Adventitious interconversion of cis- and trans-urocanic acid by laboratory light

Urocanic acid (UCA) is a chromophore in the stratum corneum. Ultraviolet radiation (ultraviolet B) has been shown to suppress mammalian cell-mediated immunity. The photoisomerization of trans-UCA to cis-UCA was proposed as the initiator of the suppression process. Cis-urocanic acid has been demonstrated to suppress immunity by a variety of experiments. Investigators should be aware that laboratory illumination may be capable of interconverting trans-UCA and cis-UCA during experimental manipulations. This possible inadvertent contamination of one isomer by the other may influence results. We demonstrated that fluorescent lamps, daylight, sunlight and incandescent lamps were able to bring about isomerization. Window glass and container materials of plastic and clear glass did not filter out effective wavelengths, but three commercial plastic diffusers on fluorescent fixtures prevented the isomerization. Because the molar extinction coefficient (epsilon) for cis-UCA is less than that of trans-UCA, we have exposed 0.1 mM trans-UCA to ambient light and monitored the change in absorbance. A method is given to calculate the percentage of trans and cis isomers from the absorbance at 277 nm when the initial purity and absorbance are known. Using this procedure, we validated the molar extinction coefficient of cis-UCA.