Recent advances in urocanic acid photochemistry, photobiology and photoimmunology

Abstract

Urocanic acid (UCA), produced in the upper layers of mammalian skin, is a major absorber of ultraviolet radiation (UVR). Originally thought to be a 'natural sunscreen', studies conducted a quarter of a century ago proposed that UCA may be a chromophore for the immunosuppression that follows exposure to UVR. With its intriguing photochemistry, its role in immunosuppression and skin cancer development, and skin barrier function, UCA continues to be the subject of intense research effort. This review summarises the photochemical, photobiological and photoimmunological findings regarding UCA, published since 1998.

cis-Urocanic acid as a potential nickel(ii) binding molecule in the human skin

“…The formation of Ni(II) complexes with cis-UCA is energetically more favoured compared to trans-UCA in the same conditions. Of note are higher Gibbs energy values for NiL 2 complexes compared with NiL, which is in agreement with absorbance 80 maxima being shifted to shorter wavelengths for Ni(cis-UCA) 2 complex, reflecting the presence of two nitrogen ligands in NiL 2 complexes.…”

“…The pH dependence of UV absorption of cis-UCA is presented in 2 , with traces of cis-UCA. Only minor changes in the spectra were observed for a sample containing 4 mM trans-UCA and 2 mM 65 Ni(II), compared to free trans-UCA, and the precipitation occurred at pH 6.5.…”

“…This explains almost the same binding stability of the first and the second trans-UCA molecule. 2 complex is higher at lower temperatures. On the other hand, the entropic contribution is higher at higher temperatures for the Ni(cis-UCA) 2 complex.…”

“…The equilibrium concentration for cis-UCA reaches 60-70% of total epidermal UCA. 2 Both UCA isomers are removed from the skin in sweat and by the desquamation process, however some UCA is transported to blood and consequently to urine. 3,4 UCA, as a 25 component of Natural Moisturizing Factor (NMF), is involved in the regulation of pH of the skin and skin hydration.…”

Membranothropic properties of the urocanic acid

“…Одним из важных факторов, влияющих на ход фотореакций УК (а, следовательно, и на последующие био-химические превращения и биомедицинские последствия), является спектральный состав УФ-излучения. Им определяется ход фото-превращения УК: транс-цис-изомеризация, ионизация либо образование триплетов УК с последующей генерацией активных форм кис-лорода [1,[4][5][6].Еще одним важным фактором, влияющим на фотопревращения УК, однако исследован-ным еще не так подробно, является реакцион-ная среда, т. е. микроокружение УК. Показано, что скорость транс-цис-изомеризации УК и соотношение концентраций изомеров в фото-стационарном состоянии линейно коррелиру-ют с полярностью растворителя (исключением является вода) [7].…”