Urocanic acid

Urocanic acid is an intermediate in the catabolism of L-histidine.

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Metabolism

It is formed from L-histidine through the action of histidine ammonialyase (also known as histidase or histidinase) by elimination of ammonium.

In the liver, urocanic acid is transformed by urocanate hydratase (or urocanase) 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.

Function

Urocanic acid was detected in animal sweat and skin where, among other possible functions, it acts as an endogenous sunscreen or photoprotectant against UVB-induced DNA damage. 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 (a histidine-rich protein). When exposed to UVB irradiation, trans-urocanic acid is converted in vitro and in vivo to the cis isomer. The cis form is known to activate suppressor T cells.

History

Urocanic acid was first isolated in 1874 by the chemist M. Jaffé from the urine of a dog,^[1] hence the name (Latin: urina = urine, and canis = dog).

See also

• Inborn error of metabolism

References

1. ^ Jaffé, M. (1874) Concerning a new constituent in the urine of dogs. Ber. Deut. Chem. Ges. 7, 1669-1673.