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00925njm 22002777a 4500 dc Palacios-Callender, Miriam author Hollis, Veronica author Frakich, Nanci author Mateo, Jesus author Moncada, Salvador author 2007-01-01 Nitric oxide (NO), generated endogenously in NO-synthase-transfected cells, increases the reduction of mitochondrial cytochrome c oxidase (CcO) at O2 concentrations ([O2]) above those at which it inhibits cell respiration. Thus, in cells respiring to anoxia, the addition of 2.5 microM L-arginine at 70 microM O2 resulted in reduction of CcO and inhibition of respiration at [O2] of 64.0+/-0.8 and 24.8+/-0.8 microM, respectively. This separation of the two effects of NO is related to electron turnover of the enzyme, because the addition of electron donors resulted in inhibition of respiration at progressively higher [O2], and to their eventual convergence. Our results indicate that partial inhibition of CcO by NO leads to an accumulation of reduced cytochrome c and, consequently, to an increase in electron flux through the enzyme population not inhibited by NO. Thus, respiration is maintained without compromising the bioenergetic status of the cell. We suggest that this is a physiological mechanism regulated by the flux of electrons in the mitochondria and by the changing ratio of O2:NO, either during hypoxia or, as a consequence of increases in NO, as a result of cell stress. J Cell Sci. 2007; 120(Pt 1):160-5 0021-9533 http://hdl.handle.net/20.500.12105/11921 17164295 10.1242/jcs.03308 Journal of cell science Cytochrome c oxidase maintains mitochondrial respiration during partial inhibition by nitric oxide.