Figure 3. Modes of mitochondrial operation that lead to O₂^•− production.

There are three modes of mitochondrial operation that are associated with O₂^•− production. In mode 1, the NADH pool is reduced, for example by damage to the respiratory chain, loss of cytochrome c during apoptosis or low ATP demand. This leads to a rate of O₂^•− formation at the FMN of complex I that is determined by the extent of FMN reduction which is in turn set by the NADH/NAD⁺ ratio. Other sites such as αKGDH may also contribute. In mode 2, there is no ATP production and there is a high Δp and a reduced CoQ pool which leads to RET through complex I, producing large amounts of O₂^•−. In mode 3, mitochondria are actively making ATP and consequently have a lower Δp than in mode 2 and a more oxidized NADH pool than in mode 1. Under these conditions, the flux of O₂^•− within mitochondria is far lower than in modes 1 and 2, and the O₂^•− sources are unclear.