Urocanic Acid Concentration and Photoisomerization in Caucasian Skin Phototypes

Abstract— To investigate the relationship between erythemal sensitivity of the skin to U V radiation and epidermal urocanic acid (UCA) concentration, 45 healthy volunteers of anamnestic skin phototypes (ASP) I-IV were studied. In 16 of the subjects, we analyzed UCA photoisomerization after graded UVB exposures. The median and mean total UCA concentration in unirradiated skin was 22.4 and 35.3 nmol/cm², and no statistically significant difference in total UCA concentrations was detectable either between ASP I through II and III through IV or between the phototested skin type (PSP) groups 1 through 2 and 3 through 4. The relative amount of the cis -isomer varied between 3 and 35%, with median and mean values of 7 and 12%, respectively. No statistically significant difference in absolute or relative cis -UCA concentrations was detectable between ASP I through II and III through IV, but a significantly lower absolute ( P < 0.009) and relative ( P < 0.002) cis -UCA concentration in unirradiated skin was recorded in PSP groups 1 through 2, compared to types 3 through 4. In all tested subjects, an erythemally weighted dose of 1 mj/cm²sufficed to cause trans - to cis -UCA isomerization. When comparing photosensitive (skin phototype I) and phototolerant (phototypes III and IV) individuals, who were irradiated with a reference 5 mJ/cm²UV dose or with fractions of 0.1-1.0 of their individual minimal erythema dose values, no skin phototype-dependent difference in ability to photoisomerize was discernible.