Neuronal mitochondrial toxicity of malondialdehyde: inhibitory effects on respiratory function and enzyme activities in rat brain mitochondria.

Malondialdehyde (MDA) is a product of oxidative damage to lipids, amino acids and DNA, and accumulates with aging and diseases. MDA can possibly react with amines so as to modify proteins and inactivate enzymes; it can also modify nucleosides so as to cause mutagenicity. Brain mitochondrial dysfunction is a major contributor to aging and neurodegenerative diseases. We hypothesize that MDA accumulated during aging targets mitochondrial enzymes so as to cause further mitochondrial dysfunction and additional contributions to aging and neurodegeneration. Herein, we investigated the neuronal mitochondrial toxic effects of MDA on mitochondrial respiration and activities of enzymes (mitochondrial complexes I-V, alpha-ketoglutarate dehydrogenase (KGDH) and pyruvate dehydrogenase (PDH)), in isolated rat brain mitochondria. MDA depressed mitochondrial membrane potential, and also showed a dose-dependent inhibition of mitochondrial complex I- and complex II-linked respiration. Complex I and II, and PDH activities were depressed by MDA at >or=0.2 micromol/mg; KGDH and complex V were inhibited by >or=0.4 and >or=1.6 micromol MDA/mg, respectively. However, MDA did not have any toxic effects on complex III and IV activities over the range 0-2 micromol/mg. MDA significantly elevated mitochondrial reactive oxygen species (ROS) and protein carbonyls at 0.2 and 0.002 micromol/mg, respectively. As for the antioxidant defense system, a high dose of MDA slightly decreased mitochondrial GSH and superoxide dismutase. These results demonstrate that MDA causes neuronal mitochondrial dysfunction by directly promoting generation of ROS and modifying mitochondrial proteins. The results suggest that MDA-induced neuronal mitochondrial toxicity may be an important contributing factor to brain aging and neurodegenerative diseases.