Chemistry, physiology and pathology of free radicals

The superoxide anion radical and other reactive oxygen species (ROS) are fo rmed in all aerobic organisms by enzymatic and nonenzymatic reactions. ROS arise in both physiological and pathological processes, but efficient mecha nisms have evolved for their detoxification. Similarly, reactive nitrogen i ntermediates (RNI) have physiological activity, but can also react with dif ferent types of molecules, including superoxide, to form toxic products. RO S and RNI participate in the destruction of microorganisms by phagocytes, a s in the formation of a myeloperoxidase-hydrogen peroxide-chloride/iodide c omplex which can destroy many cells, including bacteria. It is known that t he cellular production of ROS and RNI is controlled by different mechanisms . These free radicals can react with key cellular structures and molecules, thus altering their biological function. An imbalance between the systems producing and removing ROS and RNI may result in pathological consequences.