Conditions allowing redox-cycling ubisemiquinone in mitochondria to establish a direct redox couple with molecular oxygen

The present investigation seeks to elucidate the molecular mechanism responsible of the transformation of redox-cycling ubiquinone (UQ) from a save electron carrier to an O2·− generator as observed in toluene-treated mitochondria as well as in mitochondria exposed to conditions of organ ischemia/reperfusion. Starting from the earlier finding that for thermodynamic grounds autoxidation of ubisemiquinone (SQ·−) requires the accessibility of protons, two possibilities were considered: a) protons from the aqueous phase may penetrate into the phospholipid bilayer and react with SQ·− due to a decreased hydrophobicity of the membrane, b) the physical state of the membrane remains unchanged while the binding of redox-cycling UQ is changed such that SQ·− will come into contact with the aqueous phase in the polar head group section. Spin probes were used to follow changes of the physical order of phospholipids of the inner mitochondrial membrane. Binding changes of mitochondrial SQ·− were assessed from power saturation experiments and spin-spin interactions with a Cr3+ salt of the aqueous phase were studied to recognize orientation changes via the polar head group section of the membrane. Our results show that autoxidation of SQ·− occurs in two different ways. In the case of membrane insertion of toluene, the physical property of the membrane was affected such that protons could penetrate and allow SQ·− to undergo autoxidation. In contrast, mitochondrial respiration of cytosolic NADH accumulating during ischemia involves a low saturating SQ·− species that readily autoxidizes due to its spatial orientation close to the aqueous face of the membrane. We conclude from these observations that in line with thermodynamics autoxidation of SQ·− in mitochondria requires protons that normally have no access.