Is redox-cycling ubiquinone involved in mitochondrial oxygen activation?

In most tissues mitochondria consume more than 90% of cellular oxygen. Although the greatest part of it undergoes tetravalent reduction thereby conserving free energy changes in the form of ATP, a great deal of evidence exists in the literature that also univalently reduced dioxygen is released during respiration. Redox-cycling ubiquinone was considered most frequently to be involved in this univalent e- transfer to oxygen out of sequence however, other components of the respiratory chain could not be excluded. Our investigations on this problem questioned the role of redox-cycling ubiquinone in mitochondrial O2- formation while H2O2 is supposed to accept e- from this source. The paper provides experimental evidence that H2O2 in fact may operate as an oxidant of ubisemiquinone while dioxygen requires protons for such a reaction which are not available in the phospholipid bilayer where ubiquinone undergoes one e- redox-cycling.