The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

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The Mitochondrial Free Radical Theory of Aging
Fig. 3.3. A highly oxidizable, but necessary, juxtaposition of C=C double bonds.
3.8. Downstream Products—A Virtually Infinite Variety
One class of reaction involving LECs was not included in Table 3.3, because in order
to describe it one must describe the structure of the participating lipids in detail. Reaction
18, whereby a lipid peroxyl radical becomes a hydroperoxide, can sometimes occur
intramolecularly: the unpaired electron forms a bond with a carbon atom of the same
carbon chain, making a ring. The molecule is still a radical: a lipid radical that is also an
endoperoxide (Fig. 3.2). Molecules with an endoperoxide ring are particularly unstable
with regard to their molecular structure: the oxygens can move around the molecule,
creating even more exotic multi-ringed molecules (Fig. 3.2). 6 Eventually these complex
molecules are prone to break apart, and one of the major products (both in vitro and in
vivo) is malondialdehyde (Fig. 3.2), which can be used in biological systems as a measure
of the rate of lipid peroxidation. The other molecules most often used as measures of lipid
peroxidation are ethane and pentane, which are two of many unreactive molecules formed
by reactions 20 and 16 but are the easiest to assay because they are gaseous. 7
3.9. Fatty Acid Double Bonds: Fluidity but Oxidizability
In the above description of lipid peroxidation I have treated all lipids as equal, but they
are very far from that. Lipids are a large family of compounds, and some are much more
oxidizable than others. Unfortunately for us, the characteristic that makes lipids particularly
oxidizable is one that we cannot do without, because membranes need lipids of that type in
order to maintain physical integrity in vivo. This characteristic is the possession of carbon
chains with C=C double bonds spaced at a particular distance, as shown in Figure 3.3. The
circled hydrogen atoms, attached to the carbon in the middle, are called doubly allylic, or
bisallylic: they are particularly susceptible to removal by oxidizing LECs. 8