On the Roles of Urocanic Acid in Photoimmunosuppression: Attempted Photorepair of Urocanic Acid‐DNA Cyclobutane Adducts with DNA Photolyase

It has been reported that UV-induced immunosuppres-sion can be reversed by photoreactivation or exposure to T4 endonuclease V, two treatments that can repair cyclobutane pyrimidine dimers. These observations, together with the known role of urocanic acid (UA) in UV-induced immune suppression, prompted us to study the ability of DNA photolyase to repair UA-DNA cyclobutane photoadducts in single-stranded calf thymus DNA. We did not detect any release of UA, with a sensitivity implying that photolyase is at least 2900 times less active toward UA-DNA adducts than toward cis-syn thyminethymine dimers. This indicates that any reversal of photoimmunosuppression by photoreactivation cannot significantly involve cleavage of UA-DNA cyclobutane adducts.