Paul Brookes wrote:
> this is my chief argument against the existance of a mitochondrial NOS
> - if it was inside the mito's next to a known source of O2.- it would
> just be a peroxynitrite factory!
Unless, of course, MnSOD is so blindingly efficient that they can get
away with it. I don't think we know that it isn't.
I absolutely agree that evidence for O2.- and NO. coming together anywhere
in vivo -- in the absence of exogenous toxins -- is conspicuously lacking.
I would be interested in Sydney's view on this.
> I hadn't heard your other proposal, about cells overtaken by mutant mito's
> producing extracellular O2.- Is this really the case, as I thought all
> O2.- made by mito's was dismuted to H2O2 - you can only detect O2.-
> production by mito's if you knock out MnSOD or use sub-mitochondrial
Sure -- this extracellular O2.- is not generated by mitochondria. It is
generated (in my model, which is certainly still controversial but has
quite a bit of recent evidence in its favour) by plasma membrane enzymes
which accept electrons from cytosolic NADH and pass them to extracellular
acceptors; O2 is probably not the most assiduous such acceptor in vivo
but its abundance makes up for that. This idea was originally proposed
by Larm et al (JBC 269:30097) as a way in which OXPHOS-compromised cells
might survive long-term in vivo, as they appear to do; my contribution
was to consider the next stage, where these electrons go when they get
out of the cell and what deleterious consequences might ensue.
> The reason I say ANT doesn't have much control over ox-phos is Rod'
> Hafner's paper when he was in Martin brand's lab.
EJB 188:313 ? I have no difficulty with the idea that the ANT has only
that proportion of the control, but I think that's quite enough. I agree
that the raised proton gradient will raise the leak, but that can't cause
a TOTAL abolition of the rise in proton gradient, because such abolition
would promptly lower the leak again. (I ignore the permanent effect of
peroxidation, because in my model all that does is raise the mitochondrial
turnover rate and thus not the steady-state peroxidation levels.) The
equilibrium H+ gradient is necessarily higher than if the ANT is working,
just we can't easily say how much higher.
> From what I can tell, the
> further back along the eTC you inhibit, the greater the effect on
> superoxide production
My view is that it is fast becoming untenable to equate inhibition by
chemicals and by mutations. The paper I mentioned eariler (Guidot et al,
1993, J Biol Chem 268:26699) is very persuasive to me that loss of all
the mt-coded subunits, which of course means inhibition of Complex I,
*reduces* superoxide production -- presumably by stopping the electrons
so early that they never get to reduce vulnerable quinones.
> I think it is very important to draw a
> distinction between inhibition of electron transport at the complexes,
> which is a genuine inhibition with no "escape route" for the electrons
> except O2.- production, and inhibition by feedback from further down the
> system, in which case proton leak is an effective alleviator.
I fully agree, but in doing so we must also take into account the fact
that inhibition of electron transport causes inhibition of proton export,
thus (in my model, anyway) making the superoxide less toxic because less
of it is HO2.
> Liu et al propose that this is one mechanism by which superoxide casues
> proton leak. It is, of course, incorrect, as the amounts of O2.- produced
> are wildly insufficient to account for the levels of H+ leak observed
Not quite agreed -- Liu's model certainly needs a lot more superoxide to
be made than is observed, but as I understand it the relevant superoxide
would not BE observed -- he favours a re-export of most of it, as actual
superoxide not as HO2, by a carrier (another of our friend Vlad's ideas).
After which it may well be rapidly neutered by ferricytochrome c, so in
total we can't say anything about how much superoxide was initially made.
I still prefer straight (peroxidation-enhanced) proton leak though.
> I propose (in the ONOO- J. Neurochem. paper) that it might be due to
> lipid peroxidation
So you do, and I find myself scandalously uncited :-) I jest of course,
not least because you've made up for my failure to cite the 1977 Russian
work -- I see it is the first ever non-Russian citation of Ivanov et al!
The ability of HO2. *itself* to initiate lipid peroxidation is my main
argument, of course.
Aubrey de Grey