Reverse electron transfer and proton motive force driven NAD+ reduction

Fedor and Hirst (2018) describe RET as NAD+ reduction by complex I that is linked to succinate oxidation by complex II and driven by high Δp.³ They attribute this description to earlier work by Chance and Hollunger (1960) and by Pryde and Hirst (2011).³ Sousa and colleagues (2016) use the same term in a different setting. They say recent studies suggest that accumulated reduced quinone would trigger RET and superoxide generation by complex I.¹

Fedor and Hirst (2018) point out that RET requires an intact, well-coupled membrane, so its rate can be used to probe the membrane.⁴ They measured RET in submitochondrial particles (SMPs) exposed to the alternative oxidase (AOX) and the detergent DDM. The rate dropped precipitously above 0.25 mg AOX per mg or 0.17 mg DDM per mg.⁵ This was well above the AOX level needed for maximal catalysis. Because the individual enzyme activities stayed high, the authors read the loss of RET as catastrophic membrane failure. They attribute the failure to the DDM added along with the AOX.⁵

Sousa and colleagues (2016) review why mutations that impair complex III also lower complex I. One reading is that complex I stability depends on physical contact with other respiratory complexes. Pharmacological inhibition did not lower complex I to the same extent.¹ A second reading, which they call recent, is that low bc1 and cytochrome c oxidase levels let reduced quinone build up. That would trigger RET and superoxide production by complex I.¹ They add that stable complex I expression without complex III has been shown when ROS production by RET is inhibited directly or indirectly.²

Rose and colleagues (2020) review how fatty acids affect ROS production. Very long chain fatty acids integrate into the inner mitochondrial membrane. Hein and colleagues (2008) are cited for the finding that this lowers the membrane potential by protonophoric activity and reduces ROS generated from RET.⁶ Rose and colleagues note that fatty acids in general can promote ROS through forward electron transport and by interfering with electron transport.⁷ They state, citing Schonfeld and Wojtczak (2008), that the mechanism of this interference is not well understood.⁷