The effect of clinical UVA/B exposures on urinary urocanic acid isomer levels in individuals with caucasian type (II/III) skin types.

Terrestrial ultraviolet radiation (UVR) exposure, consisting of ultraviolet A (320-40 nm) and B (290-320 nm), results in the photoisomerizion of epidermal trans-urocanic acid (trans-UCA) to cis-urocanic acid (cis-UCA), a potential suppressor of local and systemic immune responses. This study examines urinary UCA isomers as biomarkers of UVA/B exposure. It presents results measuring both cis- and trans-UCA in human urine samples collected from a group of study subjects (skin types II/III) that underwent controlled UVA/B exposures similar to those administered in commercial suntanning parlors. The UCA isomers were purified from urine using C18 solid-phase extraction columns followed by high-performance liquid chromatography (HPLC) with UV absorbance (268 nm) detection. The UCA biomarker was expressed as the ratio of cis-UCA to trans-UCA (UCA ratio), or as cis-UCA concentration corrected for urine volume using creatinine (cis-UCA-Cr). The UCA ratio increased over baseline in the urine of individuals exposed to UVA/B. A single exposure to approximately 70 percent minimal erythema dose (MED) of UVR (95 % UVA/5 % UVB to approximately 90 % of skin area) produced a 4.75-fold increase in the UCA ratio (p< 0.001) relative to baseline. Repeated daily UV exposures of similar doses produced a minimal increase in UCA ratio above that of the single UV exposure. These findings indicate that UCA cis-trans ratio holds promise as a biomarker for recent solar UV exposure.