UROCANIC ACID CONCENTRATION AND PHOTOISOMERIZATION IN CAUCASIAN SKIN PHOTOTYPES

To investigate the relationship between erythemal sensitivity of the s kin to UV radiation and epidermal urocanic acid (UCA) concentration, 3 5 healthy volunteers of anamnestic skin phototypes (ASP) I-IV were stu died. In 16 of the subjects, we analyzed UCA photoisomerization after graded UVB exposures, The median and mean total UCA concentration in u nirradiated skin was 22.4 and 35.3 nmol/cm(2), and no statistically si gnificant difference in total UCA concentrations was detectable either between ASP I through II and III through IV or between the phototeste d skin type (PSP) groups 1 through 2 and 3 through 4. The relative amo unt of the cis-isomer varied between 3 and 35%, with median and mean v alues of 7 and 12%, respectively, No statistically significant differe nce in absolute or relative cis-UCA concentrations was detectable betw een ASP I through II and III through IV, but a significantly lower abs olute (P < 0.009) and relative (P < 0.002) cis-UCA concentration in un irradiated skin was recorded in PSP groups 1 through 2, compared to ty pes 3 through 4. In all tested subjects, an erythemally weighted dose of 1 mJ/cm(2) sufficed to cause trans- to cis-UCA isomerization, When comparing photosensitive (skin phototype I) and phototolerant (phototy pes III and IV) individuals, who were irradiated with a reference 5 mJ /cm(2) UV dose or with fractions of 0.1-1.0 of their individual minima l erythema dose values, no skin phototype-dependent difference in abil ity to photoisomerize was discernible.