Isomerization of urocanic acid after ultraviolet radiation is influenced by skin pigmentation

Exposure to ultraviolet (UV) radiation may induce erythema, DNA damage and suppression of immune responses. Melanin pigmentation offers protection aga inst the first two of these effects, but immunosuppression seems to occur i rrespective of the subject's pigmentation. Cis-urocanic acid (cis-UCA), pro duced by isomerization of trans-UCA in the stratum corneum on UV exposure, initiates some of the immunomodulatory effects of UV radiation. In the pres ent study the relationship between skin pigmentation and UCA isomerization has been examined in 28 healthy individuals of skin types I-IV. Pigmentatio n is measured in five areas of not recently exposed back skin before irradi ation with 0, 0.45, 0.9, 1.8 and 3.6 standard erythema dose (SED) of filter ed broadband UV-B (1 SED = 10 mJ cm(-2) at 298 nm). The concentration of UC A isomers is measured immediately after the irradiation. With 3.6 SED, the relative production of cis-UCA is close to the maximum obtainable, irrespec tive of skin type. A significant negative correlation is found between pigm entation and relative production of cis-UCA at 0.45 and 1.8 SED, and betwee n pigmentation and absolute production of cis-UCA at 0.45 SED. At doses of 0.45 and 0.9 SED the relative and absolute production of cis-UCA are higher in the group with skin types I and II when compared with the group with sk in types III and IV. The higher isomerization in the lightly pigmented subj ects than in the more pigmented ones may indicate that people with fair ski n are at a relatively higher risk of immunosuppression when exposed to low doses of UV radiation. (C) 1999 Elsevier Science S.A. All rights reserved.