Oxidative stress in vitiligo: photo-oxidation of pterins produces H(2)O(2) and pterin-6-carboxylic acid

Characterization and reactivity of photodimers of dihydroneopterin and dihydrobiopterin

7,8-Dihydrobiopterin (H(2)Bip) and 7,8-dihydroneopterin (H(2)Nep) belong to a class of heterocyclic compounds present in a wide range of living systems. H(2)Bip accumulates in the skin of patients suffering from vitiligo, whereas H(2)Nep is secreted by human macrophages when the cellular immune system is activated. We have investigated the photochemical reactivity of both compounds upon UV-A irradiation (320-400 nm), the chemical structures of the products and their thermal stability. The study was performed in neutral aqueous solutions. The reactions were followed by UV/Visible spectrophotometry and HPLC and the products were analyzed by means of electrospray ionization mass spectrometry and (1)H-NMR. Excitation of H(2)Bip and H(2)Nep leads to the formation, in each case, of two main isomeric dimers. The latter compounds undergo a thermal process that may consist in a retro [2 + 2]-cycloaddition and hydrolysis to yield the reactant (H(2)Bip or H(2)Nep) and a product that has incorporated a molecule of H(2)O.

HPLC analysis of tetrahydrobiopterin and its pteridine derivatives using sequential electrochemical and fluorimetric detection: application to tetrahydrobiopterin autoxidation and chemical oxidation

Tetrahydrobiopterin (BH(4)) is an essential cofactor of endothelial nitric oxide (NO) synthase and when depleted, endothelial dysfunction results with decreased production of NO. BH(4) is also an anti-oxidant being a good "scavenger" of oxidative species. NADPH oxidase, xanthine oxidase, and mitochondrial enzymes producing reactive oxygen species (ROS) can induce elevated oxidant stress and cause BH(4) oxidation and subsequent decrease in NO production and bioavailability. In order to define the process of ROS-mediated BH(4) degradation, a sensitive method for monitoring pteridine redox-state changes is required. Considering that the conventional fluorescence method is an indirect method requiring conversion of all pteridines to oxidized forms, it would be beneficial to use a rapid quantitative assay for the individual detection of BH(4) and its related pteridine metabolites. To study, in detail, the BH(4) oxidative pathways, a rapid direct sensitive HPLC assay of BH(4) and its pteridine derivatives was adapted using sequential electrochemical and fluorimetric detection. We examined BH(4) autoxidation, hydrogen peroxide- and superoxide-driven oxidation, and Fenton reaction hydroxyl radical-driven BH(4) transformation. We demonstrate that the formation of the primary two-electron oxidation product, dihydrobiopterin (BH(2)), predominates with oxygen-induced BH(4) autoxidation and superoxide-catalyzed oxidation, while the irreversible metabolites, pterin and dihydroxanthopterin (XH(2)), are largely produced during hydroxyl radical-driven BH(4) oxidation.

Blunted epidermal L-tryptophan metabolism in vitiligo affects immune response and ROS scavenging by Fenton chemistry, part 1: Epidermal H2O2/ONOO(-)-mediated stress abrogates tryptophan hydroxylase and dopa decarboxylase activities, leading to low serotonin and melatonin levels

Vitiligo is characterized by a progressive loss of inherited skin color. The cause of the disease is still unknown. To date, there is accumulating in vivo and in vitro evidence for massive oxidative stress via hydrogen peroxide (H(2)O(2)) and peroxynitrite (ONOO(-)) in the skin of affected individuals. Autoimmune etiology is the favored theory. Since depletion of the essential amino acid L-tryptophan (Trp) affects immune response mechanisms, we here looked at epidermal Trp metabolism via tryptophan hydroxylase (TPH) with its downstream cascade, including serotonin and melatonin. Our in situ immunofluorescence and Western blot data reveal significantly lower TPH1 expression in patients with vitiligo. Expression is also low in melanocytes and keratinocytes under in vitro conditions. Although in vivo Fourier transform-Raman spectroscopy proves the presence of 5-hydroxytryptophan, epidermal TPH activity is completely absent. Regulation of TPH via microphthalmia-associated transcription factor and L-type calcium channels is severely affected. Moreover, dopa decarboxylase (DDC) expression is significantly lower, in association with decreased serotonin and melatonin levels. Computer simulation supports H(2)O(2)/ONOO(-)-mediated oxidation/nitration of TPH1 and DDC, affecting, in turn, enzyme functionality. Taken together, our data point to depletion of epidermal Trp by Fenton chemistry and exclude melatonin as a relevant contributor to epidermal redox balance and immune response in vitiligo.

Expression of cytosolic NADP(+)-dependent isocitrate dehydrogenase in melanocytes and its role as an antioxidant

BACKGROUND

Cytosolic NADP(+)-dependent ICDH (IDPc) has an antioxidant effect as a supplier of NADPH to the cytosol, which is needed for the production of glutathione.

OBJECTIVE

To evaluate the expression of IDPc in melanocytes and to elucidate its role as an antioxidant.

METHODS

The knock-down of IDPc expression in immortalized mouse melanocyte cell lines (melan-a) was performed using the short interfering RNA (siRNA)-targeted gene silencing method. After confirming the silencing of IDPc expression with mRNA and protein levels, viability, apoptosis and necrosis, as well as ROS production in IDPc-silenced melanocytes were monitored under conditions of oxidative stress and non-stress. Also, the ratio of oxidized glutathione to total glutathione was examined, and whether the addition of glutathione recovered cell viability, decreased by oxidant stress, was checked.

RESULTS

The expression of IDPc in both primary human melanocytes and melan-a cells was confirmed by Western blot and RT-PCR. The silencing of IDPc expression by transfecting IDPc siRNA in melan-a cells was observed by Western blotting and real-time RT-PCR. IDPc knock-down cells showed significantly decreased cell viability and an increased number of cells under apoptosis and necrosis. IDPc siRNA-treated melanocytes demonstrated a higher intensity of DCFDA after the addition of H(2)O(2) compared with scrambled siRNA-treated melanocytes, and a lower ratio of reduced glutathione to oxidized glutathione were observed in IDPc siRNA transfected melanocytes. In addition, the addition of glutathione recovered cell viability, which was previously decreased after incubation with H(2)O(2).

CONCLUSIONS

This study suggests that decreased IDPc expression renders melanocytes more vulnerable to oxidative stress, and IDPc plays an important antioxidant function in melanocytes.

Emission properties of dihydropterins in aqueous solutions

Pterins belong to a class of heterocyclic compounds present in a wide range of living systems and accumulate in the skin of patients affected by vitiligo, a depigmentation disorder. The study of the emission of 7,8-dihydropterins is difficult because these compounds are more or less unstable in the presence of O(2) and their solutions are contaminated with oxidized pterins which have much higher fluorescence quantum yields (Φ(F)). In this work, the emission properties of six compounds of the dihydropterin family (6-formyl-7,8-dihydropterin (H(2)Fop), sepiapterin (Sep), 7,8-dihydrobiopterin (H(2)Bip), 7,8-dihydroneopterin (H(2)Nep), 6-hydroxymethyl-7,8-dihydropterin (H(2)Hmp), and 6-methyl-7,8-dihydropterin (H(2)Mep)) have been studied in aqueous solution. The fluorescence characteristics (spectra, Φ(F), lifetimes (τ(F))) of the neutral form of these compounds have been investigated using the single-photon-counting technique. Φ(F) and τ(F) values obtained lie in the ranges 3-9 × 10(-3) and 0.18-0.34 ns, respectively. The results are compared to those previously reported for oxidized pterins.

Vitiligo, reactive oxygen species and T-cells

The acquired depigmenting disorder of vitiligo affects an estimated 1% of the world population and constitutes one of the commonest dermatoses. Although essentially asymptomatic, the psychosocial impact of vitiligo can be severe. The cause of vitiligo remains enigmatic, hampering efforts at successful therapy. The underlying pathogenesis of the pigment loss has, however, been clarified to some extent in recent years, offering the prospect of effective treatment, accurate prognosis and rational preventative strategies. Vitiligo occurs when functioning melanocytes disappear from the epidermis. A single dominant pathway is unlikely to account for all cases of melanocyte loss in vitiligo; rather, it is the result of complex interactions of biochemical, environmental and immunological events, in a permissive genetic milieu. ROS (reactive oxygen species) and H2O2 in excess can damage biological processes, and this situation has been documented in active vitiligo skin. Tyrosinase activity is impaired by excess H2O2 through oxidation of methionine residues in this key melanogenic enzyme. Mechanisms for repairing this oxidant damage are also damaged by H2O2, compounding the effect. Numerous proteins and peptides, in addition to tyrosinase, are similarly affected. It is possible that oxidant stress is the principal cause of vitiligo. However, there is also ample evidence of immunological phenomena in vitiligo, particularly in established chronic and progressive disease. Both innate and adaptive arms of the immune system are involved, with a dominant role for T-cells. Sensitized CD8+ T-cells are targeted to melanocyte differentiation antigens and destroy melanocytes either as the primary event in vitiligo or as a secondary promotive consequence. There is speculation on the interplay, if any, between ROS and the immune system in the pathogenesis of vitiligo. The present review focuses on the scientific evidence linking alterations in ROS and/or T-cells to vitiligo.

Quenching of the fluorescence of aromatic pterins by deoxynucleotides

Steady-state and time-resolved studies of the fluorescence of four aromatic unconjugated pterins (pterin (Ptr), 6-(hydroxymethyl)pterin (Hmp), 6-methylpterin (Mep), and 6,7-dimethylpterin (Dmp)) in aqueous solutions in the presence of different nucleotides (2'-deoxyguanosine 5'-monophosphate (dGMP), 2'-deoxyadenosine 5'-monophosphate (dAMP), and 2'-deoxycytosine 5'-monophosphate (dCMP)) have been performed using the single-photon counting technique. The singlet excited states of acid forms of pterins are deactivated by purine nucleotides (dGMP and dAMP) via a combination of dynamic and static processes. The efficiency of the dynamic quenching is high, independently of the nature of the purine base of the nucleotide and of the chemical structure of the substituents linked to the pterin moiety. Analysis of the static quenching indicates that ground-state association between pterins and purine nucleotides takes place, but the formation of the corresponding complexes is significant only at relatively high reactant concentrations. The quenching of the fluorescence of acid forms of pterin derivatives by dCMP, a pyrimidine nucleotide, is slightly less efficient than the quenching by purine nucleotides and is purely dynamic. In alkaline media, the fluorescence quenching is much less efficient than in acidic media, the deactivation by purine nucleotides being purely dynamic, whereas quenching by dCMP is negligible. Possible mechanisms for the quenching of fluorescence of pterin derivatives by the different nucleotides are discussed.

Promoter variant in the catalase gene is associated with vitiligo in Chinese people

Vitiligo is an acquired depigmentation disorder, and reactive oxygen species have an important role in the physiology of cell damage. Reduced catalase enzyme activity and accumulation of excessive hydrogen peroxide have been observed in vitiligo. In a hospital-based case-control study of vitiligo patients (n=749) and age- and sex-matched healthy controls (n=763), we investigated three catalase (CAT) gene polymorphisms (-89A>T, 389C>T, and 419C>T) to examine whether CAT gene polymorphisms are associated with vitiligo susceptibility in the Chinese population. The case-control analysis revealed a 1.54-fold (95% confidence interval (CI) 1.25-1.91) increased risk of developing vitiligo for -89A>T genotype carriers. No evidence for any association between 389C>T and 419C>T polymorphisms in the catalase gene and vitiligo susceptibility was found. An analysis of haplotypes showed increased risk for T(-89)C(389) (odds ratio (OR) 1.90, 95% CI 1.26-2.86) and T(-89)T(389) (OR 2.80, 95% CI 1.24-6.30). Logistic regression analysis of catalase activity also showed a dose-response relationship between increased risk and decreased activity in CAT -89A>T variant genotype carriers, especially in vitiligo patients (P(trend) <0.001). Our molecular epidemiologic findings suggest that the CAT -89A>T variant genotypes were associated with a significant decrease in catalase enzyme activity and a genetic predisposition for vitiligo in Chinese people.

Comparison of plasma malondialdehyde, glutathione, glutathione peroxidase, hydroxyproline and selenium levels in patients with vitiligo and healthy controls

Background:

The etiology and pathophysiologic mechanism of vitiligo are still unclear. The relationship between increased oxidative stress due to the accumulation of radicals and reactive oxygen species and the associated changes in blood and epidermal component of vitiliginous skin have been reported many times. We investigated the possible changes of plasma malondialdehyde, glutathione, selenium, hydroxyproline and glutathione peroxidase activity levels in patients with vitiligo in order to evaluate the relationship between oxidative stress and etiopathogenesis of vitiligo.

Materials and Methods:

Plasma malondialdehyde, glutathione, hydroxyproline and glutathione peroxidase activity levels were measured by spectrophotometric methods, and HPLC was used for measurement of selenium concentrations.

Results:

Our results showed increased malondialdehyde, hydroxyproline and glutathione peroxidase activity levels in plasma of vitiligo group (P < 0.05).

Conclusion:

Support of antioxidant system via nonenzymatic antioxidant compounds and antioxidant enzymes may be useful to prevent of melanocyte degeneration which occur due to oxidative damage in vitiligo.

Photochemistry of dihydrobiopterin in aqueous solution

Dihydrobiopterin (H(2)Bip) and its oxidized analogue, biopterin (Bip), accumulate in the skin of patients suffering from vitiligo, a chronic depigmentation disorder in which the protection against UV radiation fails. The photochemistry of H(2)Bip was studied in neutral aqueous solutions upon UV-A irradiation (320-400 nm) at room temperature. The photochemical reactions were followed by UV/vis spectrophotometry, HPLC and enzymatic methods for hydrogen peroxide (H(2)O(2)) determination. Photoproducts were analyzed by means of electrospray ionization mass spectrometry. Under anaerobic conditions, excitation of H(2)Bip leads to the formation of at least two isomeric dimers with molecular masses equal to exactly twice the molecular mass of the reactant. This reaction takes place from the singlet excited state of the reactant. To the best of our knowledge, this is the first time that the photodimerization of a dihydropterin is reported. In the presence of air, the dimers are again the main photoproducts at the beginning of the reaction, but a small proportion of the reactant is converted into Bip. As the reaction proceeds and enough Bip accumulates in the solution, a photosensitized process starts, where Bip photoinduces the oxidation of H(2)Bip to Bip, and H(2)O(2) is formed. As a consequence, the rates of H(2)Bip consumption and Bip formation increase as a function of irradiation time, resulting in an autocatalytic photochemical process. In this process, Bip in its triplet excited state reacts with the ground state of H(2)Bip. The mechanisms involved are analyzed and the biological implications of the results are discussed.

Genetic polymorphisms of glutathione S-transferase and risk of vitiligo in the Chinese population

Vitiligo is a common acquired depigmenting disorder characterized by white areas on the skin. Oxidative stress is a major pathogenesis hypothesis of vitiligo. Glutathione S-transferases (GSTs) are enzymes involved in protecting cells against chemical toxicity and stress. We hypothesized that the GSTM1- and GSTT1-null genotypes and GSTP1 polymorphisms were associated with increased risk for vitiligo. In a hospital-based case-control study of 749 vitiligo patients and 763 age- and sex-frequency-matched healthy controls, GSTM1 and GSTT1 genotypes and GSTP1 (Ile104Val, Ala113Val, Gly169Asp) polymorphisms were analyzed using the multiplex PCR and PCR-restriction fragment length polymorphism technique, respectively. We found that the GSTT1-null genotype was significantly associated with the susceptibility to vitiligo and the GSTM1-null genotype also showed a trend toward vitiligo association. We further analyzed the combined effect of GSTM1-null and GSTT1-null genotypes and showed an increased risk of developing vitiligo. By contrast, no statistically significant association was found between GSTP1 polymorphisms and vitiligo risk. These results suggest that individuals with homozygous deletion of GSTT1 and/or GSTM1 have a greater predisposition to vitiligo.

Oxidative stress in experimental vitiligo C57BL/6 mice

AIM

To evaluate whether oxidative stress is implicated in melanocyte damage in vitiligo.

BACKGROUND

Vitiligo is a complex disorder characterized by gradually enlarging areas of depigmentation. A new unifying hypothesis for the etiology of this pigment disorder is proposed, in which we postulate that the final destruction of melanocytes in vitiligo results from a cascade of reactions initiated by a disregulation of melanogenesis, as the result of a breakdown in free radical defense.

METHODS

We evaluated 18 vitiligo mice and 12 controls that were age matched. Parameters of oxidative stress such as catalase (CAT), superoxide dismutase (SOD), and plasma malondialdehyde (MDA) were measured by spectrophotometry.

RESULTS

MDA levels in vitiligo mice were significantly higher than in controls (P < 0.001). CAT, SOD, and glutathione peroxidase (GPx) activities in mice were significantly lower than controls (P < 0.05 and P < 0.001, respectively).

CONCLUSION

Our results confirmed that oxidative stress plays an important role in the pathogenesis of vitiligo. Melanocyte damage in vitiligo might be linked to generalized oxidative stress. This study is the first report on antioxidant parameters in experimental vitiligo mice.

H(2)O(2) increases de novo synthesis of (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin via GTP cyclohydrolase I and its feedback regulatory protein in vitiligo

Patients with vitiligo accumulate up to 10(-3) mol/L concentrations of H(2)O(2) in their epidermis, which in turn affects many metabolic pathways in this compartment, including the synthesis and recycling of the cofactor (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin (6BH(4)). De novo synthesis of 6BH(4) is dependent on the rate-limiting enzyme GTP cyclohydrolase I (GTPCHI) together with its feedback regulatory protein (GFRP). This step is controlled by 6BH(4) and the essential amino acid L-phenylalanine. In the study presented here we wanted to investigate whether H(2)O(2) affects the GTPCHI/GFRP cascade in these patients. Our results demonstrated concentration-dependent regulation of rhGTPCHI where 100 micromol/L H(2)O(2) was the optimum concentration for the activation of the enzyme and >300 micromol/L resulted in a decrease in activity. Oxidation of GFRP and GTPCHI does not affect feedback regulation via L-phenylalanine and 6BH(4). In vitiligo a constant upregulation of 6BH(4) de novo synthesis results from epidermal build up of L-phenylalanine that is not controlled by H(2)O(2). Taking the results together, 6BH(4) de novo synthesis is controlled by H(2)O(2) in a concentration-dependent manner, but H(2)O(2)-mediated oxidation does not affect the functionality of the GTPCHI/GFRP complex.

Vitiligo e emoções

FUNDAMENTOS: O vitiligo acomete, em média, 1% da população mundial. Mais de 75% dos pacientes têm autoimagem depreciativa em relação à doença. Seu impacto emocional é muitas vezes negligenciado pelo cuidador, influenciando negativamente o prognóstico. OBJETIVO: Verificar o efeito do vitiligo sobre as emoções e discutir as últimas descobertas sobre a interação mente? corpo e seu desdobramento sobre a doença. MÉTODOS: Cem pacientes com diversas formas de vitiligo responderam, na primeira consulta, a uma pergunta sobre as emoções que a presença das manchas lhes provocava. RESULTADOS: Entre os que apresentavam manchas em áreas expostas, 80% queixaram-se de emoções desagradáveis, em contraposição a 37% dos que tinham manchas em áreas não expostas. As emoções mais referidas foram medo (71%), vergonha (57%), insegurança (55%), tristeza (55%) e inibição (53%). CONCLUSÃO: Qualquer doença crônica produz nos seres humanos uma vivência negativa propiciada pela expectativa de sofrimento. O vitiligo é um desafio à autoestima. Além de uma orientação científica adequada, o paciente de vitiligo carece de conforto emocional. A resposta e a adesão ao tratamento e até mesmo a resiliência diante de eventuais falhas terapêuticas dependem da boa relação médico-paciente. Numa época em que dispomos de respeitável terapêutica, torna-se indispensável que o dermatologista se mostre apto a avaliar seu paciente holisticamente.

New results on the photochemistry of biopterin and neopterin in aqueous solution

New photochemical studies of the reactivity of biopterin (BPT) and neopterin (NPT) in acidic (pH = 5.5) and alkaline (pH = 10.5) aqueous solutions at 350 nm and room temperature were performed. The photochemical properties of BPT are of particular interest because the photolysis of this compound takes place in the white skin patches of patients affected by vitiligo. The photochemical reactions were followed by UV/VIS spectrophotometry, HPLC, electrochemical measurement of dissolved O(2) and enzymatic methods for hydrogen peroxide (H(2)O(2)) and superoxide anion (O(2)(-)) determinations. When BPT or NPT are exposed to UVA radiation, a red intermediate, very likely 6-formyl-5,8-dihydropterin, is generated in an O(2)-independent process. That product is rapidly oxidized on admission of O(2) to yield 6-formylpterin and H(2)O(2). When the photolysis takes place in aerobic conditions, no additional pathways exist. On the other hand, in the absence of O(2), the intermediate generated is not stable and leads to the formation of many products. O(2)(-) is also generated during photo-oxidation of BPT and NPT. The quantum yields of reactant consumption depends on the O(2) concentration: the higher the O(2) concentration, the lower the quantum yields. This behavior is discussed in connection with the excited state of the pterins.

The Ca2+-binding capacity of epidermal furin is disrupted by H2O2-mediated oxidation in vitiligo

The Ca(2+)-dependent precursor convertase furin is abundantly expressed in epidermal keratinocytes and melanocytes. In this context, it is noteworthy that proopiomelanocortin (POMC) cleavage is also processed by furin, leading to ACTH, beta-lipotropin, and beta-endorphin. All prohormone convertases including furin are regulated by Ca(2+). Because numerous epidermal peptides and enzymes are affected by H(2)O(2)-mediated oxidation, including the POMC-derived peptides alpha-MSH and beta-endorphin as shown in the epidermis of patients with vitiligo, we here asked the question of whether furin could also be a possible target for this oxidation mechanism by using immunofluorescence, RT-PCR, Western blotting, Ca(2+)-binding studies, and computer modeling. Our results demonstrate significantly decreased in situ immunoreactivity of furin in the epidermis of patients with progressive vitiligo (n = 10), suggesting H(2)O(2)-mediated oxidation. This was confirmed by (45)Ca(2+)-binding studies with human recombinant furin identifying the loss of one Ca(2+)-binding site from the enzyme after oxidation with H(2)O(2). Computer simulation supported alteration of one of the two Ca(2+)-binding sites on furin. Taken together, our results implicate that the Ca(2+)-dependent proteolytic activity of this convertase is targeted by H(2)O(2), which in turn could contribute to the reduced epidermal expression of the POMC-derived peptides alpha-MSH and beta-endorphin as documented earlier in patients with vitiligo.

Oxidation of 2'-deoxyguanosine 5'-monophosphate photoinduced by pterin: type I versus type II mechanism

UV-A radiation (320-400 nm) induces damage to the DNA molecule and its components through different photosensitized reactions. Among these processes, photosensitized oxidations may occur through electron transfer or hydrogen abstraction (type I) and/or the production of singlet molecular oxygen ((1)O2) (type II). Pterins, heterocyclic compounds widespread in biological systems, participate in relevant biological processes and are able to act as photosensitizers. We have investigated the photosensitized oxidation of 2'-deoxyguanosine 5'-monophosphate (dGMP) by pterin (PT) in aqueous solution under UV-A irrradiation. Kinetic analysis was employed to evaluate the participation of both types of mechanism under different pH conditions. The rate constant of (1)O2 total quenching (k(t)) by dGMP was determined by steady-state analysis of the (1)O2 NIR luminescence, whereas the rate constant of the chemical reaction between (1)O2 and dGMP (k(r)) was evaluated from kinetic analysis of concentration profiles obtained by HPLC. The results show that the oxidation of dGMP photosensitized by PT occurs through two competing mechanisms that contribute in different proportions depending on the pH. The dominant mechanism in alkaline media involves the reaction of dGMP with (1)O2 produced by energy transfer from the PT triplet state to molecular oxygen (type II). In contrast, under acidic pH conditions, where PT and the guanine moiety of dGMP are not ionized, the main pathway for dGMP oxidation involves an initial electron transfer between dGMP and the PT triplet state (type I mechanism). The biological implications of the results obtained are also discussed.

Quinones are reduced by 6-tetrahydrobiopterin in human keratinocytes, melanocytes, and melanoma cells

Quinones are potentially dangerous substances generated from quinols via the intermediates semiquinone and hydrogen peroxide. Low semiquinone radical concentrations are acting as radical scavengers while high concentrations produce reactive oxygen species and quinones, leading to oxidative stress, apoptosis, and/or DNA damage. Recently it was recognised that thioredoxin reductase/thioredoxin (TR/T) reduces both p- and o-quinones. In this report we examine additional reduction mechanisms for p- and o-quinones generated from hydroquinone (HQ) and coenzyme Q10 and by 17beta-estradiol by the common cofactor 6(R)-L-erythro-5,6,7,8-tetrahydrobiopterin (6BH(4)). Our results confirmed that TR reduces the p-quinone 1,4 benzoquinone and coenzyme Q10-quinone back to HQ and coenzyme Q10-quinol, respectively, while 6BH(4) has the capacity to reduce coenzyme Q10-quinone and the o-quinone produced from 17beta-estradiol. 6BH(4) is present in the cytosol and in the nucleus of epidermal melanocytes and keratinocytes as well as melanoma cells and colocalises with TR/T. Therefore we conclude that both mechanisms are major players in the prevention of quinone-mediated oxidative stress and DNA damage.

Photosensitization of 2'-deoxyadenosine-5'-monophosphate by pterin

UV-A radiation (320-400 nm) induces damages to the DNA molecule and its components through photosensitized reactions. Pterins, heterocyclic compounds widespread in biological systems, participate in relevant biological processes and are able to act as photosensitizers. We have investigated the photosensitization of 2'-deoxyadenosine-5'-monophosphate (dAMP) by pterin (PT) in aqueous solution under UV-A radiation. The effect of pH was evaluated, the participation of oxygen was investigated and the products analyzed. Kinetic studies revealed that the reactivity of dAMP towards singlet oxygen (1O2) is very low and that this reactive oxygen species does not participate in the mechanism of photosensitization, although it is produced by PT upon UV-A excitation. In contrast, analysis of irradiated solutions by means of electrospray ionization mass spectrometry strongly suggested that 8-oxo-7,8-dihydro-2'-deoxyadenosine-5'-monophosphate (8-oxo-dAMP) was produced, indicating that the photosensitized oxidation takes place via a type I mechanism (electron transfer).

Singlet Oxygen (O2(1Δg)) Quenching by Dihydropterins

Pterins belong to a class of heterocyclic compounds present in a wide range of living systems. They participate in relevant biological functions and are involved in different photobiological processes. Dihydropterins are one of the biologically active forms of pterins. The photoinduced production and quenching of singlet oxygen (1O2) by a series of dihydropterins (7,8-dihydrobiopterin (DHBPT), 7,8-dihydroneopterin (DHNPT), 6-formyl-7,8-dihydropterin (FDHPT), sepiapterin (SPT), 7,8-dihydrofolic acid (DHFA), and 7,8-dihydroxanthopterin (DHXPT)) in aqueous solution at physiological pH ( approximately 7) were investigated, and the quantum yields of 1O2 production (PhiDelta) and rate constants of total quenching (kt) of 1O2 were determined. Studied compounds do not produce 1O2 under UV-A irradiation and are very efficient 1O2 quenchers. The chemical reactions between 1O2 and dihydropterin derivatives were investigated, and the corresponding rate constants (kr) were found to be particularly high. The oxidized pterin derivatives, biopterin (BPT), neopterin (NPT), 6-formylpterin (FPT), and folic acid (FA), were identified and quantified during the reaction of 1O2 with DHBPT, DHNPT, FDHPT, and DHFA, respectively. Besides the oxidation of the dihydropyrazine ring to yield the corresponding oxidized pterins, a second oxidation pathway, leading to fragmentation of the dihydropterin and formation of non-pterinic products, was identified. Mechanisms and biological implications are discussed.

Membrane Lipid Alterations as a Possible Basis for Melanocyte Degeneration in Vitiligo

The occurrence of oxidative stress has been proposed as a pathogenetic mechanism for melanocyte degeneration in vitiligo. In order to evaluate this possible correlation we focused on the lipid component of cell membranes. We observed in vitiligo melanocytes, through FACS methods, an increased median fluorescence intensity of rhodamine 123 and C11-BODIPY581/591 indicating a spontaneous higher production of reactive oxygen species (ROS) and membrane lipoperoxidation, associated with an altered pattern of cardiolipin (CL) distribution, defined on the basis of the fluorescence pattern after staining with 10-nonyl acridine orange. We confirmed membrane peroxidation by confocal and contrast-phase microscopes and demonstrated impaired activity of the mitochondrial electron transport chain (ETC) complex I. Finally, we observed increased apoptotic events following exposure to the pro-oxidant cumene hydroperoxide by Annexin V/propidium iodide fluorescence. We hypothesize that in vitiligo melanocytes lipid instability, with a defect in the synthesis or recycling of CL, induces ETC impairment and ROS production. In basal conditions melanocytes maintain the redox balance whereas following chemical or physical stress ROS-mediated membrane peroxidation is increased with a possible further CL oxidation, leading to cell death or detachment.

Oxidative stress via hydrogen peroxide affects proopiomelanocortin peptides directly in the epidermis of patients with vitiligo

The human skin holds the capacity for autocrine processing of the proopiomelanocortin (POMC)-derived peptides. Recent data demonstrated the presence and functionality of ACTH, alpha- and beta-melanocyte-stimulating hormone (MSH), and beta-endorphin in the regulation of skin pigmentation, and a role has been put forward for alpha-MSH as an effective antioxidant. In patients with vitiligo, decreased epidermal POMC processing and low alpha-MSH levels were documented previously. These patients accumulate hydrogen peroxide (H2O2) in the 10(-3) M range in their epidermis. Therefore, we examined the involvement of H2O2 on POMC-derived peptides as possible targets for oxidation by this reactive oxygen species. To address this, we employed immunofluorescence labelling, dot blot analysis, Fourier transform Raman spectroscopy, functionality studies, and computer simulation of the peptide structures. We demonstrate H2O2-mediated oxidation of epidermal ACTH, alpha-MSH, and beta-endorphin in vitiligo owing to oxidation of methionine residues in the sequences of these peptides. Moreover, we show that oxidized beta-endorphin loses its function in the promotion of pigmentation in melanocytes. These changes are reversible upon the reduction of H2O2 levels by a pseudocatalase PC-KUS. Moreover, oxidation of alpha-MSH can be prevented by the formation of a 1:1 complex with the abundant cofactor (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin. Thus, using vitiligo, we demonstrate that H2O2 can affect pigmentation via epidermal POMC peptide redox homeostasis.

A review and a new hypothesis for non-immunological pathogenetic mechanisms in vitiligo

Vitiligo is an acquired depigmenting disorder characterized by the loss of functioning epidermal melanocytes because of multifactorial and overlapping pathogenetic mechanisms. Besides the immunological approach, the study of the metabolic deregulations leading to toxic damage of the melanocytes appears to be more and more relevant. It was only last year that the first in vitro evidence supporting the link and the temporal sequence between the immune response and the cellular oxidative stress was provided, suggesting that the intrinsic damage of the melanocytes is primitive. What can be the guide line of the multiple altered metabolisms? A compromised membrane could render the cell sensitive to the external and internal agents differently, usually ineffective on the cell activity and survival. The primitive altered arrangement of the lipids may affect the transmembrane housing of proteins with enzymatic or receptorial activities, also conferring on them antigenic properties.

Photophysics and photochemistry of pterins in aqueous solution

Pterins belong to a family of heterocyclic compounds present in a wide range of living systems and participate in relevant biological functions. Interest in the photochemistry and photophysics of this group of compounds has increased since the participation of pterin derivatives in different photobiological processes has been suggested or demonstrated in recent decades. This account describes and connects basic studies on the fluorescence emission, the photooxidation, and the photosensitizing properties of oxidized six-substituted pterins in aqueous solution under UV-A irradiation. The biological implications of these studies are also discussed.

Estrogens can contribute to hydrogen peroxide generation and quinone-mediated DNA damage in peripheral blood lymphocytes from patients with vitiligo

To date there is ample in vivo and in vitro evidence for increased epidermal and systemic hydrogen peroxide (H(2)O(2)) levels in vitiligo, which can be reduced with a topical application of a pseudocatalase-K.U. Schallreuter (PC-KUS) leading to the recovery of epidermal catalase levels as well as other enzymes in peripheral blood cells. Recently, the generation of H(2)O(2) by oxidative metabolism of estrogens and other aromatic steroids was documented. Therefore, it was tempting to follow estrogen-generated H(2)O(2) and its possible effect on DNA damage in peripheral blood lymphocytes from patients with vitiligo before and after the reduction of epidermal H(2)O(2) with pseudocatalase PC-KUS compared to controls. For this purpose, 20 Caucasian patients were grouped in treated responders (group A, n=11) and untreated active/acute disease (group B, n=9) and compared to Caucasian healthy controls (group C, n=7). Consequently, epidermal catalase protein expression in full skin biopsies was assessed using immunofluorescence labelling together with determination of basal H(2)O(2) levels in peripheral blood lymphocytes. To test the influence of estrogen on H(2)O(2) generation and DNA damage, freshly prepared peripheral blood lymphocytes from all three groups were used for the alkaline comet assay in the presence and absence of catalase. The results of this study demonstrated that reduction of epidermal H(2)O(2) leads to both increased epidermal catalase protein expression as well as decreased H(2)O(2) concentrations in lymphocytes. Moreover, a direct estrogen-mediated DNA damage was identified in both patient groups, which was absent in healthy controls. This effect was not abolished by catalase pointing to direct quinone-mediated DNA damage by estrogens in peripheral blood lymphocytes in vitiligo.

Endogenous UVA-photosensitizers: mediators of skin photodamage and novel targets for skin photoprotection

Endogenous chromophores in human skin serve as photosensitizers involved in skin photocarcinogenesis and photoaging. Absorption of solar photons, particularly in the UVA region, induces the formation of photoexcited states of skin photosensitizers with subsequent generation of reactive oxygen species (ROS), organic free radicals and other toxic photoproducts that mediate skin photooxidative stress. The complexity of endogenous skin photosensitizers with regard to molecular structure, pathways of formation, mechanisms of action, and the diversity of relevant skin targets has hampered progress in this area of photobiology and most likely contributed to an underestimation of the importance of endogenous sensitizers in skin photodamage. Recently, UVA-fluorophores in extracellular matrix proteins formed posttranslationally as a consequence of enzymatic maturation or spontaneous chemical damage during chronological and actinic aging have been identified as an abundant source of light-driven ROS formation in skin upstream of photooxidative cellular stress. Importantly, sensitized skin cell photodamage by this bystander mechanism occurs after photoexcitation of sensitizers contained in skin structural proteins without direct cellular photon absorption thereby enhancing the potency and range of phototoxic UVA action in deeper layers of skin. The causative role of photoexcited states in skin photodamage suggests that direct molecular antagonism of photosensitization reactions using physical quenchers of photoexcited states offers a novel chemopreventive opportunity for skin photoprotection.

Decreased phenylalanine uptake and turnover in patients with vitiligo

The human epidermis has the full machinery for autocrine L-phenylalanine turnover to L-tyrosine in keratinocytes and melanocytes. Phenylalanine hydroxylase (PAH) activities increase linearly with inherited skin colour (skin phototype I-VI, Fitzpatrick classification) yielding eightfold more activities in black skin compared to white skin. Moreover, UVB irradiation (1 MED) significantly increases epidermal PAH activities 24 h after exposure. Importantly, L-phenylalanine uptake and turnover in the pigment forming melanocytes is vital for initiation of melanogenesis. In this context it was shown that the uptake of this amino acid is regulated by calcium. The depigmentation disorder vitiligo provides a unique model to follow impaired L-phenylalanine turnover in the skin as well as in serum because affected individuals hold an impaired epidermal 6BH4 de novo synthesis/recycling and regulation including low epidermal PAH activities. After overnight fasting and oral loading with L-phenylalanine (100 mg/kg body weight), 29.6% of 970 patients tested (n=287/970) yielded serum phenylalanine/tyrosine ratios >or=4 and 35.3% (n=342/970) had mild to moderate hyperphenylalaninaemia (HPA), while 9.3% (n=90/970) had both serum L-phenylalanine levels >or=2.0 mg/dl and phe/tyr ratios >or=4.0. Isolated HPA was found in 26% (n=252/970), whereas 20.3% had only increased ratios (n=197/970). None of the patients had phenylketonuria and the family history for this metabolic disease was negative. The IQ followed normal Gaussian distribution. In vitro L-phenylalanine uptake/turnover studies on primary epidermal melanocytes originating from these patients demonstrated a significantly decreased calcium dependent L-phenylalanine uptake and turnover compared to healthy control cells. Based on our observation, we would like to propose that phenylalanine uptake/turnover is under tight control by calcium which in turn could offer an additional novel mechanism in the aetiology of HPA.

Relevance of thyroiditis and of other autoimmune diseases in children with vitiligo

BACKGROUND

Studies that clearly define the possible association of childhood vitiligo with autoimmune and/or endocrine diseases are lacking.

OBJECTIVE

To examine the presence of autoimmune disorders, in particular of thyroid disease, in paediatric patients with vitiligo and investigate the utility of such screening in these patients.

METHODS

One hundred and twenty-one paediatric patients (40 males, 81 females) with vitiligo were grouped in segmental and non-segmental vitiligo. All patients were screened for thyroid disease.

RESULTS

13 out of 121 patients had different degrees of thyroid parameter alterations. These patients were all affected by the non-segmental type while none of those with the segmental form presented thyroid alterations.

CONCLUSION

In paediatric patients with non-segmental vitiligo, a significant incidence of thyroid dysfunction was found. Since vitiligo usually appears before the development of the thyroid disease, it may be useful to screen thyroid autoantibodies in all paediatric patients with non-segmental vitiligo who present symptoms related to thyroid disease.

Photooxidation of Pterin in Aqueous Solutions: Biological and Biomedical Implications

Studies of the photochemical reactivity of pterin (= 2-aminopteridin-4(3H)-one; PT) in acidic (pH 5.0-6.0) and alkaline (pH 10.2-10.8) aqueous solutions have been performed. The photochemical reactions were followed by UV/VIS spectrophotometry, thin layer chromatography (TLC), high-performance liquid chromatography (HPLC), and an enzymatic method for H2O2 determination. PT is not light-sensitive in the absence of molecular oxygen, but it undergoes photooxidation in the presence of O2, yielding several nonpteridinic products. The quantum yields for PT disappearance were found to be 8.2 (+/-0.6) x 10(-4) and 1.2 (+/-0.2) x 10(-3) in acidic and alkaline media, respectively. H2O2 was detected and quantified in irradiated solutions of PT; and its importance from a biomedical point of view is discussed. The rate constant of the chemical reaction between singlet oxygen ((1)O2) and PT was determined to be 2.5 (+/-0.2) x 10(5) l mol(-1) s(-1) in alkaline medium, and the role of (1)O2 in the photooxidation of pterin was evaluated.

Human phenylalanine hydroxylase is activated by H2O2: a novel mechanism for increasing the l-tyrosine supply for melanogenesis in melanocytes

Epidermal phenylalanine hydroxylase (PAH) produces L-tyrosine from the essential amino acid L-phenylalanine supporting melanogenesis in human melanocytes. Those PAH activities increase linearly in the different skin phototypes I-VI (Fitzpatrick classification) and also increase up to 24h after UVB light with only one minimal erythemal dose. Since UVB generates also H(2)O(2), we here asked the question whether this reactive oxygen species could influence the activity of pure recombinant human PAH. Under saturating conditions with the substrate L-phenylalanine (1x10(-3)M), the V(max) for enzyme activity increased 4-fold by H(2)O(2) (>2.0x10(-3)M). Lineweaver-Burk analysis identified a mixed activation mechanism involving both the regulatory and catalytic domains of PAH. Hyperchem molecular modelling and Deep View analysis support oxidation of the single Trp(120) residue to 5-OH-Trp(120) by H(2)O(2) causing a conformational change in the regulatory domain. PAH was still activated by H(2)O(2) in the presence of the electron donor/cofactor 6(R)-L-erythro-5,6,7,8-tetrahydrobiopterin despite slow oxidation of this cofactor. In vivo FT-Raman spectroscopy confirmed decreased epidermal phenylalanine in association with increased tyrosine after UVB exposure. Hence, generation of H(2)O(2) by UVB can activate epidermal PAH leading to an increased L-tyrosine pool for melanogenesis.

Oxidant-antioxidant enzymes and lipid peroxidation in generalized vitiligo

Vitiligo is a common pigmentary disorder of the skin with selective destruction of melanocytes. The pathogenetic mechanisms in vitiligo have not been completely clarified. The aim of our investigation was to evaluate the oxidative stress in the pathogenesis of generalized vitiligo. Twenty-seven patients with generalized vitiligo and 24 phototype-, age-, and sex-matched healthy controls were included in this study. We analysed serum levels of malondialdehyde (MDA), nitric oxide (NO), and serum activities of superoxide dismutase (SOD) and xanthine oxidase (XO) in the patients with vitiligo and in the controls. We found significantly higher levels of MDA and XO activity (P < 0.001 and P < 0.05, respectively), and a significantly lower level of serum SOD activity (P < 0.05) in patients with vitiligo compared with the controls. However, the increase in the level of serum NO was insignificant (P > 0.05). These results suggest that lipid peroxidation of cellular membrane of melanocytes by free radicals may have a significant role in the pathogenesis of generalized vitiligo.

On the Etiology of Contact/Occupational Vitiligo

Vitiligo is an acquired depigmentary disorder of the skin that results from the selective destruction of melanocytes, generally during the second decade of life and affecting approximately 1% of the population worldwide. Loss of cutaneous pigment appears to render the skin susceptible to premature aging and cancer. In addition this disease can be socially devastating for afflicted individuals. The etiology of vitiligo is poorly understood. The present dogma suggests that genetic factors render the melanocyte fragile thus predisposing individuals to developing vitiligo. When subjected to instigating factors, these susceptible, fragile melanocytes undergo apoptosis. Autoimmune factors then perpetuate the removal of the melanocyte component from the skin. In the majority of cases the instigating factors are not known (idiopathic vitiligo), however a small sub-set of individuals develop contact/occupational vitiligo following exposure to particular chemicals. Many of these chemicals have been implicated in both contact/occupational vitiligo and chemical leukoderma. Both conditions present with well-defined, depigmented skin lesions that develop following exposure. Only in the case of vitiligo does the depigmentation spread beyond the areas of contact, probably via an immune-mediated mechanism. The largest class of chemicals known to trigger contact/occupational vitiligo is the phenolic/catecholic derivatives. Many have been demonstrated to be preferentially cytotoxic to melanocytes, with high-dose exposure resulting in the initiation of apoptosis. Phenolic/catecholic derivatives are structurally similar to the melanin precursor tyrosine, and therefore tyrosinase was originally implicated as a mediator of cytotoxicity. However, our data suggests that tyrosinase-related protein-1, rather than tyrosinase, facilitates toxicity, possibly by catalytic conversion of the compounds, which results in the generation of radical oxygen species. The ensuing oxidative stress then triggers activation of cellular free radical scavenging pathways to prevent cell death. Genetic inability of melanocytes to tolerate and/or respond to the oxidative stress may underlie the etiology of contact/occupational vitiligo.

Activation/deactivation of acetylcholinesterase by H2O2: more evidence for oxidative stress in vitiligo

Previously it has been demonstrated that the human epidermis synthesises and degrades acetylcholine and expresses both muscarinic and nicotinic receptors. These cholinergic systems have been implicated in the development of the epidermal calcium gradient and differentiation in normal healthy skin. In vitiligo severe oxidative stress occurs in the epidermis of these patients with accumulation of H2O2 in the 10(-3)M range together with a decrease in catalase expression/activity due to deactivation of the enzyme active site. It was also shown that the entire recycling of the essential cofactor (6R)-l-erythro-5,6,7,8-tetrahydrobiopterin via pterin-4a-carbinolamine dehydratase (PCD) and dihydropteridine reductase (DHPR) is affected by H2O2 oxidation of Trp/Met residues in the enzyme structure leading to deactivation of these proteins. Using fluorescence immunohistochemistry we now show that epidermal H2O2 in vitiligo patients yields also almost absent epidermal acetylcholinesterase (AchE). A kinetic analysis using pure recombinant human AchE revealed that low concentrations of H2O2 (10(-6)M) activate this enzyme by increasing the Vmax>2-fold, meanwhile high concentrations of H2O2 (10(-3)M) inhibit the enzyme with a significant decrease in Vmax. This result was confirmed by fluorescence excitation spectroscopy following the Trp fluorescence at lambdamax 280nm. Molecular modelling based on the established 3D structure of human AchE supported that H2O2-mediated oxidation of Trp(432), Trp(435), and Met(436) moves and disorients the active site His(440) of the enzyme, leading to deactivation of the protein. To our knowledge these results identified for the first time H2O2 regulation of AchE. Moreover, it was shown that H2O2-mediated oxidation of AchE contributes significantly to the well-established oxidative stress in vitiligo.

Indirect oxidation of 6-tetrahydrobiopterin by tyrosinase

6-Tetrahydrobiopterin is known to bind to an allosteric site of tyrosinase to directly inhibit the enzyme. However, simultaneous measurements of ultraviolet-visible absorption spectra and oxygen consumption led us to conclude that the inhibition was due to oxidation of 6-tetrahydrobiopterin by dopaquinone. Immediately after addition of 6-tetrahydrobiopterin, tyrosinase stopped producing dopachrome from either tyrosine or dopa. Duration of inhibition was proportional to the concentration of added 6-tetrahydrobiopterin and the enzyme activity was fully restored after the inhibition. Surprisingly, there was a rapid consumption of oxygen during the inhibition period. In addition, absorption spectra indicated that the only reaction that occurred during the inhibition was oxidation of 6-tetrahydrobiopterin to 7,8-dihydrobiopterin. In the absence of tyrosine or dopa, tyrosinase did not oxidize 6-tetrahydrobiopterin, suggesting that a reaction intermediate between dopa and dopachrome was a target for the inhibition. We propose a new mechanism in which dopa is oxidized to dopaquinone and the latter, instead of producing dopachrome, is reduced back to dopa by 6-tetrahydrobiopterin.

Perturbed 6-tetrahydrobiopterin recycling via decreased dihydropteridine reductase in vitiligo: more evidence for H2O2 stress

To date there is ample evidence that patients with vitiligo accumulate millimolar concentrations of hydrogen peroxide (H2O2) in their epidermis as well as in their blood lymphocytes/monocytes. Several enzymes are affected by this H2O2 including catalase, glutathione peroxidase, and 4 alpha-carbinolamine dehydratase. The latter enzyme disrupts the recycling of the essential cofactor (6R)-L-erythro-5,6,7,8-tetrahydrobiopterin (6BH4) for the aromatic amino acid hydroxylases as well as the nitric oxide synthases. In this report we have elucidated the influence of H2O2 on dihydropteridine reductase (DHPR), the last enzyme in the 6BH4-recycling process. Here we show for the first time that concentrations of less than 30 microM H2O2 increase DHPR activities, whereas levels greater than 30 microM H2O2 deactivate the enzyme based on the oxidation of Met146 and Met151 in the sequence, consequently leading to disruption of the NADH-dependent enzyme active site. This oxidation was confirmed by Fourier transform-Raman spectroscopy yielding the expected SO band at 1025 cm-1 characteristic of methionine sulfoxide. Hence these results unmasked a novel regulatory mechanism for DHPR enzyme activity. Moreover, we also demonstrated that DHPR activities in whole blood of patients with vitiligo are significantly decreased in untreated patients, whereas activities are normalized after removal of epidermal H2O2 with a topical pseudocatalase (PC-KUS). Taken together, these new data add more evidence to a systemic involvement of H2O2 in the pathomechanism of vitiligo.