Urocanic acid
Urocanic acid is an intermediate in the catabolism of L-histidine. The cis-urocanic acid isomer is rare.
Ultraviolet radiation absorption
Urocanic acid is a chromophore with a strong, broad, and largely structureless absorption band that spans the UV-C and UV-B regions of the electromagnetic spectrum. Experimental measurements consistently find the absorption maximum, λmax, in the range of 267 nm to 280 nm, with the precise value being sensitive to its solvent's pH and polarity.Metabolism
It is formed from L-histidine through the action of histidine ammonialyase by elimination of ammonium.In the liver, urocanic acid is transformed by urocanate hydratase to 4-imidazolone-5-propionic acid and subsequently to glutamic acid.
Clinical significance
Inherited deficiency of urocanase leads to elevated levels of urocanic acid in the urine, a condition known as urocanic aciduria.An important role for the onset of atopic dermatitis and asthma has been attributed to filaggrin, a skin precursor of urocanic acid.
Urocanic acid is thought to be a significant attractant of the nematode parasite Strongyloides stercoralis, in part because of relatively high levels in the plantar surfaces of the feet, the site through which this parasite often enters the body.
Function
Urocanic acid is found in animal sweat and skin. its concentration varies greatly between people, from 4 nM per cm2 to 34 nM per cm2 with only small differences between areas other than at the sole of the foot and between sun-exposed and unexposed areas. Concentration does not link with age, sex, skin tone, photosensitivity, and stratum corneum thickness.Among other functions, urocanic acid acts as an endogenous sunscreen or photoprotectant against UVB-induced DNA damage. However, some studies have questioned this.
Urocanic acid is found predominantly in the stratum corneum of the skin and it is likely that most of it is derived from filaggrin catabolism. When exposed to UVB irradiation, trans-urocanic acid is converted in vitro and in vivo to cis-urocanic acid. The cis form is known to activate regulatory T cells. Measuring cis-UCA provides a sensitive means at sub-erythemal doses to detect UVB, and so offers a potential method to assess the UVR protection provided by suncreams.
Some studies attribute filaggrin an important role in keeping the skin surface slightly acidic, through a breaking down mechanism to form histidine and subsequently trans-urocanic acid, however others have shown that the filaggrin–histidine–urocanic acid cascade is not essential for skin acidification.