The oxidation-reduction kinetics of cytochromes b, c1 and c in initially fully reduced mitochondrial membranes are in agreement with the Q-cycle hypothesis.

Stopped-flow experiments were performed to distinguish between two hypotheses, the Q-cycle and the SQ-cycle, each describing the pathway of electron transfer in the QH2:cytochrome c oxidoreductases. It was observed that, when mitochondrial membranes from the yeast Saccharomyces cerevisiae were poised at a low redox potential with appropriate amounts of sodium dithionite to completely reduce cytochrome b, the kinetics of oxidation of cytochrome b showed a lag period of maximally 100 ms. Under the same experimental conditions, the oxidation-reduction kinetics of cytochromes c + c1 showed transient behaviour. These results do not support the presence of a mobile species of semiquinone in the QH2:cytochrome c oxidoreductases, as envisaged in the SQ-cycle, but are consistent with a Q-cycle mechanism in which the two quinone-binding domains do not exchange electrons directly on the timescale of turnover of the enzyme.