PHOTOISOMERIZATION SPECTRUM OF UROCANIC ACID IN HUMAN SKIN AND IN-VITRO - EFFECTS OF SIMULATED SOLAR AND ARTIFICIAL ULTRAVIOLET-RADIATION

Ultraviolet (UV) irradiation of trans-urocanic acid (UCA), a major UV absorbing component of the epidermis, leads to the formation of cis-UC A, which mediates immunosuppressive effects. In this study, the net yi eld of cis-UCA was measured after the photoisomerization of urocanic a cid by narrow UV wavebands (spectral range 295-405 nm), with the irrad iation doses related to solar irradiance at sea level. The formation o f cis-UCA in Caucasian skin (in vivo), as well as in aqueous solution (in vitro), was determined by HPLC analysis. The same irradiation cond itions were met in both components of the study. The in vivo experimen ts showed high efficiency of cis-UCA formation in the spectral region of 305-341 nm, whereas high efficiency in vitro was found at 305 and 3 26 nm. At 350 and 363 nm, cis-UCA was formed in vivo, but not in vitro . At longer test wavelengths up to 405 nm, no significant formation of cis-UCA was detectable. The established partition between UVB and UVA at 320 nm is not relevant for the isomerization pattern of UCA. Addit ional studies revealed substantial cis-UCA formation in human skin by UVA phototherapy lamps. Furthermore, raised levels of 295 nm irradiati on doses, a possible effect of stratospheric ozone depletion, were fou nd to increase the cis-UCA yield. Our results demonstrate that the for mation of cis-UCA in the skin with common exposures takes place over a broad spectrum range of UVB and WA, up to at least 363 nm. These find ings emphasize the potency of WA to isomerize UCA, and they may contri bute to further elucidation of the effects of phototherapy and sunbath ing.