Synergistic inhibition of respiration in brain mitochondria by nitric oxide and dihydroxyphenylacetic acid (DOPAC) Implications for Parkinson's disease

The inhibition of mitochondrial respiration by nitric oxide ((NO)-N-center dot) at cytochrome c oxidase level has been established as a physiological regulatory mechanism of mitochondrial function. Given, on the one hand, the potential involvement of (NO)-N-center dot and dopamine metabolism in mitochondrial dysfunction associated with neurodegeneration and, on the other hand, the reported interaction of (NO)-N-center dot with dihydroxyphenylacetic acid (DOPAC), a major mitochondrial-associated dopamine metabolite, we examined the combined effects of (NO)-N-center dot and DOPAC on the respiratory chain of isolated rat brain mitochondria. Whereas dopamine or DOPAC induced no measurable effects on the mitochondrial respiration rate, a mixture of (NO)-N-center dot with DOPAC inhibited the rate in a way stronger than that exerted by (NO)-N-center dot. This effect was noticed with actively respiring (state 3) and resting (state 4) mitochondria. At variance with DOPAC, dopamine failed to potentiate (NO)-N-center dot inhibitory effects. The inhibition was dependent on the concentration of both compounds, (NO)-N-center dot and DOPAC, and exhibited characteristics similar to those exerted by (NO)-N-center dot, namely: it was reversible and dependent on the concentration of oxygen. Analysis of respiratory enzymatic activities demonstrated a selective inhibition at the level of cytochrome c oxidase (complex IV). Insights into the chemical mechanisms underlying the inhibitory effect were inferred from experiments using metmyoglobin (a ligand for (NO)-N-center dot and derived species, such as nitroxyl anion) and ferrocyanide (a reductant of (NO)-N-center dot, producing nitroxyl anion). Whereas metmyoglobin decreased the inhibition, ferrocyanide potentiated the inhibition. Moreover, a mixture of ferrocyanide with (NO)-N-center dot reproduced the effects exerted by the mixture of (NO)-N-center dot with DOPAC. The results are consistent with the notion of a reaction of (NO)-N-center dot with DOPAC producing a nitric oxide-derived compound(s), which inhibit O-2 uptake at the cytochrome oxidase level. Although the mechanism in question remains to be clearly elucidated it is suggested that the (NO)-N-center dot/DOPACdependent inhibition of cytochrome oxidase may involve nitroxyl anion. The significance of these observations for mitochondrial dysfunction inherent in Parkinson's disease is discussed. (c) 2005 Elsevier Ltd. All rights reserved.