Sources and responses to reactive oxygen and nitrogen species (ROS and RNS) in the skin. These highly reactive entities are generated as a result of normal intracellular metabolism in mitochondria and peroxisomes, and by a variety of cytosolic enzyme systems. The skin is peculiarly rich of enzymatic and nonenzymatic systems for the regulation of overall ROS and RNS levels and hence for the maintenance of physiological homeostasis, due to its direct contact with strong pro-oxidative physical and chemical insults from the environment. These include UV and ionizing radiations, and a variety of pollutants. During chronic inflammatory events, the persistent release of potent cytokines by infiltrating leukocytes contribute to perturb the redox balance, essentially through the upregulated expression of numerous enzymes involved in the regulation of this balance. Lower-than-normal levels of ROS and RNS leads to impaired cell proliferation and reduced host defence. Also an increase in their levels is detrimental for the skin, leading to damage to cell components and eventually acceleration of ageing and age-related diseases.
