The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

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The Mitochondrial Free Radical Theory of Aging
11.2.1. Superoxide Stimulation: Theory and Experiment
First the theoretical work. 2b Coenzyme Q (CoQ), the molecule which transfers electrons
from Complexes I and II to Complex III, exists in three forms: ubiquinone (Q),
ubisemiquinone (Q• — ) and ubiquinol (QH2). The middle of these is a LEC, and is thought
to be the respiratory chain component most prone to lose its lonely electron to oxygen,
forming superoxide (O2• — ). When free in the membrane, CoQ almost never exists as
Q• — , but it is believed to exist transiently in that state while bound to Complexes I and
III,* the proton pumps with which it exchanges electrons. 3-5a In particular, it probably
exists as Q• — at a crucial stage in its interaction with Complex III: the point when it passes
from the Rieske protein to cytochrome b. The reason why that point is crucial is that
cytochrome b is the only subunit of Complex III that is encoded by the mtDNA. Thus, a
knockout of cytochrome b—or, of course, of any tRNAs—would leave CoQ molecules
stranded in the radical state (see Fig. 11.1), and thus would cause a rise in superoxide
production. This would occur even when Complex I is also failing, because CoQ would
still be receiving electrons from the nuclear-coded sources (mainly Complex II). A weakness
of this logic is that it assumes that the Rieske protein would assemble into Complex III
adequately to be able to accept electrons from ubiquinol, which seems not to be so in yeast; 5b
and, indeed, yeast with no mtDNA seem to make less superoxide than do wild-type. 5c This
may also be so in mammals, 5d though the opposite has also been reported. 5e But,
importantly, all these studies used dividing cells, in which SOS is not applicable; moreover,
it does not help to explain the raised tolerance to hyperoxia of mitochondria whose only
defect is in complex IV. 5c
The experimental evidence that malfunction of the respiratory chain increases the release
of superoxide is mainly from in vitro studies of the effects of inhibitors. Many chemicals
have been identified which block the respiratory chain; they are often effective antibiotics,
since at appropriate doses they work more powerfully on the respiration machinery of
bacteria than on mitochondria. Much careful biochemistry, over several decades, has
identified the sites at which these various drugs act: not only on which enzyme of the
respiratory chain, but whereabouts on that enzyme. 6 These experiments can be coupled
with measurements of the rate of formation of superoxide, and can thereby lead to an
indication of where in the respiratory chain the superoxide is being made. The conclusion is
in accordance with that of the theoretical analysis outlined above: the site of interaction of
CoQ with Complex III is a major source, 7-9 as is the site of interaction of CoQ with Complex
I, which may operate in a similar way at one or both membrane surfaces. 5a A recent study 10
presents compelling evidence that the contribution from a functional Complex III is slight
when respiration is rapid, but that does not imply that it would be slight when Complex III
was dysfunctional.
11.2.2. Perhydroxyl-Initiated Peroxidation In Vivo
So, what hope is there for SOS? Recall first that superoxide is relatively unreactive, and
has long been known not to initiate lipid peroxidation itself 11 (see Section 3.5). Now: in
order to maintain SOS as a plausible mechanism, while accepting that mutant mitochondria
tend to generate more superoxide, one must argue that the rate of lipid peroxidation is not
a function solely of the rate of superoxide production. This could be either because
* Interestingly, the other enzymes which donate electrons to ubiquinone—Complex II, fatty acyl CoA dehydrogenase
and s,n-glycerophosphate dehydrogenase—appear not to contribute to LEC production. This is
probably because they do not form Q• — at any stage, but instead transfer two electrons in unison from FADH2
to ubiquinone. The more intricate strategy employed by Complexes I and III is needed in order to couple the
electron transport to proton pumping.