Recent investigations suggest that complex I is a potent source of reactive oxygen species. Complex I can produce superoxide (as well as hydrogen peroxide), through at least two different pathways. During forward electron transfer, only very small amounts of superoxide are produced (probably less than 0.1% of the overall electron flow).
During reverse electron transfer, complex I might be the most important site of superoxide production within mitochondria, with around 3-4% of electrons being diverted to superoxide formation. Reverse electron transfer, the process by which electrons from the reduced ubiquinol pool (supplied by succinate dehydrogenase, glycerol-3-phosphate dehydrogenase, electron-transferring flavoprotein or dihydroorotate dehydrogenase in mammalian mitochondria) pass through complex I to reduce NAD to NADH, driven by the inner mitochondrial membrane potential electric potential. Although it is not precisely known under what pathological conditions reverse-electron transfer would occur in vivo, in vitro experiments indicate that this process can be a very potent source of superoxide when succinate concentrations are high and oxaloacetate or malate concentrations are low. This can take place during tissue ischaemia, when oxygen delivery is blocked.
Superoxide is a reactive oxygen species that contributes to cellular oxidative stress and is linked to neuromuscular diseases and aging. NADH dehydrogenase produces superoxide by transferring one electron from FMNH (or semireduced flavin) to oxygen (O). The radical flavin leftover is unstable, and transfers the remaining electron to the iron-sulfur centers. It is the ratio of NADH to NAD that determines the rate of superoxide formation.