The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

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The Mitochondrial Free Radical Theory of Aging
cease activity and allow the oxygen supply to catch up, so that the lactate can be turned back
into pyruvate and its mitochondrially-mediated derivatives.
But the toxicity of lactate is not the reason OXPHOS-less cells in culture die: it is not
present in nearly high enough quantities to be toxic, since after excretion by the cells it is
diluted out in the culture medium. The problem is the NADH. It turns out that a net zero
conversion of NAD + to NADH is not good enough for the cell: it relies on OXPHOS for a
significant conversion the other way. This is because certain vital cellular processes, such as
the synthesis of serine (and consequently various other amino acids), turn NAD + into
NADH. 14 Other equally vital processes do the opposite (usually with NADP rather than
NAD), so it is actually a rather close-run thing whether the cell can balance the books;
facultative aerobes, such as the fungi that exhibit suppressiveness (see Section 6.3), can balance
them when pushed, so they can survive (albeit with severely retarded growth) with the
net zero recycling of NADH that glycolysis provides.
Now it should be clear why pyruvate worked (see Fig. 9.1). The pyruvate created from
glucose was being turned into lactate and excreted; but now, the exogenous, imported
pyruvate could also be turned into lactate and excreted. This supplied the cell with the net
conversion of NADH to NAD + that it needed in order to go about the rest of its business.
9.3. The Plasma Membrane Oxidoreductase
This was the state of knowledge until 1993. In that year, experiments in the laboratory
of Alfons Lawen discovered a variety of other chemicals which could substitute for pyruvate:
that is, they could keep ρ 0 cells alive without pyruvate, so long as the medium also contained
glucose and uridine. 15 They were eager acceptors of electrons, so they were presumably
working by taking electrons from intracellular NADH, thereby recycling it. But the really
curious thing about these chemicals—ferricyanide, for example—was that they were
impermeant to the cell membrane. Since the NADH is inside the cell at all times (being
also impermeant), there must be some intermediate system in the membrane itself, which
transfers electrons across it.
It was no accident that Lawen investigated such chemicals. Many years previously, a
system had been discovered 16,17 which potentially has the capacity to achieve, for anaerobic
cells, the same effect as the exchange of lactate and pyruvate with their environment. It has
been found in all cell types yet examined; it is called the Plasma Membrane OxidoReductase,
or PMOR. This system is able to accept electrons from intracellular NADH—thereby
regenerating the all-important NAD + —and export them out of the cell, as long as some
chemical is present in the extracellular medium which can then absorb them. Evidently,
despite presumably operating only at low rates in aerobically respiring cells, it is able to
pump electrons at a sufficient rate in anaerobic cells that there is no need for a pyruvate/
lactate exchange with the culture medium.
9.4. Getting the Most ATP Without Oxygen
There is, however, a very big difference between a pyruvate/lactate exchange and a PMOR
in terms of the anaerobic cell’s energy supply. One of the enzymes that Müller-Höcker and
many others since have tested for in muscle fibers 18,19 is succinate dehydrogenase (SDH),
otherwise known as Complex II. They have assayed for it because it has important similarities
to, and differences from, the proton-pumping respiratory chain enzymes, so is a good
“control” for the validity of the experimental technique. Like the proton pumps, it is
embedded in the mitochondrial inner membrane; but unlike them, it is entirely encoded by
nuclear genes. Müller-Höcker and others found that, in line with MiFRA, it was never affected
in the anaerobic fibers they examined. Moreover, they found that it is actually present at