleotide reduction/oxidation (reduced) reaction substrates NAD + Nicotinamide adenine dinucleotide (oxidized) FADH2 Flavin adenine dinucleotide (reduced) FAD Flavin adenine dinucleotide (oxidized) Non-LECs: antioxidant SOD Superoxide dismutase enzymes CAT Catalase GPx Glutathione peroxidase GR Glutathione reductase 39

An Introduction to <strong>Free</strong> <strong>Radical</strong>s<br />

Table 3.2. Molecules <strong>of</strong> relevance to MiFRA<br />

LECs that are free radicals O2• — Superoxide<br />

HO• Hydroxyl radical<br />

HO2• Perhydroxyl radical<br />

L• Lipid radical<br />

LOO• Lipid peroxyl radical<br />

LO• Lipid alkoxyl radical<br />

Q• — Ubisemiquinone<br />

TocO• Tocopheryl radical<br />

Asc• — Ascorbate radical<br />

GS• Glutathiyl radical<br />

LECs that are not free Fe 2+ • Ferrous iron ion<br />

radicals Cu + • Cuprous copper ion<br />

cyt-c 2+ • Reduced (ferrous) cytochrome c<br />

Non-LECs: simple H + Proton, or hydrogen cation<br />

hydrogen/oxygen OH — Hydroxide anion<br />

derivatives O2 Molecular oxygen<br />

H2O Water<br />

Hydrogen peroxide<br />

H2O2<br />

Non-LECs: respiratory Q Ubiquinone<br />

chain components QH2 Ubiquinol<br />

cyt-c 3+ Oxidized (ferric) cytochrome c<br />

Non-LECs: lipids and L, LH, LH2 Lipid<br />

peroxidation LOOH Lipid hydroperoxide<br />

derivatives LO Lipid aldehyde<br />

Non-LECs: antioxidants TocOH Tocopherol (or Vitamin E)<br />

(non-enzymatic) AscH2 Ascorbate (or ascorbic acid<br />

and derivatives or Vitamin C)<br />

Asc Dehydroascorbate<br />

GSH Glutathione (reduced)<br />

GSSG Glutathione disulphide<br />

Non-LECs: metal ions Fe 3+ Ferric iron ion<br />

Cu 2+ Cupric iron ion<br />

Non-LECs: NADH Nicotinamide adenine dinucleotide<br />

reduction/oxidation (reduced)<br />

reaction substrates NAD + Nicotinamide adenine dinucleotide<br />

(oxidized)<br />

FADH2<br />

Flavin adenine dinucleotide<br />

(reduced)<br />

FAD Flavin adenine dinucleotide<br />

(oxidized)<br />

Non-LECs: antioxidant SOD Superoxide dismutase<br />

enzymes CAT Catalase<br />

GPx Glutathione peroxidase<br />

GR Glutathione reductase<br />

39

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