Role of endogenous and exogenous antioxidants in the defence against functional damage and lipid peroxidation in rat liver mitochondria.

Mitochondria are cellular organelles where the generation of reactive oxygen species may be high. They are, however, effectively protected by their high capacities of antioxidative systems, as enzymes and either water or lipid soluble low molecular weight antioxidants. These antioxidative defence systems can be effectively regenerated after or during an oxidative stress as long as the mitochondria are in an energized state. Energization of mitochondria mainly depends on the availability of suitable respiratory substrates which can provide hydrogen for the reduction of either the glutathione- or alpha-tocopherol-system, since GSH is regenerated by glutathione reductase with the substrate NADPH and the alpha-tocopheroxyl-radical likely by reduced coenzyme Q. It was shown that mitochondria do not undergo damages as long as they can keep a high energy state. The delicate balance between prooxidative/antioxidative activities can be shifted towards oxidation, if experimentally prooxidants were added. After exhaustion of the antioxidative defence systems damages of mitochondrial functions become expressed followed by membrane injuries along with the oxidation and degradation of mitochondrial lipids and proteins leading finally to the total degradation of the mitochondria. Extramitochondrial antioxidants may assist the mitochondrial antioxidative defence systems in a complex way, whereby particularly ascorbic acid can act both as prooxidant and as antioxidant.