Photochemical reactions involved in the phototoxicity of the anticonvulsant and antidepressant drug lamotrigine (Lamictal)

Lamotrigine (LTG) [3,5-diamino-6-(2,3-dichlorophenyl)-1,2,4-triazine], an anticonvulsant and antidepressant drug Lamictal, produces a (photo)toxic response in some patients. LTG absorbs UV light, generating singlet oxygen (1O2) with a quantum yield of 0.22 in CH2Cl2, 0.11 in MeCN and 0.01 in D2O. A small production of superoxide radical anion was also detected in acetonitrile. Thus, LTG is a moderate photosensitizer producing phototoxicity and oxidizing linoleic acid. LTG is a weak 1O2 quencher (k(q) = 3.2 x 10(5) M(-1) s(-1) in MeCN), but its photodecomposition products in dimethyl sulfoxide (DMSO) quenched 1O2 very efficiently. Upon intense UV irradiation from a xenon lamp, LTG was photobleached rapidly in DMSO and slowly in acetonitrile, alcohol and water. The rate increased significantly when laser pulses at 266 nm were employed. The photobleaching products generated 1O2 twice as strongly as LTG. Photobleaching was usually accompanied by the release of chloride anions, which increased in the presence of ascorbic acid. This suggests the formation of aryl radicals via dechlorination, a process which may be responsible for the photoallergic response observed in some patients. Our results demonstrate that LTG is a moderate generator of 1O2 prone to photodechlorination, especially in a reducing environment, which can contribute to the reported phototoxicity of LTG.

EPR studies of in vivo radical production by 3,3',5,5'-tetrabromobisphenol A (TBBPA) in the Sprague-Dawley rat

Brominated flame retardants (BFRs) are present in many consumer products ranging from fabrics to plastics and electronics. Wide use of flame retardants can pose an environmental hazard and it is of interest to determine the mechanism of their toxicity. Of all the BFRs, 3,3',5,5'-tetrabromobisphenol A (TBBPA) is produced in the largest volume. Previous studies by Szymanska et al. (2000) have shown that TBBPA is hepatotoxic in rats. We report here that when TBBPA (100 or 600 mg/kg) dissolved in DMSO and alpha-(4-pyridyl-1-oxide)-N-t-butylnitrone (POBN) was administered ip to male Sprague-Dawley rats the POBN/CH(3) spin adduct was detected by electron paramagnetic resonance (EPR) in the bile. When (13)C-DMSO was employed the POBN/C(13)H(3) adduct was observed. Also present in the bile was the 2,6-dibromobenzosemiquinone radical derived from 2,6-dibromohydroquinone, a known metabolite of TBBPA. Reaction of the 2,6-dibromobenzosemiquinone radical with oxygen would generate superoxide from which hydrogen peroxide can form by dismutation. The hydroxyl radical generated via the Fenton reaction from hydrogen peroxide reacts in vivo with DMSO to give the methyl radical which is trapped by POBN. These observations suggest that the hepatotoxicity of TBBPA in rats may be due to the in vivo generation of the hydroxyl radical as a result of redox reactions involving the TBBPA metabolite 2,6-dibromohydroquinone and its corresponding semiquinone radical.

Oxidation of flame retardant tetrabromobisphenol A by singlet oxygen

Wide use of flame retardants can pose an environmental hazard, and it is of interest to investigate how they may degrade. We report here that 3,3',5,5'-tetrabromobisphenol A (TBBPA) is subject to photosensitized oxidation involving singlet molecular oxygen ((1)O2). By using visible light and rose bengal or methylene blue as 102 photosensitizers, we have found that TBBPA is a 102 quencher. The quenching rate constant, k(q), depends on TBBPA ionization (pK = 7.4). In acetonitrile, where TBBPA is undissociated, the kq value is 6.1 x 10(5) M(-1) s(-1) for a TBBPA monomer and decreases to 2.9 x 10(4) M(-1) s(-1) for TBBPA dimers and/or aggregates. TBBPA dissociates in aqueous solutions, and its kq value is 1.44 x 10(9) M(-1) s(-1) in alkaline solution, decreasing to 3.9 x 10(8) M(-1) s(-1) at pH 7.2. The strong 102 quenching by TBBPA anion initiates an efficient oxidation of TBBPA, which results in oxygen consumption in aqueous micellar (e.g., Triton X-100) solutions containing photosensitizer. This oxygen consumption is mediated by transient radical species, which we detected by using EPR spectroscopy. We observed two major radicals and one minor radical generated from TBBPA by reaction with 102 at pH 10. One was identified as the 2,6-dibromo-p-benzosemiquinone radical (a2H = 2.36 G, g = 2.0056). A second radical (aH = 2.10 G, g = 2.0055) could not be identified butwas probably a 2,6-dibromo-p-benzosemiquinone radical containing an EPR-silent substituent at the 3-position. Spin trapping with 5,5-dimethyl-1-pyrroline N-oxide (DPMO) showed that other minor radicals (hydroxyl, carbon-centered) are also generated during the reaction of TBBPA with (1)O2. The photosensitized production of radicals and oxygen consumption were completely inhibited by the azide anion, an efficient physical (1)O2 quencher. Because TBBPA is a stable compound that at neutral pH does not absorb much of the atmosphere-filtered solar radiation, its photosensitized oxidation by (1)O2 may be the key reaction initiating or mediating TBBPA degradation in the natural environment.

Photochemistry and Photocytotoxicity of Alkaloids from Goldenseal (Hydrastis canadensis L.). 2. Palmatine, Hydrastine, Canadine, and Hydrastinine

Goldenseal is an herb that is widely used in dietary supplements, eye washes, and skin lotions. The presence of Goldenseal root powder in dietary supplements and the topical application of Goldenseal preparations raise the possibility that an adverse phototoxic reaction may result from an interaction between its constituent alkaloids and light in exposed tissues. We have previously shown that berberine, the major alkaloid in Goldenseal powder, in combination with UVA causes DNA damage and cell death in HaCaT keratinocytes [(2001) Chem. Res. Toxicol. 14, 1529]. We have studied the photochemical and photobiological properties of four minor alkaloids found in Goldenseal, namely, hydrastine, palmatine, canadine, and hydrastinine. UVA radiation of palmatine in aqueous solutions generated no (1)O(2), but in CH(2)Cl(2), copious amounts of (1)O(2) were detected (Phi = 0.2). Palmatine also photogenerated oxygen-centered radicals, (*)OH and O(2)(*)(-) in aerated aqueous buffer and acetonitrile, respectively, as detected by the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO). In nitrogen-sparged acetonitrile containing DMPO, we observed the neutral palmatine radical formed by one-electron reduction. UVA irradiation (4 J/cm(2)) of HaCaT keratinocytes in the presence of palmatine (50 microM) resulted in a 50% decrease in cell viability but no DNA damage as measured by the comet assay. UVA irradiation of hydrastine, hydrastinine, or canadine (50 microM) did not cause DNA damage or cell death in keratinocytes. Although palmatine is photoactive, it is present in such small amounts in Goldenseal root powder that the phototoxicity of the herb is most likely due to berberine, the major constituent alkaloid.

Chronic UVA irradiation of human HaCaT keratinocytes induces malignant transformation associated with acquired apoptotic resistance

Ultraviolet A (UVA, 315-400 nm), constituting about 95% of ultraviolet irradiation in natural sunlight, represents a major environmental challenge to the skin and is clearly associated with human skin cancer. It has proven difficult to show direct actions of UVA as a carcinogen in human cells. Here, we demonstrate that chronic UVA exposures at environmentally relevant doses in vitro can induce malignant transformation of human keratinocytes associated with acquired apoptotic resistance. As evidence of carcinogenic transformation, UVA-long-treated (24 J/cm(2) once/week for 18 weeks) HaCaT (ULTH) cells showed increased secretion of matrix metalloproteinase (MMP-9), overexpression of keratin 13, altered morphology and anchorage-independent growth. Malignant transformation was established by the production of aggressive squamous cell carcinomas after inoculation of ULTH cells into nude mice (NC(r)-nu). ULTH cells were resistant to apoptosis induced not only by UVA but also by UVB and arsenite, two other human skin carcinogens. ULTH cells also became resistant to apoptosis induced by etoposide, staurosporine and doxorubicin hydrochloride. Elevated phosphorylation of protein kinase B (PKB, also called AKT) and reduced expression of phosphatase and tensin homologue deleted on chromosome 10 (PTEN) were detected in ULTH cells. The resistance of ULTH cells to UVA-induced apoptosis was reversed by either inhibition of phosphatidylinositol 3-kinase (PI-3K) or adenovirus expression of PTEN or dominant negative AKT. These data indicate that UVA has carcinogenic potential in human keratinocytes and that the increased AKT signaling and decreased PTEN expression may contribute to this malignant transformation. Further comparisons between the transformed ULTH and control cells should lead to a better understanding of the mechanism of UVA carcinogenesis and may help identify biomarkers for UVA-induced skin malignancies.

Cleavage of epidermal growth factor receptor by caspase during apoptosis is independent of its internalization

Epidermal growth factor receptor (EGFR) plays a critical role in cell proliferation, differentiation, and transformation. EGFR downregulation attenuates its signaling intensity and duration to maintain cellular homeostasis. Here, we report that during apoptosis EGFR is cleaved by activated caspase-3 or related proteases at its C-terminus domain. EGFR downregulation by activation of caspases is neither stimulus- nor cell type-specific. EGFR internalization during apoptosis required dynamin and cholesterol since dominant-negative dynamin (K44A) or cholesterol depletion by methyl-beta-cyclodextrin prevented EGFR internalization. However, EGFR downregulation did not require its internalization. The EGFR cleavage fragment was detected in the membrane blebs in addition to the cell pellets. Mutations at the consensus sequence (DXXD) at the C-terminus domain revealed that DVVD1012 and to a lesser extent DNPD1172 may be target sites for active recombinant caspase-3 in vitro and activated caspase-3 or related proteases in vivo. We have detected the N-terminus and C-terminus fragments in vitro and in vivo. A cleavage-deficient EGFR mutant delayed apoptosis process. We conclude that the evolutionarily conserved C-terminus domain of EGFR is the target of caspases and subjected to degradation during apoptosis to shut down its signaling.

Photochemical characterization of water samples from Minnesota and Vermont sites with malformed frogs: potential influence of photosensitization by singlet molecular oxygen (1O(2)) and free radicals on aquatic toxicity

Environmental pollutants activated by UV sunlight may have contributed to the recent decline in frog populations and the concomitant increase in malformations in the USA and abroad. UV radiation is able to mutate DNA and to initiate photosensitization processes that generate mutagenic and biologically disruptive oxygen transients. We have examined water from selected sites in Minnesota and Vermont using singlet molecular oxygen (1O(2)), detected by its phosphorescence and free radicals detected by spin trapping, as markers for photosensitization. Water from a pond in Minnesota with malformed frogs, which also causes malformations in the laboratory, photosensitized more 1O(2), even though it absorbed less UV light compared to water from a site that did not cause malformations. This suggested that unknown natural or pollutant agents were present, and that photosensitization may be involved. Although UV irradiation of the two Minnesota water samples in the presence of the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) revealed the presence of the DMPO/*OH, DMPO/*H(e(aq)-) and DMPO/*C(unknown) adducts there were no qualitative or quantitative differences between them. We also examined water samples from several sites in Vermont, and compared them by measuring the quantum yield of 1O(2) photosensitization. While all the Vermont samples produced a small amount of 1O(2), there was no clear correlation with the incidence of frog malformations. However, the samples differed strongly in absorption spectra and the ability to quench 1O(2). These factors may determine how much UV light is absorbed and converted into chemical reactions. Our results show that photochemical characterization of 1O(2) photosensitization is possible in untreated natural water samples. Photosensitization falls into the category of global factors that may be closely associated with the effects of UV irradiation of the Earth's environments. Thus, photosensitization might be an important component in global amphibian malformation and decline. The observation of 1O(2) emission directly from natural water may also provide new opportunities to investigate the involvement of 1O(2) in other complex environmental processes.

Interaction of singlet molecular oxygen with double fluorescent and spin sensors

Double fluorescent and spin sensors were recently used to detect transient oxidants via simultaneous fluorescence change and production of the nitroxide radical detected by electron paramagnetic resonance. One such oxidant, singlet molecular oxygen ((1)O(2)), was detected in thylakoid membrane using these probes. In the present study, we investigated the total (physical and chemical) quenching of (1)O(2) phosphorescence by sensors composed of the 2,5-dihydro-2,2,5,5-tetramethyl-1H-pyrrole moiety attached to xanthene or dansyl fluorophores. We found that the quenching rate constants were in the range (2-7) x 10(7) M(-1)s(-1) in acetonitrile and D(2)O. Quenching of (1)O(2) is usually an additive process in which different functional groups may contribute. We estimated that the (1)O(2) quenching by the amine fragments was ca. one to two orders of magnitude lower than that for the complete molecules. Our data suggest that the incorporation of a fluorescent chromophore results in additional strong quenching of (1)O(2), which may in turn decrease the nitroxide yield via the (1)O(2) chemical path, possibly having an effect on quantitative interpretations. We have also found that probes with the dansyl fluorophore photosensitized (1)O(2) upon UV excitation with the quantum yield of 0.087 in acetonitrile at 366 nm. This result shows that care must be taken when the dansyl-based sensors are used in experiments requiring UV irradiation. We hope that our results will contribute to a better characterization and wider use of these novel double sensors.

Interaction of Singlet Molecular Oxygen with Melatonin and Related Indoles¶

Singlet molecular oxygen (1O2) is one of the major agents responsible for (photo)oxidative damage in biological systems including human skin and eyes. It has been reported that the neural hormone melatonin (MLT) can abrogate 1O2-mediated cytotoxicity through its purported high antioxidant activity. We studied the interaction of MLT with 1O2 in deuterium oxide (D2O), acetonitrile and methanol by measuring the phosphorescence lifetime of 1O2 in the presence of MLT and related indoles for comparison. Rose bengal (RB) was used as the main 1O2 photosensitizer. The rate constant (kq) for the total (physical and chemical) quenching of 1O2 by MLT was determined to be 4.0 x 10(7) M(-1) s(-1) in D2O (pD 7), 6.0 x 10(7) M(-1) s(-1) in acetonitrile, and 6.1 x 10(7) M(-1) s(-1) in methanol-d1. The related indoles, tryptophan, 5-hydroxyindole, 5-methoxytryptamine, 5-hydroxytryptamine (5-OH-T, serotonin), 6-hydroxymelatonin (6-OH-MLT) and 6-chloromelatonin quenched 1O2 phosphorescence with similar kq values. We also compared the photosensitized photobleaching rate of MLT with that of other indoles, which revealed that MLT is the most sensitive to 1O2 bleaching. Hydroxylation of the indole moiety in 5-OH-T and 6-OH-MLT makes them more sensitive to photodegradation. In the absence of exogenous photosensitizers MLT itself can generate 1O2 with low quantum yield (0.1 in CH3CN) upon UV excitation. Thus, the processes we investigated may occur in the skin and eyes during physiological circadian rhythm (photo)signaling involving MLT and other indoles. Our results indicate that all the indoles studied, including MLT, are quite efficient yet very similar 1O2 quenchers. This directly shows that the exceptional antioxidant ability proposed for MLT is unsubstantiated when merely chemical mechanism(s) are considered in vivo, and it must predominantly involve humoral regulation that mobilizes other antioxidant defenses in living organisms.

Photocarcinogenesis in the Tg.AC mouse: lomefloxacin and 8-methoxypsoralen

The Tg.AC mouse is a good predictor of carcinogenic potential when the test article is administered by dorsal painting (Tennant et al. (1995) Environ. Health Perspect. 103, 942). We have used lomefloxacin (LOME) and 8-methoxypsoralen (8-MOP) in combination with UVA to determine whether the Tg.AC transgenic mouse also responds to parenterally administered photocarcinogens. Female Tg.AC mice were given LOME (25 mg/kg intraperitoneal in normal saline) followed by UVA (25 J/cm2) 1-2 h later, five times every 2 weeks on a repetitive schedule. Other groups received LOME, UVA or vehicle alone. After 16 weeks, the mean numbers of papillomas/mouse +/- SD (% responding) were: saline, 0.3 +/- 0.5 (33%); UVA + saline, 1.3 +/- 0.6 (100%); LOME, 1.9 +/- 1.6 (86%) and LOME-UVA, 1.5 +/- 1.9 (64%). Only the 100% incidence of tumors in the UVA group and the maximum tumor yields in the LOME and UVA groups are significant (P < 0.05) when compared with the control. In a second study, Tg.AC mice were administered the classical photocarcinogen 8-MOP (8 mg/kg intragastric in corn oil) followed by 2 J/cm2 UVA 1-2 h later, five times every 2 weeks on a repetitive schedule. The second group received 8-MOP, whereas the third was exposed to UVA alone. Papillomas began to appear at 2 weeks in the 8-MOP-UVA group, and after 17 weeks the mean numbers of papillomas/mouse +/- SD (% responding) were: 8-MOP-UVA, 6.9 +/- 8.6 (93%); UVA + corn oil, 1.1 +/- 1.2 (69%) and 8-MOP, 1.1 +/- 1.6 (50%). The maximum tumor yield in the 8-MOP-UVA group was significantly higher (P < 0.01) than that in the other two groups. Our findings suggest that more studies need to be done before the Tg.AC mouse can be used with confidence to identify parenterally administered photocarcinogens.

Photosensitized oxidation of 2',7'-dichlorofluorescin: singlet oxygen does not contribute to the formation of fluorescent oxidation product 2',7'-dichlorofluorescein

2',7'-Dichlorofluorescin (DCFH) is often employed to assess oxidative stress in cells by monitoring the appearance of 2',7'-dichlorofluorescein (DCF), its highly fluorescent oxidation product. We have investigated the photosensitized oxidation of DCFH in solution and elucidated the role played by singlet molecular oxygen (1O(2)) in this reaction. We used rose bengal (RB), protoporphyrin, and DCF as photosensitizers. Irradiation (550 nm) of RB (20 microM) in 50 mM phosphate (pH 7.4) in the presence of DCFH (50 microM) resulted in the rapid formation of DCF, measured as an increase in its characteristic absorbance and fluorescence. The oxidation rate was faster in deoxygenated solution, did not increase in D(2)O, and even increased in the presence of sodium azide. The presence of antioxidants that react with 1O(2), thus removing oxygen, accelerated DCF formation. Such results eliminate any potential direct involvement of 1O(2) in DCF formation, even though DCFH is an efficient (physical) quencher of 1O(2) (k(q) = 1.4 x 10(8) M(-1)s(-1) in methanol). DCF is also a moderate photosensitizer of 1O(2) with a quantum yield of circa phi = 0.06 in D(2)O and phi = 0.08 in propylene carbonate, which unequivocally indicates that DCF can exist in a triplet state upon excitation with UV and visible light. This triplet can initiate photo-oxidization of DCFH via redox-and-radical mechanism(s) similar to those involving RB (vide supra). Our results show that, upon illumination, DCF can function as a moderate photosensitizer initiating DCFH oxidation, which may prime and accelerate the formation of DCF. We have also shown that, while 1O(2) does not contribute directly to DCF production, it can do so indirectly via reaction with cellular substrates yielding peroxy products and peroxyl radicals, which are able to oxidize DCFH in subsequent dark reactions. These findings suggest that DCFH should not be regarded as a probe sensitive to singlet molecular oxygen, and that care must be taken when using DCFH to measure oxidative stress in cells as a result of both visible and UV light exposure.

Quenching of singlet molecular oxygen (1O2) by azide anion in solvent mixtures

The azide ion is a strong physical quencher of singlet molecular oxygen (1O2) and is frequently employed to show involvement of 1O2 in oxidation processes. Rate constants (k(q)) for the quenching of 1O2 by azide are routinely used as standards to calculate k(q) values for quenching by other substrates. We have measured k(q) for azide in solvent mixtures containing deuterium oxide (D2O), acetonitrile (MeCN), 1,4-dioxane, ethanol (EtOH), propylene carbonate (PC), or ethylene carbonate (EC), mixtures commonly used for many experimental studies. The rate constants were calculated directly from 1O2 phosphorescence lifetimes observed after laser pulse excitation of rose bengal (RB), used to generate 1O2. In aqueous mixtures with MeCN and carbonates, the rate constant increased nonlinearly with increasing volume of organic solvent in the mixtures. k(q) was 4.78 x 10(8) M(-1) s(-1) in D2O and increased to 26.7 x 10(8) and 27.7 x 10(8) M(-1) s(-1) in 96% MeCN and 97.7% EC/PC, respectively. However, in EtOH/D2O mixtures, k(q) decreased with increasing alcohol concentration. This shows that a higher solvent polarity increases the quenching efficiency, which is unexpectedly decreased by the proticity of aqueous and alcohol solvent mixtures. The rate constant values increased with increasing temperature, yielding a quenching activation energy of 11.3 kJ mol(-1) in D2O. Our results show that rate constants in most solvent mixtures cannot be derived reliably from k(q) values measured in pure solvents by using a simple additivity rule. We have measured the rate constants with high accuracy, and they may serve as a reliable reference to calculate unknown k(q) values.

Photochemistry and photocytotoxicity of alkaloids from Goldenseal (Hydrastis canadensis L.) 1. Berberine

Goldenseal is an herb which is widely used for many medical applications such as in eyewashes and skin lotions and which is currently undergoing testing by the National Toxicology Program. The main alkaloid constituent of Goldenseal is berberine. The topical application of Goldenseal or berberine to the skin or eyes raises the possibility that an adverse phototoxic reaction may result from an interaction between the alkaloid and light. We have therefore studied the photochemistry of berberine in different solvents and its phototoxicty to HaCaT keratinocytes. Irradiation of berberine in aqueous solutions does not generate (1)O(2), but in CH(2)Cl(2), (1)O(2) is produced with a quantum yield phi = 0.34. With the aid of the electron paramagnetic resonance (EPR) spin trapping technique and 5,5-dimethyl-1-pyrroline N-oxide (DMPO), we have detected oxygen-centered radicals photogenerated by berberine in water and acetonitrile. In the latter solvent and in the absence of oxygen, the neutral berberine radical formed by one electron reduction was observed. Methanol radicals were detected by EPR in water/alcohol low-temperature glasses irradiated in the berberine long-wavelength absorption band. In such alcoholic glasses, we have also detected an EPR signal from the berberine triplet at 77 K, in contrast to aqueous glasses where neither triplet nor radicals were detectable. Our data show that, although a weak photosensitizer in water, berberine is able to produce both (1)O(2) and radical species in a nonpolar environment. UVA irradiation of HaCaT keratinocytes in the presence of 50 microM berberine resulted in an 80% decrease in cell viability and a 3-fold increase in DNA damage as measured by the Comet assay. These findings suggest that exposure to sunlight or artificial light sources emitting UVA should be avoided when topical preparations derived from Goldenseal or containing berberine are used.

The effect of short intermittent light exposures on the melatonin circadian rhythm and NMU-induced breast cancer in female F344/N rats

We investigated the effects of altered endogenous nighttime melatonin concentrations on mammary tumor production in an N-nitroso-N-methylurea (NMU)-induced breast cancer model in female Fischer 344 (F344)/N rats. Experiments were designed 1) to evaluate whether short-duration intermittent exposures to light at night would affect the nocturnal rise of melatonin, resulting in a decrease in nighttime serum melatonin concentrations, 2) to evaluate whether any suppression of nighttime serum melatonin concentrations could be maintained for a period of weeks, and 3) to determine the effects of suppressed serum melatonin concentrations on the incidence and progression of NMU-induced breast cancer. In vivo studies were used to assess serum melatonin concentrations after 1 day and 2 and 10 weeks of nightly administration of short-duration intermittent light exposure at night and incidence of NMU-induced tumors. Five 1-minute exposures to incandescent light every 2 hours after the start of the dark phase of the light: dark cycle decreased the magnitude of the nocturnal rise of serum melatonin concentrations in rats by approximately 65%. After 2 weeks of nightly intermittent light exposures, an average decrease of the peak nighttime serum melatonin concentrations of approximately 35% occurred. The amelioration continued and, at 10 weeks, peak nighttime serum melatonin concentrations were still decreased, by approximately 25%. Because peak endogenous nighttime serum melatonin values could be moderately suppressed for at least 10 weeks, a 26-week NMU mammary tumor study was conducted. Serum melatonin concentrations and incidence, multiplicity, and weight of NMU-induced mammary tumors were assessed. A group of pinealectomized (Px) animals was also included in the tumor study. No effect on the development of mammary tumors in an NMU-induced tumor model in rats occurred when endogenous nighttime serum melatonin concentrations were moderately suppressed by short-duration intermittent light exposures at night. At necropsy, there were no alterations in mammary tumor incidence (28/40 NMU controls, 28/40 NMU + light, 31/40 NMU + Px), multiplicity (2.18 tumors/tumor-bearing NMU control, 1.89 NMU + light, 2.39 NMU + Px), or average tumor weight (1.20 g NMU control, 1.19 g NMU + light, 0.74 g NMU + Px). Tumor burden had no effect on the serum melatonin cycle. At 26 weeks, however, animals exposed to intermittent light at night exhibited approximately 3-fold higher serum melatonin concentrations as compared with controls. Additionally, rats that had been pinealectomized at 4 weeks of age had serum melatonin concentrations that were markedly higher than the expected baseline concentrations for pinealectomized rats (<15 pg/ml), suggesting the reestablishment of a melatonin cycle. This finding was unexpected and suggests that melatonin can be produced by an organ or tissue other than the pineal gland.

Photochemical studies on xanthurenic acid

The tryptophan metabolite xanthurenic acid (Xan) has been isolated from aged human cataractous lenses. The photophysical properties of Xan were examined to determine if it is a potential chromophore for age-related cataractogenesis. We found that Xan produces singlet oxygen (psi delta = 0.17 in CD3OD) with the same efficiency as the lenticular chromophore N-formyl kynurenine and quenches singlet oxygen at a rate similar (2.1 x 10(7); CD3OD) to other tryptophan metabolites found in the eye. As the mechanisms of induction of cataracts may also involve redox reactions, the interactions of hydrated electrons (e(aq)-), the azide radical (N3*) and hydroxyl radical (OH*) with Xan were studied using the technique of pulse radiolysis. The reaction rate constants of e(aq)-, N3* and OH* with Xan were found to be of the same order of magnitude as other tryptophan metabolites. The rate constant for reaction of Xan with e(aq)- solvated electrons was found to be diffusion controlled (k = 1.43 x 10(10) M(-1) s(-1); the reaction with N3* was very fast (k = 4.0 x 10(9) M(-1) s(-1)); and with OH* was also near diffusion controlled (k = 1.0 x 10(10) M(-1) s(-1)). Superoxide O2*- production by irradiated Xan in methanol was detected by electron paramagnetic resonance and substantiated by determining that the enhanced rate of oxygen consumption of Xan irradiated in the presence of furfuryl alcohol was lowered by superoxide dismutase.

Photophysical studies on melatonin and its receptor agonists

Previous work has demonstrated that melatonin inhibits the growth of both dermal and uveal melanoma cells. Recent clinical trials have found that melatonin is an efficacious treatment for metastatic dermal melanoma. The goal of this study was to provide further insight into the oncostatic mechanism(s) of melatonin. The inhibition of the growth of uveal melanoma cells is dose-dependent (0.1-10 nM) within the range of endogenous melatonin concentrations (2 nM) found in the human aqueous humor. We know that this inhibition of growth is receptor-mediated, at least in part, because uveal melanoma cell growth was also blocked by the agonists of melatonin receptors. There are two known membrane receptors for melatonin (Mel(1a) and Mel(1b)) and one known nuclear receptor (Mel2). To determine if singlet oxygen production and/or quenching contributed to the growth inhibition of melatonin, we examined the photophysical properties of melatonin and its agonists. Using flash photolysis, we determined that melatonin and its membrane receptor agonist 6-chloromelatonin (Mel(1a-b), Lilly, Indianapolis, IN) produced very little singlet oxygen (psidelta = 0.073 and psidelta = 0.01, respectively). There was no detectable singlet oxygen phosphorescence at 1,270 nm for the nuclear receptor agonist CG-52608 (Mel2, Novartis, Basel, Switzerland). In contrast, the agonist of the Mel(1b) receptor, S-20098 (Servier, Paris, France), produced singlet oxygen with a quantum efficiency of psidelta = 0.34. Singlet oxygen was quenched by melatonin and 6-chloromelatonin at approximately the same rate (6.1 x 10(7) M(-1)s(-1) and 6.0 x 10(7) M(-1)s(-1) in CD3OD), while the rate of quenching for the nuclear receptor agonist CG-52608 and membrane receptor agonist S-20098 was less (2.2 x 10(7) M(-1)s(-1) and 1.5 x 10(7) M(-1) s(-1), respectively). It appears that the production of singlet oxygen by melatonin would not be sufficient to directly block the proliferation of melanoma cells, but may activate gene products that could contribute to the oncostatic effect.

Photooxidation of lens alpha-crystallin by hypericin (active ingredient in St. John's Wort)

Hypericin is the active ingredient in the over the counter antidepressant medication St. John's Wort. Hypericin produces singlet oxygen and other excited state intermediates that indicate it should be a very efficient phototoxic agent in the eye. Furthermore it absorbs in the UV and visible range, which means it can potentially damage both the lens and the retina. Lens alpha-crystallin, isolated from calf lenses, was irradiated in the presence of hypericin (5 x 10(-5) M, 10 mM ammonium bicarbonate, pH 7.0) and in the presence and absence of light (> 300 nm, 24 mW/cm2). Hypericin-induced photosensitized photopolymerization as assessed by sodium dodecylsulfate-polyacrylamide gel electrophoresis. Further analysis of the oxidative changes occurring in alpha-crystallin using mass spectrometry showed specific oxidation of methionine, tryptophan and histidine residues, which increased with irradiation time. Hypericin did not damage the lens protein in the dark. Damage to alpha-crystallin could undermine the integrity of the lens directly by protein denaturation and indirectly by disturbing chaperone function. Therefore, in the presence of light, hypericin can induce changes in lens protein that could lead to the formation of cataracts. Appropriate precautions should be taken to protect the eye from intense sunlight while on this antidepressant medication.

Investigation of the mechanism of action of microperoxidase-11, (MP11), a potential anti-cataract agent, with hydrogen peroxide and ascorbate

The interaction of hydrogen peroxide, ascorbate and microperoxidase-11 (MP11), a ferriheme undecapeptide derived from cytochrome c, has been investigated using spectrophotometry, oxymetry, electron paramagnetic resonance (EPR), and mass spectroscopy techniques. It is shown that in 50 m M phosphate pH 7. 0-7.4 in the absence of other reactants H(2)O(2)induces a concentration-dependent decrease in absorption at the Soret band (399 nm) of the microperoxidase, with concomitant H(2)O(2)decomposition and oxygen evolution. The reaction causes irreversible heme degradation, concomitant with loss of enzymatic activity. Ascorbate effectively protects MP11 from degradation and inhibits oxygen evolution. At ascorbate concentrations greater than that of H(2)O(2), microperoxidase degradation is almost completely prevented. Mass spectrometry showed that H(2)O(2)oxidizes the microperoxidase to a monooxygenated product, which did not form if ascorbate was included in the reaction system. There appears to be a 1:1 relationship between H(2)O(2)degradation and ascorbate oxidation. EPR experiments revealed that an ascorbate radical was formed during the reaction. These reactions may be described by a scheme where a putative 'compound I' of the microperoxidase is reduced by ascorbate back to the original redox state (ferric) of the peroxidase in two one-electron steps, concomitantly with oxidation of the ascorbate to an ascorbate radical or in one two-electron transfer step forming dehydroascorbate. In the absence of ascorbate, the 'compound I' reacts further with the peroxide causing microperoxidase degradation and partial oxygen evolution. These observations are relevant to the interaction of ferrihemes with H(2)O(2)and ascorbic acid and may be pertinent for the potential application of MP11 as an anti-cataract agent.

Dihydrocercosporin singlet oxygen production and subcellular localization: a possible defense against cercosporin phototoxicity in Cercospora

Fungi in the genus Cercospora produce cercosporin, a potent singlet oxygen (1O2)-generating photosensitizer that plays a critical role in the ability of these fungi to parasitize plants. Although plants, mice, bacteria and many fungi are sensitive to cercosporin, Cercospora species are resistant to its toxicity. The cellular resistance of these fungi to cercosporin has been correlated with fungal cell surface reducing ability and the ability to maintain cercosporin in a chemically reduced state. As a model for reduced cercosporin we employed a reduced, acetylated derivative (hexaacetyl-dihydrocercosporin, HAC) that we tested for 1O2 production in a range of solvents. We found that as a 1O2 photosensitizer, HAC was only moderately effective in organic solvents (phi SO = 0.14-0.18) and very poor in water (phi SO = 0.02-0.04). By contrast, the 1O2 quantum yield of cercosporin itself was unaffected by solvent (phi SO = 0.84-0.97). To investigate the localization of reduced cercosporin in fungal cells, we developed a fluorescence assay using laser scanning confocal microscopy. This assay showed a uniform green fluorescence, indicative of reduced cercosporin, in the cytoplasm of hyphal cells treated with cercosporin. We hypothesize that the main protection mechanism against cercosporin phototoxicity in the fungus consists of transformation of cercosporin to a reduced state and localization of this reduced form in the aqueous compartment of the cell, thus decreasing intracellular 1O2 production to levels that can be tolerated by the fungus. In addition, we have, for the first time, directly detected 1O2 phosphorescence from fungal culture, either stained with the photosensitizer rose bengal or actively synthesizing cercosporin, demonstrating 1O2 production in vivo and from cercosporin in culture.

Vitamin B6 (pyridoxine) and its derivatives are efficient singlet oxygen quenchers and potential fungal antioxidants

Vitamin B6 (pyridoxine, 1) and its derivatives: pyridoxal (2), pyridoxal 5-phosphate (3) and pyridoxamine (4) are important natural compounds involved in numerous biological functions. Pyridoxine appears to play a role in the resistance of the filamentous fungus Cercospora nicotianae to its own abundantly produced strong photosensitizer of singlet molecular oxygen (1O2), cercosporin. We measured the rate constants (kq) for the quenching of 1O2 phosphorescence by 1-4 in D2O. The respective total (physical and chemical quenching) kq values are: 5.5 x 10(7) M-1 s-1 for 1; 7.5 x 10(7) M-1 s-1 for 2, 6.2 x 10(7) M-1 s-1 for 3 and 7.5 x 10(7) M-1 s-1 for 4, all measured at pD 6.2. The quenching efficacy increased up to five times in alkaline solutions and decreased approximately 10 times in ethanol. Significant contribution to total quenching by chemical reaction(s) is suggested by the degradation of all the vitamin derivatives by 1O2, which was observed as declining absorption of the pyridoxine moiety upon aerobic irradiation of RB used to photosensitize 1O2. This photodegradation was completely stopped by azide, a known physical quencher of 1O2. The pyridoxine moiety can also function as a redox quencher for excited cercosporin by forming the cercosporin radical anion, as observed by electron paramagnetic resonance. All B6 vitamers fluoresce upon UV excitation. Compounds 1 and 4 emit fluorescence at 400 nm, compound 2 at 450 nm and compound 3 at 550 nm. The fluorescence intensity of 3 increased approximately 10 times in organic solvents such as ethanol and 1,2-propanediol compared to aqueous solutions, suggesting that fluorescence may be used to image the distribution of 1-4 in Cercospora to understand better the interactions of pyridoxine and 1O2 in the living fungus.

Considerations on photochemical genotoxicity: report of the International Workshop on Genotoxicity Test Procedures Working Group

Recent toxicological observations have caused concern regarding the need to test, for example, pharmaceuticals and cosmetic products for photochemical genotoxicity. The objective of this report is to give assistance on how to adapt existing test methods to investigate the potential of light-absorbing compounds to induce genotoxic effects on photoactivation. In general, the Organization for Economic Co-Operation & Economic Development (OECD) draft guideline on in vitro phototoxicity testing served as a basis for consideration. Concomitant exposure of the cells to the test compound and solar simulated light was considered appropriate as the initial, basic test condition. Optimization of the exposure scheme, e.g., a change of the irradiation spectrum, might be indicated depending on the initial test results. Selection of test compound concentrations should be based on results obtained with the dark version of the respective test system but might have to be modified if phototoxic effects are observed. Selection of the irradiation dose has to be performed individually for each test system based on dose-effect studies. The irradiation should induce per se a small, reproducible toxic or genotoxic effect. The report includes a specification of necessary controls, discusses factors that might have an impact on the irradiation characteristics, and gives a rationale for the omission of an external metabolic activation system. It also addresses the question that physicochemical and pharmacokinetic properties might trigger the need to test a chemical for photochemical genotoxicity. Relevant experimental observations are presented to back up the recommendations. The working group did not reach a consensus as to whether a single, adequately perfomed in vitro test for clastogenicity would be sufficient to exclude a photogenotoxic liability or whether a test battery including a gene mutation assay would be needed for product safety testing regarding photochemical genotoxicity.

Evidence for free radical formation during the oxidation of 2'-7'-dichlorofluorescin to the fluorescent dye 2'-7'-dichlorofluorescein by horseradish peroxidase: possible implications for oxidative stress measurements

The oxidation of 2'-7'-dichlorofluorescin (DCFH) to the fluorescent 2'-7'-dichlorofluorescein (DCF) by horseradish peroxidase (HRP) was investigated by fluorescence, absorption, and electron spin resonance spectroscopy (ESR). As has been previously reported, HRP/H2O2 oxidized DCFH to the highly fluorescent DCF. However, DCF fluorescence was still observed when H2O2 was omitted, although its intensity was reduced by 50%. Surprisingly, the fluorescence increase, in the absence of exogenous H2O2, was still strongly inhibited by catalase, demonstrating that H2O2 was present and necessary for DCF formation. H2O2 was apparently formed during either chemical or enzymatic deacetylation of 2'-7'-dichlorofluorescin diacetate (DCFH-DA), probably by auto-oxidation. Spectrophotometric measurements clearly showed that DCFH could be oxidized either by HRP-compound I or HRP-compound II with the obligate generation of the DCF semiquinone free radical (DCF*-). Oxidation of DCF*- to DCF by oxygen would yield superoxide (O2*-). ESR spectroscopy in conjunction with the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) revealed the presence of both superoxide and hydroxyl radicals in the DCFH/H2O2/HRP system. Both radicals were also detected in the absence of added H2O2, although the intensities of the resultant adducts were decreased. This work demonstrates that DCF fluorescence cannot be used reliably to measure O2*- in cells because O2*- itself is formed during the conversion of DCFH to DCF by peroxidases. The disproportionation of superoxide forms H2O2 which, in the presence of peroxidase activity, will oxidize more DCFH to DCF with self-amplification of the fluorescence. Because the deacetylation of DCFH-DA, even by esterases, can produce H2O2, the use of this probe to measure H2O2 production in cells is problematic.

A highly conserved sequence is a novel gene involved in de novo vitamin B6 biosynthesis

The Cercospora nicotianae SOR1 (singlet oxygen resistance) gene was identified previously as a gene involved in resistance of this fungus to singlet-oxygen-generating phototoxins. Although homologues to SOR1 occur in organisms in four kingdoms and encode one of the most highly conserved proteins yet identified, the precise function of this protein has, until now, remained unknown. We show that SOR1 is essential in pyridoxine (vitamin B6) synthesis in C. nicotianae and Aspergillus flavus, although it shows no homology to previously identified pyridoxine synthesis genes identified in Escherichia coli. Sequence database analysis demonstrated that organisms encode either SOR1 or E. coli pyridoxine biosynthesis genes, but not both, suggesting that there are two divergent pathways for de novo pyridoxine biosynthesis in nature. Pathway divergence appears to have occurred during the evolution of the eubacteria. We also present data showing that pyridoxine quenches singlet oxygen at a rate comparable to that of vitamins C and E, two of the most highly efficient biological antioxidants, suggesting a previously unknown role for pyridoxine in active oxygen resistance.

Merocyanine 540 solubilized as an ion pair with cationic surfactant in nonpolar solvents: spectral and photochemical properties

Merocyanine 540 (MC) is an anionic dye that is used to photopurge the bone marrow of leukemia cells. Under these conditions it is localized mostly in cell membranes, which may affect its photochemical reactivity. We investigated the photochemistry of MC dissolved as a hydrophobic ion pair with a hexadecyltrioctadecylammonium cation in cyclohexane, trimethylpentane and toluene as well as in propylene carbonate, CH3CN, C2H5OH and D2O. In organic solvents, the absorption and fluorescence spectra of MC were strongly red-shifted compared with aqueous solutions. The fluorescence was also more intense despite aggregation that occurred in some solvents. Aggregation strongly affects the spectral and photochemical properties of MC, especially in aliphatic hydrocarbons in which distinctive H-type aggregates are formed. Hydrophobic MC is a moderate photosensitizer of singlet molecular oxygen (1O2). The following values for 1O2 quantum yields were calculated based on 1O2 phosphorescence relative to 1O2 generation by Rose Bengal: approximately 0.12 in trimethylpenthane, approximately 0.13 in cyclohexane, 0.045 in EtOH, 0.039 in toluene, 0.007 in CH3CN and approximately 3 x 10(-4) in D2O. The H-aggregates of MC in cyclohexane and trimethylpentane are better 1O2 producers than monomeric MC. The above 1O2 quantum yields are corrected for self-quenching because MC is an efficient 1O2 quencher (17 x 10(7) M-1 s-1 in CH3CN, 6.8 x 10(7) M-1 s-1 in D2O, 5.2 x 10(7) M-1 s-1 in EtOH, and 1.4 x 10(7) M-1 s-1 in toluene). Merocyanine undergoes photodegradation, a solvent-dependent process that proceeds faster when the dye is aggregated. The initial photodegradation rate is much slower in organic solvents than in water, but photodegradation products accumulated during longer irradiation may increase the rate in most solvents. Higher photostability and better photosensitization by MC in hydrophobic nonpolar solvents suggest that the killing of leukemia cells via a photodynamic mechanism may operate mostly in cell membranes. In contrast, any cytotoxic products from photodecomposition may be important in hydrophilic cell compartments. Our data show the spectral and photochemical properties of MC in a pure hydrophobic environment.

Photophysical studies on antimalarial drugs

Most drugs used in the treatment of malaria produce phototoxic side effects in both the skin and the eye. Cutaneous and ocular effects that may be caused by light include changes in skin pigmentation, corneal opacity, cataract formation and other visual disturbances including irreversible retinal damage (retinopathy) leading to blindness. The mechanism for these reactions in humans is unknown. We irradiated a number of antimalarial drugs (amodiaquine, chloroquine, hydroxychloroquine, mefloquine, primaquine and quinacrine) with light (lambda > 300 nm) and conducted electron paramagnetic resonance (EPR) and laser flash photolysis studies to determine the possible active intermediates produced. Each antimalarial drug produced at least one EPR adduct with the spin-trap 5,5-dimethyl-1-pyrroline N-oxide in benzene: superoxide/hydroperoxyl adducts (chloroquine, mefloquine, quinacrine, amodiaquine and quinine), carbon-centered radical adducts (all but primaquine), or a nitrogen-centered radical adduct only (primaquine). In ethanol all drugs except primaquine produced some superoxide/hydroperoxyl adduct, with quinine, quinacrine, and hydroxychloroquine also producing the ethoxyl adduct. As detected with flash photolysis and steady-state techniques, mefloquine, quinine, amodiquine and a photoproduct of quinacrine produced singlet oxygen ([symbol: see text]delta = 0.38; [symbol: see text]delta = 0.36; [symbol: see text]delta = 0.011; [symbol: see text]delta = 0.013 in D2O, pD7), but only primaquine quenched singlet oxygen efficiently (2.6 x 10(8) M-1 s-1 in D2O, pD7). Because malaria is a disease most prevalent in regions of high light intensity, protective measures (clothing, sunblock, sunglasses or eye wraps) should be recommended when administering antimalarial drugs.

Oxidation of biological electron donors and antioxidants by a reactive lactoperoxidase metabolite from nitrite (NO2-): an EPR and spin trapping study

We report that a lactoperoxidase (LPO) metabolite derived from nitrite (NO2-) catalyses one-electron oxidation of biological electron donors and antioxidants such as NADH, NADPH, cysteine, glutathione, ascorbate, and Trolox C. The radical products of the reaction have been detected and identified using either direct EPR or EPR combined with spin trapping. While LPO/H2O2 alone generated only minute amounts of radicals from these compounds, the yield of radicals increased sharply when nitrite was also present. In aerated buffer (pH 7) the nitrite-dependent oxidation of NAD(P)H by LPO/H2O2 produced superoxide radical, O2*-, which was detected as a DMPO/*O2H adduct. We propose that in the LPO/H2O2/NO2-/biological electron donor systems the nitrite functions as a catalyst because of its preferential oxidation by LPO to a strongly oxidizing metabolite, most likely a nitrogen dioxide radical *NO2, which then reacts with the biological substrates more efficiently than does LPO/H2O2 alone. Because both nitrite and peroxidase enzymes are ubiquitous our observations point at a possible mechanism through which nitrite might exert its biological and cytotoxic action in vivo, and identify some of the physiological targets which might be affected by the peroxidase/H2O2/nitrite systems.

Electron paramagnetic resonance and spin trapping investigations of the photoreactivity of human lens proteins

Oxidation of cysteine, glutathione and ascorbate by photoexcited proteins from normal and cataractous lenses was investigated using electron paramagnetic resonance in combination with spin trapping. We report that illumination of these proteins in pH 7 buffer with light > 300 nm in the presence of thiols (RSH) and a spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO), afforded DMPO/S-cysteine and DMPO/SG adducts, suggesting the formation of the corresponding thiyl radicals. In a nonbuffered aqueous solution, illumination of the proteins and glutathione also produced superoxide detected as a DMPO/O2H adduct. Irradiation of these proteins in the presence of ascorbate generated ascorbate radical. We conclude that chromophores present in the natural normal and cataractous lenses are capable of initiating photooxidative processes involving endogenous thiols and ascorbic acid. This observation may be pertinent to UV-induced development of cataract.

Photoreduction of the fluorescent dye 2'-7'-dichlorofluorescein: a spin trapping and direct electron spin resonance study with implications for oxidative stress measurements

The photoreduction of 2'-7'-dichlorofluorescein (DCF) was investigated in buffer solution using direct electron spin resonance (ESR) and the ESR spin-trapping technique. Anaerobic studies of the reaction of DCF in the presence of reducing agents demonstrated that during visible irradiation (lambda > 300 nm) 2'-7'-dichlorofluorescein undergoes one-electron reduction to produce a semiquinone-type free radical as demonstrated by direct ESR. Spin-trapping studies of incubations containing DCF, 5,5-dimethyl-1-pyrroline N-oxide (DMPO) and either reduced glutathione (GSH) or reduced NADH demonstrate, under irradiation with visible light, the production of the superoxide dismutase-sensitive DMPO/*OOH adduct. In the absence of DMPO, measurements with a Clark-type oxygen electrode show that molecular oxygen is consumed in a light-dependent process. The semiquinone radical of DCF, when formed in an aerobic system, is immediately oxidized by oxygen, which regenerates the dye and forms superoxide.

Photoproduction and direct spectral detection of singlet molecular oxygen (1O2) in keratinocytes stained with rose bengal

In vivo, keratinocyte skin cells are exposed to photooxidative processes, some of which can be mediated by singlet molecular oxygen (1O2), a species that is very difficult to detect spectrally in cells. We photosensitized 1O2 in cultured HaCaT keratinocytes stained with rose bengal (RB) that localizes exclusively inside the keratinocyte hydrophobic regions, as evidenced by strongly red-shifted absorbance and intense fluorescence. We used keratinocytes grown in a monolayer on a plastic coverslip and in suspension. The phosphorescence spectrum (1200-1350 nm) from 1O2 was strongest when the coverslip containing RB-stained keratinocytes was irradiated in air. The spectral intensity decreased when the coverslip was immersed in D2O during irradiation and was almost completely quenched when it was irradiated while immersed in water. Water not only shortens the 1O2 lifetime but also reabsorbs part of the 1O2 phosphorescence, processes that do not occur when 1O2 is produced in a keratinocyte layer exposed to air. Because the RB was inside keratinocytes, singlet oxygen must also be produced inside the keratinocytes. However, the sensitivity to the extracellular environment suggests that most of the detectable 1O2 phosphorescence originates from those 1O2 molecules that escaped from the cell through its membrane into D2O or into the air, where 1O2 has longer lifetimes. Our results confirm directly that 1O2 is indeed photosensitized in living cells by RB. They also suggest that keratinocyte monolayers may be a good cell model to examine in vitro the production of 1O2 by other photosensitizers of environmental and photomedical interest.

Effect of magnetite particles on photoinduced and nonphotoinduced free radical processes in human erythrocytes

Magnetite (Fe3O4) encapsulated in polystyrene microspheres dramatically decreased the time for 50% hemolysis (t1/2) of human erythrocytes irradiated (lambda > 300 nm) in the presence of ketoprofen (0.1 mM). The magnetic microspheres were present at a very low concentration (0.002%) such that on average there was only one particle per four erythrocytes. No such effect was seen when nonmagnetic microspheres were employed or when the equivalent concentration of soluble iron (FeCl3) was present. A decrease in t1/2 was also observed when the magnetic microspheres were added after UVA/ketoprofen treatment or when they were present during hemolysis initiated by thermolysis of 2,2'-azobis(2-amidinopropane). These findings may be attributed to an increase in the membrane concentration of lipid radicals as a result of a magnetic field-induced increase in radicals escaping from triplet radical pairs.

Photocleavage of DNA by the fluoroquinolone antibacterials

We have determined the relative efficiencies for the formation of single strand breaks (ssbs) after the UVA irradiation of pBR322 DNA and various fluoroquinolone (fleroxacin, lomefloxacin, norfloxacin) and naphthyridine (nalidixic acid, enoxacin) antibacterials. After correcting for the differences in absorption, the relative order for DNA photocleaving activity under anaerobic conditions is: fleroxacin, lomefloxacin > nalidixic acid >> norfloxacin > enoxacin. In general, fluoroquinolones having fluorine substituents at the C-6 and C-8 positions (lomefloxacin and fleroxacin) are 10-fold more efficient in generating ssbs than those having only a C-6 fluorine atom (norfloxacin). The effect of oxygen on photoinduced DNA damage caused by these antibacterials is complex, but our data imply that active oxygen species are not necessary for DNA scission by these molecules, and indeed, may sometimes inhibit it. Lomefloxacin ethyl ester, which cannot undergo decarboxylation, is as active as lomefloxacin itself. Thus the free radical generated by decarboxylation is unlikely to be the active species involved in photoinduced fluoroquinolone DNA cleavage. For lomefloxacin and fleroxacin, DNA damage probably results from the generation of a carbene at C-8 as a result of photoinduced loss of their F8 atom as fluoride upon UVA irradiation. Fluoroquinolones lacking a C-8 fluorine atom must operate by a different mechanism. While photocleavage of pBR322 DNA does not necessarily mean that duplex DNA will be cleaved under the same conditions, nevertheless lomefloxacin and fleroxacin, the two most photogenotoxic fluoroquinolones, did cause the most damage to the plasmid DNA.

Lactoperoxidase-catalyzed oxidation of melanin by reactive nitrogen species derived from nitrite (NO2-): an EPR study

The reaction of synthetic DOPA melanin (DM) with lactoperoxidase (LPO), hydrogen peroxide, and nitrite (NO2-) has been investigated using EPR. We observed that in the presence of nitrite LPO/H2O2 generated large amount of melanin radicals, as evidenced by a strong, up to 11-fold, increase in the intensity of the melanin EPR signal. In contrast, when nitrite was omitted the increase was much less, ca. 30%, which, nevertheless, indicates that DM can be metabolized directly by LPO/H2O2. When the nitrite was present, the concentration of melanin radicals was linearly dependent on [NO2-] (for [NO2-] <5 mM), and increased when [LPO] and [H2O2] increased (at constant [NO2-]). We propose that the mechanism for the generation of melanin radicals by the LPO/H2O2/nitrite system involves oxidation of NO2- by LPO/H2O2 to a reactive metabolite, most likely the nitrogen dioxide radical (.NO2), which subsequently reacts with melanin 5,6-dihydroxyindole subunits producing the respective semiquinone radicals. Because melanin and .NO2 generating systems (nitrite, peroxidase enzymes, hydrogen peroxide) may coexist in cells in vivo, our results suggest that melanin could function as a natural scavenger of this highly reactive nitrogen species. This property may be relevant to the physiological functions of the melanin pigments in vivo.

Influence of solvent polarity and proticity on the photochemical properties of norfloxacin

The fluoroquinolone antibacterial norfloxacin (NF) is a moderate photosensitizer of singlet molecular oxygen (1O2). We have studied photosensitization by NF as a function of medium polarity and proticity in solvent mixtures. We have used 1,4-dioxane and propylene carbonate mixtures to keep proticity constant while modulating polarity, and water/D2O and ethylene carbonate mixtures to alter proticity without large changes in polarity. The absorption spectrum of NF was little affected by solvent changes, as compared to the fluorescence spectrum that exhibited as much as a 50 nm blue-shift, e.g. 1,4-dioxane versus D2O. The quantum yield of NF fluorescence saturated at an almost 10 times higher value (approximately 0.14) when proticity was increased by added water, up to 0.2 mol fraction, to ethylene carbonate. Less pronounced, the increasing polarity in 1,4-dioxane/propylene carbonate mixtures affected the fluorescence yield much less. Norfloxacin produces 1O2 and is able to quench 1O2. The rate constant for 1O2 quenching is 4.5 x 10(7) M-1 s-1 in propylene carbonate but decreases ca four times in D2O. The quantum yield of 1O2 photogeneration was also up to five times higher in solvents that were both protic and polar than vice versa. Our data show that NF is more photochemically active in an environment that is both protic and polar. This suggests the involvement of polar excited state(s) and possible proton/hydrogen transfer during photoexcitation. Similar processes may initiate the phototoxic response reported in some patients treated with the fluoroquinolone drugs. The phototoxicity of NF and other fluoroquinolone antibiotics may strongly depend on their localization in hydrophilic or hydrophobic cell/tissue regions.

The effect of static magnetic fields on the photohemolysis of human erythrocytes by ketoprofen

Ultraviolet irradiation (lambda > 300 nm) of the nonsteroidal anti-inflammatory agent ketoprofen (KP, 3-benzoyl-alpha-methylbenzoacetic acid) in aqueous solution, pH 7.4, results in heterolytic decarboxylation of the drug to give 3-ethylbenzophenone (EtBP). Ketoprofen caused the photohemolysis of human erythrocytes probably as a result of lipid peroxidation. Application of a static magnetic field (250-1500 G) during UV (> 300 nm) irradiation of KP and erythrocytes significantly decreased the time required for photohemolysis. This observation suggests that KP-induced photohemolysis involves the initial generation of a triplet radical pair derived from the reaction of triplet state KP (or 3-EtBP) with erythrocyte component(s) probably lipids. The magnetic field increases the concentration and/or lifetime of free radicals that escape from the radical pair so that the critical radical concentration needed to initiate membrane damage and cause cell lysis is reached sooner. Spin-trapping studies with 2,6-dibromo-1-nitrosobenzene-4-sulfonate confirmed that the application of an external static magnetic field increased the concentration of radicals released during the photolysis of either KP or 3-EtBP dissolved in organized media such as sodium dodecylsulfate micelles.

Fluoroquinolone antimicrobials: singlet oxygen, superoxide and phototoxicity

The fluoroquinolone antibacterial agents possess photosensitizing properties that lead to phototoxic responses in both human and animal subjects. The phototoxicity order reported in humans is: fleroxacin > lomefloxacin, pefloxacin >> ciprofloxacin > enoxacin, norfloxacin and ofloxacin. Studies both in vivo and in vitro have related this phototoxicity to the generation of reactive oxygen species including hydrogen peroxide and the hydroxyl radical. We determined the quantum yields of singlet oxygen generation (phi delta) by detection of the singlet oxygen (1O2) luminescence at 1270 nm for several fluoroquinolones, naphthyridines and other structurally related compounds. All the fluoroquinolones examined have low phi delta values ranging from 0.06 to 0.09 in phosphate buffer at pD 7.5. We also determined the 1O2 quenching constants for these compounds and their values were on the order of 10(6) M-1 s-1, except for lomefloxacin whose rate constant was 1.8 x 10(7) M-1 s-1. The phi delta values were significantly decreased in a solvent of lower polarity such as methanol (0.007 < or = phi delta < or = 0.02). The production of 1O2 by these antibiotics did not correlate with the order reported for their phototoxicity. We also measured the photogeneration (lambda > 300 nm) of superoxide by these antibacterials in dimethylsulfoxide using electron paramagnetic resonance and the spin trap 5,5-dimethyl-1-pyrroline N-oxide. Although there is not a one-to-one correspondence between the relative rates of superoxide generation and the phototoxicity ranking of the fluoroquinolones, the more phototoxic compounds tended to produce superoxide at a faster rate. Nevertheless, the magnitudes of the observed differences do not appear sufficient to explain the range of fluoroquinolone phototoxicity potencies in human and animal subjects in general and the high activity of fleroxacin and lomefloxacin in particular. For these latter drugs the photoinduced loss of the F8 atom as fluoride and the concomitant generation of a highly reactive carbene at C-8 provide a more plausible mechanism for their potent phototoxic and photocarcinogenic properties.

Anthralin stimulates keratinocyte-derived proinflammatory cytokines via generation of reactive oxygen species

OBJECTIVE AND DESIGN

Topical application of anthralin, used in the treatment of psoriasis, is often accompanied by severe skin inflammation, presumably due to free radical products of the drug. The role of inflammatory cytokines and their induction by anthralin-derived reactive oxygen species were studied in cultures of normal human keratinocytes (NHKs).

MATERIALS AND METHODS

Anthralin was added to cultures of NHKs in the presence or absence of various antioxidants, including superoxide dismutase, tetramethylthiourea, N-acetylcysteine and vitamin E and relative changes in cytokine secretion and in the number of mRNA transcripts were examined. In addition, NHKs were either treated with neutralizing antibodies to tumor necrosis factor (TNF)-alpha or transfected with a CAT-linked IL-8 promoter to establish the direct effects of anthralin on chemokine synthesis.

RESULTS

Anthralin, at concentrations between 5 microM and 25 microM, caused a marked increase in granulocyte macrophage-colony stimulating factor (GM-CSF), interleukin (IL)-6, IL-8 and TNFalpha synthesis that was selectively inhibited by specific antioxidants. Furthermore, anthralin induced chemokine secretion without the need of primary cytokines.

CONCLUSIONS

Taken together, these studies suggest that oxygen radicals generated from anthralin are responsible for the induction of inflammatory cytokines which, in turn contributes to their dermal toxicity.

Lactoperoxidase-catalyzed oxidation of the anticancer agent mitoxantrone by nitrogen dioxide (NO2.) radicals

Mitoxantrone [1,4-dihydroxy-5,8-bis[[2-[(2-hydroxyethyl)amino]ethyl] amino]-9,10-anthracenedione, MXH2] is a novel anticancer agent frequently employed in the chemotherapy of leukemia and breast cancer. Earlier studies have shown that metabolic oxidation to reactive 1,4-quinone or/and 5,8-diiminequinone intermediates may be an important mechanism of activation of this agent, pertinent to its cytotoxic action in vivo. Here we report that in the presence of nitrite ions (NO2-), MXH2 undergoes oxidation by the mammalian enzyme lactoperoxidase (LPO) and hydrogen peroxide and that the process proceeds at a rate that is proportional to NO2- concentration. In contrast, when MXH2 was exposed to LPO/H2O2 in the absence of nitrite, oxidation of the drug was either completely absent or markedly inhibited. These experiments were carried out using concentrated solutions of MXH2 (approximately 100 microM) at near neutral pH where dimers of the drug predominate. We propose that oxidation of MXH2 is mediated by an LPO/ H2O2 metabolite of NO2-, most likely the .NO2 radical. Because in mitoxantrone therapy the drug is administered intravenously, it is directly exposed to nitrogen oxides and other free radicals produced by blood components. It is therefore possible that the ability of mitoxantrone to react with the nitrogen dioxide radical may be relevant to the biological action of the drug in vivo.

Photochemical reactions and phototoxicity of sterols: novel self-perpetuating mechanisms for lipid photooxidation

Sterols are important lipid components that may contribute to phototoxicity. We have found that phototoxic response in earthworms is related to sterols extractable with lipophilic solvents. The photochemically active compounds in worm lipids are 5,7,9(11),22-ergostatetraen-3 beta-ol (9-DHE) and 5,7,9(11)-cholestartien-3 beta-ol (9-DDHC), respectively. Human skin lipids are known to contain 9-DHE. We have also found 9-DDHC in human skin, which is reported here for the first time. In the presence of an excess of the corresponding 5,7-dienes (ergosterol of 7-dehydrocholesterol), these photoactive sterols constitute a self-regenerating source of singlet molecular oxygen (1O2) during irradiation in vivo or in vitro with UVA (315-400 nm). The quantum yield for photosensitization of 1O2 by 9-DHE was estimated to be 0.09. The 1O2 is scavenged by the dienes and the rate constant for 1O2 quenching by ergosterol was found to be 1.2 x 10(7) M-1 s-1 in methyl t-butyl ether (MTBE). This scavenging ultimately leads to the production of 5,8-endoperoxide and hydrogen peroxide. Photochemically induced superoxide radical was also produced on irradiation of sterol 5,7,9-trienes and trapped with the spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO). The production of singlet oxygen, peroxides and radicals by the sterols may be significant in the cell damaging and tumor promoting action of UVA light on skin.