Photochemistry and photobiology of urocanic acid.

E-Urocanic acid is a metabolite of histidine which accumulates in the skin and is excreted in sweat. It has been of interest to photobiologists and photodermatologists because of its intense absorption band at approximately 270 nm, a feature suggestive of a role as a natural photoprotecting agent for DNA. Early work concentrated on the E----Z isomerization resulting from UV excitation. Recent studies have revealed additional, potentially significant, photobiological properties, i.e. photochemical binding to DNA and an apparent involvement of the Z isomer in the phenomenon of photoimmunosuppression.