The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

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The Mitochondrial Free Radical Theory of Aging
Fig. 11.6. Variation in proton-motive force ("p") according to the electrodic hypothesis.
of the membrane). This force will be composed entirely of pH difference, since all ions
other than protons are unaffected by the organisation of water and will therefore move to
equilibrate any difference of electrical potential between surface and bulk. Therefore, the
pH at the outer surface of the inner membrane will be lowered by respiration, and conversely
raised by the inhibition of respiration that arises when the mitochondrion becomes
genetically unable to build the respiratory chain. The effects on the rate of lipid peroxidation
then follow as discussed in Section 11.2.
11.3.6. The Gap in the Electrodic Hypothesis
Nonetheless, this revision of the chemiosmotic theory has not been accepted. There is
a major gap in its logic, which was in fact identified in 1981 by Mitchell himself. 51 It is so
conclusive that Kell also rapidly discarded the electrodic model in favour of a much more
complex scheme 52 which invokes additional, still unidentified proteinaceous components
of the proton circuit. That model also has difficulties; it will not be explored further here.
The problem with the electrodic model becomes apparent when one bears in mind
that it is proposed to apply to mitochondria at steady state. That is, not non-respiring
ones, but ones whose rate of respiration (and ATP synthesis) is constant. The organised
orientation of the surface water will impede proton movement perpendicular to the
membrane, but—this is the crucial point—it will not abolish it. Thus, if there is a protonmotive
force between the surface and the adjacent bulk on one or both sides of the
membrane (δpc or δpm)—as there must be if ∂p is to exceed Δp (Fig. 11.6)—then there
will be a net flow of protons down that proton gradient, which, however slow, will inexorably