The sunscreening effect of urocanic acid

Urocanic acid (UCA), present in the stratum corneum, is a major absorber of ultraviolet (UV) radiation and, on UV exposure, is induced to isomerize from the naturally occurring trans-isomer to the cis-isomer. Cis-UCA has been shown to have immunosuppressive properties, while trans-UCA may act as a natural sunscreen due to its UV-absorbing properties. The photoprotective capacity of UCA was investigated in this study. Minimal erythema dose (MED) was determined on normal buttock skin in 36 healthy subjects and the concentration of UCA isomers was measured on the skin adjacent to the test site. On the contralateral buttock. MED was determined 20 min after application of trans-UCA 5% in a cream base. The UCA cream gave a sun protection factor of 1.58. The amount of UCA applied was, however, 20-200 times higher than the amount of UCA found in normal skin, making a sunscreening effect of naturally occurring UCA very low. This was further supported, by a lack of correlation between naturally occurring UCA and the UV sensitivity of each subject determined by the MED.