Nitrite reductase activity is a novel function of mammalian mitochondria

Nitrite, which is the major stable degradation product of nitric oxide, exi sts in all tissues capable of nitric oxide synthesis from L-arginine. The p resent study provides experimental evidence that nitrite in contact with re spiring mitochondria accepts reducing equivalents from the ubiquinone cycle of the respiratory chain. Univalent reduction of nitrite was totally inhib ited by myxothiazol. We therefore conclude on the involvement of redox cycl ing that ubisemiquinone is associated with the bc(1) complex. Recycling of nitric oxide degradation products via these electron carriers may become a threat to energy-linked respiration since nitric oxide in direct contact wi th mitochondria was shown to slow the energy-linked respiration down and to trigger a mitochondrial source for superoxide radicals. Until now, the exi stence of nitrite reductase activity was only demonstrated in plants and ba cteria, In addition, the present observation elucidates the existence of a nitric oxide synthase-independent nitric oxide source. (C) 1999 Federation of European Biochemical Societies.