Protons (44) Does fatty acid oxidation really drive reverse electron transport and superoxide generation at complex I?

This post has been extended and adjusted quite considerably in the light of further information. The first five comments in the comments section are from pre update.

I suppose I should say now that I am particularly interested in data which trash the Protons hypothesis. I am so deeply biased in its favour that contradictory evidence has to be taken very seriously. Hence the initial post (preserved and embedded below) and the current extension of it based on another paper, also via Mike. It just goes to show how deeply selective people can be with the information which they pass on and how limited they are in coming forward with what they really think is happening. Personally, I'm interested in how stuff works. That's what I write about. Any agenda comes from the biases I have about how well the Protons hypothesis, largely self generated, fits most of the data.

Needless to say, other papers (Back in Protons 3) using mitochondrial preparations show they generate significant amounts of superoxide using palmitoyl carnitine. Anyway, here we go with the edited post:


The original post:

Well, should I develop any leisure time not taken up with the beach, crabbing, canoeing or any one of the hundreds of school holiday activities which are on-going, I have some serious reading to do!

From Mike Eades:

Mitochondrial fatty acid oxidation and oxidative stress: lack of reverse electron transfer-associated production of reactive oxygen species

The group seem pretty good and are supportive of succinate and mtG3Pdh driven RET, but not of ETFdh driven RET. You can imaging how much that gives me to think about! Needless to say, in view of the age of the paper, the group has interesting stuff published more recently which may have something to say about FFA oxidation and ROS generation.

Life is never as simple as you might like it to be!!!!

More to come, will take time.

Peter

End of original post.


It's worth adding this quote from the results to make things absolutely clear:

"The rate of ROS release from heart mitochondria oxidizing carnitine esters of long- and medium-chain fatty acids was much lower than that in the presence of succinate (Fig. 1A, B, C and D) and comparable to that with NAD-linked substrates, pyruvate or glutamate (not shown). An increase of acylcarnitine concentration from 0.5 mM up to 5 mM (examined with butyryl- and octanoylcarnitine) did not enhance ROS production (not shown)".

You can get ROS to be produced in this preparation, but only by using an ETC inhibitor. That's not physiological. Okay.

Now here is some more current (2013) thinking from Schönfeld and Reiser. This is the Schönfeld, as in the first author of the above (2010) paper. Here is what he says in this review:


Why does brain metabolism not favor burning of fatty acids to provide energy? - Reflections on disadvantages of the use of free fatty acids as fuel for brain


"This should be substantiated by the following quantitative analysis: during complete degradation of one glucose molecule, two molecules FADH2 and 10 molecules of NADH are formed, which corresponds to a FADH2/NADH ratio of 0.2. In contrast, b-oxidation of palmitic acid generates 15 molecules of FADH2 and 31 molecules of NADH, with an FADH2/NADH ratio of approx 0.5. Consequently, during b-oxidation there is competition of NADH and FADH2 electrons for oxidized ubiquinone as electron acceptor. This situation would most likely enhance oxidative stress in neurons for two reasons. Thus, slow NADH oxidation maintained the redox state of the electron carriers upstream of complex III in a highly reduced state, a situation similar to rotenone inhibition of complex I. Such situation enhances the superoxide generations by ETC. Moreover, at a high FADH2/NADH ratio, more FADH2 becomes oxidized by the electron transfer flavoprotein-ubiquinone oxidoreductase, a reaction known to be a potent source for superoxide generation".

Let's zoom in:

"Consequently, during b-oxidation there is competition of NADH and FADH2 electrons for oxidized ubiquinone as electron acceptor".

The whole quote and most especially the crucial snippet could have been lifted almost directly from the Protons thread. This is exactly the argument I made for the use of lactate rather than palmitate in neurons. This is simply one facet of the overall Protons concept, which is largely based on the FADH2/NADH ratio.

NB In the 2010 paper there was no difference in total ROS generated between feeding the mitochondria on pyruvate or palmitoyl canitine. Go figure!

Bear in mind that in his 2010 paper Schönfeld found very low generation of superoxide from any fatty acid source (using heart and liver mitochondria) and, although the group have some info since then from brown adipose tissue mitochondrial ROS, they don't appear to have generic data to support Schönfeld's (roughly correct) Protons-like hypothesis above. You can read their quote as well as I can. FADH2 via ETFdh is accepted as driving ROS generation via CoQ reduction. i.e. ROS are generated in proportion to FADH2 which is generated in proportion to the length and saturation of beta oxidised FFAs. They don't specify RET, the ROS may come from ETFdh directly, but I can live with that (should it turn out to be correct). It's the CoQ reduction and FADH2 input that speak to me.

They didn't find anything like this in their 2010 paper comparing ROS from butyric acid to octanoic acid to palmitic acid! All three substrates generated ROS comparable to pyruvate despite the FADH2/NADH ratio being very different.

My presumption is that Schönfeld considers his version of the Protons FADH2/NADH concept to be correct and I'd be willing to bet he even knows exactly why the 2010 model doesn't show this.

But he ain't sayin' nuffing. There's a lot of it about.

Summary: I don't think I'm about to discard my pet hypothesis quite yet!

Peter