Nonenzymatic nitric oxide formation during UVA irradiation of human skin: experimental setups and ways to measure.

Many of the local ultraviolet (UV)-induced responses, including erythema and edema formation, inflammation, premature aging, and immune suppression, can be influenced by nitric oxide synthase (NOS)-produced NO, which plays a pivotal role in cutaneous physiology. Besides enzyme-mediated NO production, UV radiation triggers an enzyme-independent NO formation in human skin. This occurs due to decomposition of photoreactive nitrogen oxides like nitrite and S-nitrosothiols, which are present in human skin at relatively high concentrations and lead to high-output formation of bioactive NO. This enzyme-independent NO formation opens a new field in cutaneous physiology and will extend our understanding of mechanisms contributing to skin aging, inflammation, and cancerogenesis but also functional protection. Therefore, it is of high interest to examine the chemical storage forms of these potential NO-generating agents in skin, the mechanisms and kinetics of their decomposition, and their biological relevance.