[Urocanic acid and its role in the photoimmunomodulation process]

Urocanic acid (UCA) is a metabolite of the amino acid histidine. It represents an important chromatophore in epidermis, which can absorb ultraviolet rays in UVB and UVA region and sequentially convert it from trans- to cis-isomer. Cis-isomer is not further degraded; it accumulates in the skin and is excreted with sweat and in shedding keratin scales. UCA has several important functions, the regulation of the homeostasis of the acidic cutaneous surface, the terminal differentiation of epidermal cells and namely the immunomodulatory role. As and immunomodulator UCA can suppress contact allergic reaction and the delayed hypersensitivity of the organism. It can affect reactions mediated by Th-lymphocytes, cytokine system, Langerhans cells, and by some neuropeptides. UCA is related to the development of non-pigmented skin tumors (basaliomas) and indirectly also to pigmented tumors. Cis-UCA can inhibit both the local and systemic resistance to infectious agents. In the immunomodulation some adductive compounds with another important cutaneous chromatophore DNA can participate.