The photoaging of human skin
Free radicals and the aging process
Free radicals are highly reactive chemical species carrying an unpaired electron in their outer orbit. They abstract electrons from the surrounding molecules (lipids, proteins, DNA) in order to complete their own electron structure, thus inducing cellular damage. Such reactions are strongly implicated in the development / pathogenesis of such chronic diseases as atherosclerosis, diabetes, rheumatoid arthritis, cancer, skin and neurodegenerative disorders, as well as in the aging process.
To the free-radical family belong Reactive Oxygen Species (ROS) such as superoxide (*O2-), hydrogen peroxide (H2O2), lipid peroxides (LOO*, LO*, LOOH), hydroxyl radical (*OH) and singlet oxygen (Δ-O2); Reactive Nitrogen Species such as nitric oxide (NO*) and peroxinitrite (ONOO*); and Reactive Chlorine Species such as the hypochlorite anion (OCl- ).
Important internal and external sources of free radicals contributing to the aging process of human tissues are described in Fig. 1, but reactive oxygen species (ROS) are also generated in cell-free systems including O2, H2O2, transition metals as Fe2+/Fe3+, Cu1+/Cu2+, thiols, ascorbate, xanthine/ xanthine oxidase, chelating agents, xenobiotics and others.