PEROXYNITRITE AND BRAIN MITOCHONDRIA - EVIDENCE FOR INCREASED PROTON LEAK

Peroxynitrite has been reported to inhibit irreversibly mitochondrial respiration. Here we show that three sequential additions of 200 mu M peroxynitrite (initial concentration) to rat brain mitochondria (0.2 m g of protein/ml) significantly stimulated state 4 respiration and that further additions progressively inhibited it. No stimulation of state 3 respiration or of the maximal enzymatic activities of the respirato ry chain complexes was observed on identical peroxynitrite exposure. S tate 4 respiration is a consequence of the proton permeability of the mitochondrial inner membrane, and we demonstrate that the peroxynitrit e-induced stimulation of state 4 respiration is accompanied by a decre ased mitochondrial membrane potential, suggesting an increase in this proton leak. Cyclosporin A did not affect the stimulation, suggesting no involvement of the mitochondrial permeability transition pore. The stimulation was prevented by the lipid-soluble vitamin E analogue Trol ox, suggesting the involvement of lipid peroxidation, a proposed mecha nism of peroxynitrite cytotoxicity. Lipid peroxidation has previously been reported to increase membrane bilayer proton permeability. The hi gh polyunsaturate content of brain mitochondrial phospholipids may pre dispose them to peroxidation, and thus a peroxynitrite-induced, lipid peroxidation-mediated increase in proton leak may apply particularly t o brain mitochondria and to certain neurodegenerative disorders though t to proceed via mechanisms of mitochondrial oxidative damage.