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Editor’s Note: Our regular Tech Edge columnist Mindy Goldstein, PhD, welcomes the following contribution from colleague Paolo U. Giacomoni, PhD, also of Estée Lauder.
Life is accompanied by a wealth of chemical reactions. As far as life on earth is concerned, one of the most relevant reactants is molecular oxygen (O2). While oxygen is required by all animals in order to live, it can also be extremely toxic by itself and under the form of reactive oxygen species (ROS) such as H2O2 (hydrogen peroxide), O2∙– (superoxide anion), OH∙ (hydroxyl radical) or 1O2 (singlet oxygen). These ROS provoke spontaneous, rapid and nonspecific chemical reactions that can wreak havoc on biological systems. In the human body, ROS can be formed intrinsically as a result of normal cellular metabolism or extrinsically as a result of environmental stressors.
Superoxide anion, which is generated in the course of cellular respiration (oxidative phosphorylation), and nitric oxide (NO), which is involved in a variety of processes, including vasodilation, neurotransmission and immune defense, are among the ROS generated by normal physiological chemical reactions. NO can be associated with pro-oxidant or antioxidant activity depending on existing conditions. NO itself is not a strong oxidant, but becomes so after reacting rapidly with superoxide anion to give ONOO- (peroxynitrite). Peroxynitrite decomposes to OH∙ (hydroxyl radical) and NO2 (nitrogen dioxide), both of which trigger chain peroxidation via H-abstraction (Figure 1).
This could occur, for example, at sites of inflammation, where macrophages are recruited and activated. When macrophages are activated they undergo a respiratory burst that produces and releases large amounts of nitric oxide.
One of the most widespread stressors on the surface of earth is UV radiation from the sun. Significant experimental evidence has been accumulated about the damaging action of solar radiation and its mechanisms. One of the crucial observations was made when growth media was exposed to UVA radiation. The media became toxic to the cultured mammalian and human cells. In attempt to understand the mechanisms of this UVA cytotoxity, it was observed that when tryptophan is exposed to near UV light (300–400 nm) in aerated water, hydrogen peroxide is generated.