Background:

Urocanic acid, produced in the upper layers of mammalian skin, is a major absorber of ultraviolet radiation (UVR).

Urocanic acid (UCA) is formed in the upper layers of the epidermis where filaggrin, a histidine-rich filamentous protein produced after caspase-14 cleavage of profilaggrin, is broken down by proteinases into component amino acids[1].

[1]. Gibbs NK, et al. Recent advances in urocanic acid photochemistry, photobiology and photoimmunology. Photochem Photobiol Sci. 2008 Jun;7(6):655-67.