Eureka moment when I tripped over this gem by Vinogradov
"The redox potential of one binuclear [FeS] center (N-1a) is so negative that it can not be reduced by NADH"
Couple this with this group's conclusion:
results lead us to propose a model of thermodynamic control of
mitochondrial ROS production which suggests that the ROS-generating site
of complex I is the Fe-S centre N-1a".
reduce N-1a to generate superoxide using NADH at "normal"
concnetrations. The easiest way you can generate superoxide at N-1a is
by reverse electron flow through complex 1 under conditions of a strong
membrane potential and a high FADH2 input, in this case using succinate.
Very satisfying. They also point out that, if you can get the NAD+/NADH
ratio high enough you can get it far enough from its electrical mid
point to pass electrons "down gradient" to N-1a. At ratios of less than
about 3 parts NADH to one part NAD+ the transfer is uphill and isn't
going to happen. As they say:
"...the reduction of the *ROS site [they consider that it is probably N-1a] is regulated by the NADH/NAD+ ratio rather than the NADH level (eqns 7 and 8)..."
sort of takes us back to B3 and some cancer cells which have probably
lost N-1a so fail to develop insulin resistance, ie they don't limit
their energy generation to their needs. They also develop metastatic
aggressiveness in proportion to their elevated NADH:NAD+ ratio (even if
they expressed it as a reduced NAD+/NADH ratio!). I wrote that post a
while ago, time to check it and hit publish...
Vinogradov pointed out in his review paper that very few labs have the
massively expensive and complex gear to look at this sort of redox
research and both of the papers discussed here are from groups who know
each other, Vinogradov being thanked for reading through the manuscript
of the second paper. But I think they are correct.