Urocanic acid photobiology. Photooxidation and superoxide formation

Interaction and reactivity of urocanic acid towards peroxyl radicals

The capacity of urocanic acid to interact with peroxyl radicals has been evaluated in several systems: oxidation in the presence of a free radical source (2,2'-azobis(2-amidinopropane; AAPH), protection of phycocyanin bleaching elicited by peroxyl radicals, and Cu(II)- and AAPH-promoted LDL oxidation. The results indicate that both isomers (cis and trans) are mild peroxyl radical scavengers. For example, trans-urocanic acid is nearly 400 times less efficient than Trolox in the protection of the peroxyl radical promoted bleaching of phycocyanin. Regarding the removal of urocanic acid by peroxyl radicals, nearly 100 muM trans-urocanic acid is required to trap half of the produced radicals under the employed conditions (10 mM AAPH, 37 degrees C). Competitive experiments show that the cis-isomer traps peroxyl radicals 30% less efficiently than the trans-isomer. Given the high concentrations that trans-urocanic acid reaches in skin, its capacity to trap peroxyl radicals could contribute to the protection of the tissue towards ROS-mediated processes. Furthermore, both isomers, and particularly the cis-isomer, protect LDL from Cu(II)-induced oxidation.

Theoretical investigation of the photosensitization mechanisms of urocanic acid

The photosensitization mechanisms of urocanic acid (UA), the main skin chromophores of ultraviolet (UV) light, are investigated by means of theoretical calculations. The results indicate that the direct photooxidative damage to DNA bases by triplet state UA through electron transfer reaction is not favorable on thermodynamic grounds. However, UA can photogenerate various reactive oxygen species (ROS, e.g., (1)O(2), O(2)(-)) theoretically and the ROS-generating mechanisms are illustrated as follows. Firstly, the (1)O(2)-generating pathway involves direct energy transfer between triplet state UA and (3)O(2). Secondly, UA gives birth to O(2)(-) through two pathways: (i) direct electron transfer between triplet state UA and (3)O(2); (ii) electron transfer between anion radical of UA (generated through autoionization reactions) and (3)O(2).

Photooxidations Initiated or Sensitized by Biological Molecules: Singlet Oxygen Versus Radical Peroxidation in Micelles and Human Blood Plasma¶

Biomolecules common in blood plasma, including 2-methyl-1,4-naphthoquinone (vitamin K-0, 2), 2,3-dimethoxy-5-methyl-1,4-benzoquinone (ubiquinone-0, 3), bilirubin, 4, and urocanic acid, 5, were used as photoactivators for the photooxidation of methyl linoleate (ML) in 0.50 M sodium dodecyl sulfate micelles to mimic a bioenvironment. UV irradiation of 2 in this system initiated H-atom abstraction from ML (Type-I mechanism). The evidence includes kinetics of oxygen uptake, inhibition of oxidation by an antioxidant ((R)-(+)-6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid [Trolox], 7) and the analysis of four geometric hydroperoxides formed (cis, trans to trans, trans ratio of 0.5). In contrast, irradiation with a singlet-oxygen sensitizer, 3,5-di-t-butyl-1,2-benzoquinone, 1, formed six isomers by a Type-II mechanism, yielding a cis, trans to trans, trans isomer ratio of 6. Peroxidation activated by 3 or 4 with visible light occurred by a singlet-oxygen pathway (Type-II mechanism), as shown by kinetics of oxygen uptake and the effect of quenchers. In contrast, peroxidation in the presence of 5 in this system initiated H-atom abstraction from ML as shown by oxygen uptake and inhibition by Trolox. A comparison of thermal free-radical peroxidation with direct photooxidation of human blood plasma samples showed important differences. Blood plasma resisted thermal peroxidation because of natural antioxidants or on the addition of Trolox. In contrast, direct photooxidation involved singlet oxygen, according to the effect of quenchers and the lack of inhibition by antioxidants.

Formation of singlet oxygen by urocanic acid by UVA irradiation and some consequences thereof

Singlet oxygen-initiated decomposition of urocanic acid (UCA) (3-(1H-imidazol-4(5)-yl)-2-propenoic acid) was used to successfully confirm the report that UCA generates singlet oxygen when irradiated with ultraviolet A light (UVA). The UCA-generated singlet oxygen converts UCA to one or more products that then catalyze the further destruction of the UCA with UVA light by singlet oxygen formation. Some nicking of the phiX-174 supercoiled plasmid DNA was observed when UCA was irradiated with UVA to complete destruction of the starting material, and the product mixture was then mixed with the plasmid in the dark. More extensive nicking was seen when the photoproduct mixture and the plasmid were irradiated with UVA light. An "aged" (4 days) solution of UCA photoproduct no longer caused nicking in the dark but retained the capability to nick the plasmid when irradiated. There is evidence for the presence of hydroperoxides in the UCA photolysis product mixture, and the quenching studies with 2-propanol indicate that free radicals are involved in the plasmid-nicking photochemistry. Singlet oxygen does not appear to play a role in the nicking of the plasmid.

Photogeneration and quenching of reactive oxygen species by urocanic acid

Urocanic acid, UCA, is characterized by two electronic transitions in the UV-B (280-320 nm) which comprise its broad absorption spectrum and give rise to wavelength-dependent isomerization quantum yields. The absorption spectrum of UCA extends into the UV-A (320-400 nm). Given the UV-A component of sunlight is significantly greater than the UV-B component it is hypothesized even weak UV-A photochemistry of UCA could be important for in vivo responses to UV radiation. Degenerate pump-probe experiments performed on t-UCA at several wavelengths in the UV-A reveal an excited-state absorption that undergoes a rapid, approximately 1 ps decay. Photoacoustic experiments performed on both the cis and trans isomers reveal the formation of a long-lived intermediate following UV-A excitation. The efficiency and action spectra for this latter photoactive process are presented and are similar for both isomers of UCA. Cholesterol hydroperoxide assays designed to investigate the nature of the UV-A photoreactivity of t-UCA confirm the production of reactive oxygen species. The bimolecular rate constant for the quenching of singlet oxygen by t-UCA is determined to be 3.5 x 10(6) M(-1) s(-1). Taking into consideration recent theoretical calculations and jet expansion studies of the electronic structure of gas-phase t-UCA, a model is proposed to explain the isomerization and photoreactivity of t-UCA in solution over the UV-A region.

Effects of low concentrations of cis- and trans-urocanic acid on cytokine elaboration by keratinocytes

The urocanic acid cis isomer (cis-UCA) is a possible cutaneous photoreceptor for the immunomodulatory phenomena that follow ultraviolet B irradiation. Several experiments in animals show an inhibitory action of cis-UCA on cellular immunity. However, the action of cis-UCA on the synthesis of cytokines in keratinocytes remains unknown. Long-term cultures of normal human keratiocytes were prepared in a serum-free medium, and stimulated with 1 microgram/ml of phorbol 12-myristate 13-acetate (TPA) and UCA or UVB-UCA (10-100 micrograms/ml). Synthesis of the following cytokines was measured using ELISA and Northern blot techniques: TNF-alpha, IL-1 alpha, IL-1 beta, IL-6, IL-8 and TGF-beta 1. TPA increased TNF-alpha protein levels in culture supernatants. No changes in Il-1 alpha and IL-1 beta protein levels were detected in basal culture supernatant after TPA stimulus. TPA augmented RNA expression for TNF-alpha, IL-1 alpha, IL-1 beta and TGF-beta 1. UCA isomers did not induce cytokine changes in protein synthesis. Expression of IL-1 alpha and IL-1 beta genes was increased after exposure to 100 micrograms/ml UVB-UCA (70 micrograms/ml cis-UCA). A slight increase in TNF-alpha RNA expression was detected when the dose of UVB-UCA reached 100 micrograms/ml. No effects on cytokine synthesis were found after UCA stimulus. These results suggest that low doses of cis-UCA do not effect cytokine synthesis by keratinocytes.

Photosensitized inactivation of infectious DNA by urocanic acid, indoleacrylic acid and rhodium complexes

Naked, infectious single-stranded (ss) and double-stranded (ds) DNA from phages S13 and G4 were irradiated with 308 nm UV radiation in the absence and presence of several photobiologically active compounds: E- and Z-urocanic acid (E- and Z-UA), their methyl esters (E- and Z-MU), E- and Z-indoleacrylic acid (E- and Z-IA), cis-dichloro-bis(1,10-phenanthroline)rhodium(III) chloride (cDCBPR) and tris(1,10-phenanthroline)rhodium (III) perchlorate (TPR). E-urocanic acid protects against cyclobutane pyrimidine dimer formation in ssDNA but concomitantly photosensitizes the formation of other lesions that inactivate ssDNA. Z-urocanic acid also protects ssDNA against such dimerization but without the associated sensitized damage. The methyl ester isomers behave similarly. There is no such differential activity observed for the IA isomers, both of which sensitize the inactivation of ssDNA. Photostationary state mixtures of both UA and IA efficiently sensitize the inactivation of dsDNA, and cDCBPR strongly protects ssDNA from UV damage, while TPR is a significant sensitizer. Both of these metal complexes sensitize the inactivation of dsDNA slightly. For all compounds, cyclobutane pyrimidine dimers were the predominant lethal lesions produced by sensitization of the dsDNA, but they were not the major lethal lesions created by sensitization of the ssDNA. In the case of dsDNA, both UA and IA created pyrimidine dimers with a high degree of potential for mutagenesis, as determined by an assay that monitors the frequency of mutations following the spontaneous deamination of cytosine in photodimers.

Photochemical covalent binding of urocanic acid to polynucleic acids

Photolysis of E-[ring-2-14C]urocanic acid (UA) with native or denatured calf thymus DNA leads to covalent binding of the radiolabel to the nucleic acid. A similar observation is made upon photolysis of the labeled UA with the polyribonucleotides, in which case a strong preference is observed for binding to poly[U]. DNA or poly[U], which had been reacted with UA and purified by dialysis and multiple precipitations, releases UA upon further irradiation with 254 nm light (as expected for cyclobutane adducts). Quantum efficiencies for binding of the UA to native DNA have been measured at 308 and 266 nm and are 0.30 x 10(-5) and 1.3 x 10(-4), respectively, at comparable reactant concentrations. The large increase at the shorter wavelength (where DNA absorption is more competitive) is taken as evidence for the primary role of a DNA excited state in initiating the binding reaction(s).