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 r esponsible of the transformation of redoxcycling ubiquinone (UQ) from a save electron carrier to an O-2(.-) 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(.-)) req uires 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 th e membrane, b) the physical state of the membrane remains unchanged wh ile the binding of redox-cycling UQ is changed such that SQ(.-) will c ome into contact with the aqueous phase in the polar head group sectio n. Spin probes were used to follow changes of the physical order of ph ospholipids of the inner mitochondrial membrane. Binding changes of mi tochondrial SQ(.-) were assessed from power saturation experiments and spin-spin interactions with a Cr3+ Salt of the aqueous phase were stu died 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, t he physical property of the membrane was affected such that protons co uld penetrate and allow SQ(.-) to undergo autoxidation. In contrast, m itochondrial respiration of cytosolic NADH accumulating during ischemi a involves a low saturating SQ(.-) species that readily autoxidizes du e 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.