The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

The Search for How Mutant mtDNA is Amplified
organism, on the basis that if it were disadvantageous then it would have been evolved away.
This has led to the proposal that it increases the potential for regulation of metabolism:
organisms may benefit from being able to vary rather rapidly the rate of OXPHOS within a
cell, and this is achieved by the presence of “futile cycles” (of which proton leak would be
one) which act as extra loci of control on the steady-state rate. This is sound in principle as
an explanation for having some leak, but it does not appear to explain why some animals
have far more leak than others. Another proposal, 20 which I believe to be logically flawed, is
that proton leak exists as a safety valve to dissipate the mitochondrial proton gradient at
times when the cell is at rest, i.e., using little ATP; the idea is that a high gradient causes
greater production of superoxide, which is undesirable. The flaw in this is that the gradient
can be controlled in other ways that do not involve the wastage of nutrients which leak does:
these include the permeability transition pore, which is available as an emergency safety
valve, and control of the rate of the precursors to the respiratory chain (the TCA cycle, etc.)
which would have a longer lag but are less drastic. This challenge applies equally to a third
proposal: that leak exists to maintain the cellular NAD + /NADH ratio at a value compatible
with various biosynthetic reactions such as amino acid synthesis. Cells have a variety of
systems to use for this purpose, including the reversible conversion of pyruvate to lactate,
which likewise do not waste nutrients in the way leak does.
This compels one to revisit the precept upon which the search for such uses of leak
was founded: that if it were genuinely a bad thing it would have disappeared during
evolution. The alternative viewpoint is that it really is a bad thing, but that it is a side-effect
of some vital process, and evolution has not found a way to maintain that vital process
without having leak too. A candidate for the vital process in question is readily to hand:
OXPHOS itself. It is generally accepted that one side-effect of OXPHOS—superoxide
production—is a bad thing, so evolution already has an imperfect track record in this
regard.
But how could leak be a side-effect of OXPHOS, let alone an inevitable one? Remarkably,
a very direct answer to this question has been available in the literature for over 25 years.
LEC-mediated damage affects all macromolecules: in particular, it affects lipids, the molecules
that make up the mitochondrial membranes. The only missing link, therefore, is whether
membranes that have suffered LEC-mediated damage are leakier to protons. If they are,
then we have an explanation for proton leak as a deleterious, but inevitable, side-effect of
mitochondrial function. And indeed, lipid peroxidation does make membranes leakier. 21,22
A couple of very recent results confirm this in different ways. If one isolates the lipids
from a preparation of mitochondria and reconstructs them as vesicles—usually termed
liposomes—then one can measure their proton leak and compare it to that of intact
mitochondria. The remarkable finding is that liposomes prepared in this way exhibit only
about 5% of the leak of intact mitochondria. 23 It is hard to see how this can result from the
absence of the non-lipid components of mitochondrial membranes (transmembrane
proteins and coenzyme Q), since they are also highly hydrophobic so should repel protons;
but if most leak is caused by peroxidation then a simple explanation emerges. Peroxidation
proceeds as a chain reaction (see Section 3.7), so the products of peroxidation in an intact
mitochondrion will be concentrated around the occasional point at which a reaction was
initiated. These locally very high levels of membrane damage will constitute “pin-pricks” in
the membrane, through which protons will flow rapidly. Synthesis of liposomes, on the
other hand, will homogenise the lipid, spreading the peroxidation products evenly through
it; thus they will have a uniformly low concentration which will only negligibly ease the
passage of protons.
The second recent study showing the effect of peroxidation was an X-ray diffraction
analysis, which allowed the detection of peroxidation-induced changes in membrane
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