[Mitochondria as an source of reactive oxygen species].
Atmospheric oxygen molecules are reduced to water through four consecutive electron transfer reactions in mitochondria. As a consequence of these processes, various by-products (called reactive oxygen species, ROS), including superoxide radical, hydrogen peroxide, hydroxyl radicals, and singlet oxygen are released. Under physiological conditions, ROS are rapidly inactivated and do not induce disruption of cell function. Abnormal function of the respiratory chain, resulting from deficiency, mutation, or inhibition of its elements, leads to elevated ROS production. Complexes I and III of the mitochondrial chain are the most important sites of ROS formation; however, this can be affected by extracellular conditions. Excessive ROS generation leads to decreased efficiency of cells, tissues, and whole organs and is responsible for acceleration of aging and the development of various diseases.