Oxidation of ubiquinol by peroxynitrite: implications for protection of mitochondria against nitrosative damage

A major pathway of nitric oxide utilization in mitochondria is its conversi on to peroxynitrite, a species involved in biomolecule damage via oxidation , hydroxylation and nitration reactions. In the present study the potential role of mitochondrial ubiquinol in protecting against peroxynitrite-mediat ed damage is examined and the requirements of the mitochondrial redox statu s that support this function of ubiquinol are established. (1) Absorption a nd EPR spectroscopy studies revealed that the reactions involved in the ubi quinol/peroxynitrite interaction were first-order in peroxynitrite and zero -order in ubiquinol, in agreement with the rate-limiting formation of a rea ctive intermediate formed during the isomerization of peroxynitrite to nitr ate. Ubiquinol oxidation occurred in one-electron transfer steps as indicat ed by the formation of ubisemiquinone. (2) Peroxynitrite promoted, in a con centration-dependent manner, the formation of superoxide anion by mitochond rial membranes. (3) Ubiquinol protected against peroxynitrite-mediated nitr ation of tyrosine residues in albumin and mitochondrial membranes, as sugge sted by experimental models, entailing either addition of ubiquinol or expa nsion of the mitochondrial ubiquinol pool caused by selective inhibitors of complexes III and IV. (4) Increase in membrane-bound ubiquinol partially p revented the loss of mitochondrial respiratory function induced by peroxyni trite. These findings are analysed in terms of the redox transitions of ubi quinone linked to both nitrogen-centred radical scavenging and oxygen-centr ed radical production. It may be concluded that the reaction of mitochondri al ubiquinol with peroxynitrite is part of a complex regulatory mechanism w ith implications for mitochondrial function and integrity.