Hug D H, Hunter J K
Bacteriology Research Laboratory, Department of Veterans Affairs Medical Center, Iowa City, IA 52246.
Photochem Photobiol. 1994 Mar;59(3):303-8. doi: 10.1111/j.1751-1097.1994.tb05038.x.
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.
尿刊酸（UCA）是角质层中的一种发色团。紫外线辐射（紫外线B）已被证明可抑制哺乳动物的细胞介导免疫。反式UCA向顺式UCA的光异构化被认为是抑制过程的起始因素。通过各种实验已证明顺式尿刊酸可抑制免疫。研究人员应意识到，在实验操作过程中，实验室照明可能会使反式UCA和顺式UCA相互转化。一种异构体可能会因这种无意的污染而影响结果。我们证明荧光灯、日光、阳光和白炽灯都能引起异构化。窗户玻璃以及塑料和透明玻璃制成的容器材料都不能滤除有效波长，但荧光灯具上的三种商用塑料扩散器可防止异构化。由于顺式UCA的摩尔消光系数（ε）小于反式UCA，我们将0.1 mM的反式UCA暴露于环境光下，并监测吸光度的变化。当已知初始纯度和吸光度时，给出了一种根据277 nm处的吸光度计算反式和顺式异构体百分比的方法。使用该程序，我们验证了顺式UCA的摩尔消光系数。
