Previous studies in mammalian models indicate that the rate of mitochondrial reactive oxygen species ROS production and the ensuing modification of mitochondrial DNA (mtDNA) link oxidative stress to aging rate. However, there is scarce information concerning this in relation to caloric restriction (CR) in the brain, an organ of maximum relevance for ageing. Furthermore, it has never been studied if CR started late in life can improve those oxidative stress-related parameters. In this investigation, rats were subjected during 1 year to 40% CR starting at 24 months of age. This protocol of CR significantly decreased the rate of mitochondrial H(2)O(2) production (by 24%) and oxidative damage to mtDNA (by 23%) in the brain below the level of both old and young ad libitum-fed animals. In agreement with the progressive character of aging, the rate of H(2)O(2) production of brain mitochondria stayed constant with age. Oxidative damage to nuclear DNA increased with age and this increase was fully reversed by CR to the level of the young controls. The decrease in ROS production induced by CR was localized at Complex I and occurred without changes in oxygen consumption. Instead, the efficiency of brain mitochondria to avoid electron leak to oxygen at Complex I was increased by CR. The mechanism involved in that increase in efficiency was related to the degree of electronic reduction of the Complex I generator. The results agree with the idea that CR decreases aging rate in part by lowering the rate of free radical generation of mitochondria in the brain.

Aging is characterized by decrements in maximum function and accumulation of mitochondrial DNA mutations, which are best observed in organs such as the brain that contain post-mitotic cells. Oxygen radicals are increasingly considered responsible for part of these aging changes. Comparative studies of animals with different aging rates have shown that the rate of mitochondrial oxygen radical generation is directly related to the steady-state level of oxidative damage to mitochondrial DNA and is inversely correlated with maximum longevity in higher vertebrates. The degree of unsaturation of tissue fatty acids also correlates inversely with maximum longevity. These are the two known traits connecting oxidative stress with aging. Furthermore, caloric restriction, which decreases the rate of aging, proportionately decreases mitochondrial oxygen radical generation, especially at complex I. These findings are reviewed, highlighting the results obtained in the brain.

The effect of long-term caloric restriction and aging on the rates of mitochondrial H(2)O(2) production and oxygen consumption as well as on oxidative damage to nuclear (nDNA) and mitochondrial DNA (mtDNA) was studied in rat liver tissue. Long-term caloric restriction significantly decreased H(2)O(2) production of rat liver mitochondria (47% reduction) and significantly reduced oxidative damage to mtDNA (46% reduction) with no changes in nDNA. The decrease in ROS production was located at complex I because it only took place with complex I-linked substrates (pyruvate/malate) but not with complex II-linked substrates (succinate). The mechanism responsible for that decrease in ROS production was not a decrease in mitochondrial oxygen consumption because it did not change after long-term restriction. Instead, the caloric restricted mitochondria released less ROS per unit electron flow, due to a decrease in the reduction degree of the complex I generator. On the other hand, increased ROS production with aging in state 3 was observed in succinate-supplemented mitochondria because old control animals were unable to suppress H(2)O(2) production during the energy transition from state 4 to state 3. The levels of 8-oxodG in mtDNA increased with age in old animals and this increase was abolished by caloric restriction. These results support the idea that caloric restriction reduces the aging rate at least in part by decreasing the rate of mitochondrial ROS production and so, the rate of oxidative attack to biological macromolecules like mtDNA.