Abstract

Urocanic Acid (trans) is a metabolic product of histidine and accumulates in the epidermis as a result of the absence of the enzyme urocanase in the upper layer of the skin¹. Trans-urocanic acid absorbs in both the UVA and UVB regions (Fig. 1) and is proposed to be a major ultraviolet protection agent in the skin because its absorption spectrum covers a similar wavelength region as DNA²,³. The entire physiological implications of urocanic acid are, however, more complicated. Upon exposure to UVA and UVB radiation, trans-urocanic acid isomerizes into cis-urocanic acid (Scheme 1), and the latter has been reported to be a mediator for UV induced immunosuppression⁴ and is also a suspected carcinogen.

© 1996 Optical Society of America