The Mitochondrial Free Radical Theory of Aging - Supernova: Pliki

142
The Mitochondrial Free Radical Theory of Aging
the aqueous medium, and for some phospholipids this group is acidic. Being acidic means
that each head group easily loses a proton, leaving it negatively charged so that it attracts
free protons into its vicinity. The pH in the water right next to a typical phospholipid
membrane, therefore, is considerably lower than in the bulk medium further away from the
membrane 14 (Fig. 11.2). The inner mitochondrial membrane has about 20% acidic
phospholipids, which gives a pH at the surface about one unit* lower than in the bulk, 17a so
there will indeed be plenty of HO2• there. Being neutral, HO2• can also pass through the
membrane into the matrix; this may be an important mechanism of proton leak (see Section
8.4), 17b though unlike straightforward “water wire” transfer of protons it may not be
promoted by lipid peroxidation.
11.2.3. Perhydroxyl and SOS
Now for the relevance to SOS. It comes from the fact that pH is, quite simply,
concentration of protons in the aqueous medium, and protons are what the respiratory
chain pumps. A mtDNA mutation that inhibits the respiratory chain will, therefore, lower
the proton concentration—that is, raise the pH—at the outer surface of its host
mitochondrion’s inner membrane. This will reduce the protonation of superoxide to HO2•
and consequently lower the rate of lipid peroxidation.** It has been argued 18 that all superoxide
production must be on the matrix side of the inner membrane, but the evidence cited
in support of this 9,19 actually only tells us that some is made inside.
Moreover, there is experimental support for this effect of respiration rate on HO2•
levels. Experiments in 1995 in the laboratory of Joe McCord, the codiscoverer of superoxide
dismutase, investigated the effect of respiration on non-enzymatic dismutation of superoxide.
Recall from Table 3.3 that non-enzymatic superoxide dismutation does not occur as a reaction
between two superoxide anions, but instead either between two perhydroxyls or between
one superoxide and one perhydroxyl. (The two-superoxide reaction is not totally absent, in
fact, but it is at least six orders of magnitude slower than the others). 20 Therefore, the rate of
non-enzymatic dismutation is a measure of pH. And indeed, McCord’s laboratory established
that non-enzymatic dismutation of externally generated O2• — in the presence of isolated
mitochondria was much faster when their respiration was rapid than when it was inhibited. 21
On top of all this, a perusal of Table 3.3 shows us that even the levels of HO• may fall
when respiration slows! This is worth explaining, because the simulation noted above 12 is
currently the only work showing that HO2• initiates most lipid peroxidation, and many
biologists (with history on their side, I fully acknowledge) are very wary of simulations. The
reason is that reaction 8 in Table 3.3, which is essentially the only way that HO• is made in
vivo, requires not only superoxide but also H2O2, which is created by superoxide dismutation.
Now, the intermembrane space is one place in the cell where superoxide may be plentiful
but superoxide dismutase is absent,*** because the cytosolic version of the enzyme is too
* The literature is strewn with statements of this pH difference, varying from 2.5 15 to only 0.3; 16 but these
have discussed either unrealistic membrane composition or the pH not quite at the surface. The calculation
cited here 17a is based firmly on well-supported measurements of the relevant biological parameters.
** The same argument implies that loss of respiratory capacity will lower the pH at the inner surface of the
inner membrane, so increasing the levels of HO2• there. This increase will be much less, however, than the
decrease in HO2• levels at the outer surface, since the pH inside will always remain above the value of about
6.5 generated by the head groups. Thus the net HO2• concentration on either side of the membrane will be
reduced by an inhibition of respiration.
*** SOD was originally reported 22 to be present in the intermembrane space, but more recent studies 23,24a
have refuted this. The existence of a compartment lacking SOD may explain why birds have paradoxically
low antioxidant enzyme levels (see Section 6.5.6). 24b