Perturbed 6-Tetrahydrobiopterin Recycling via Decreased Dihydropteridine Reductase in Vitiligo: More Evidence for H2O2 Stress

Abstract

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.

Beta Adrenergic Receptors in Keratinocytes

“…Third, epidermal keratinocytes derived from the affected skin of patients with vitiligo (and not the clinically normal appearing skin in these patients), demonstrate increased expression of the beta2AR in culture [12]. The expression of these receptors on keratinocytes are regulated by extracellular levels of calcium [12] as well as by 6-biopterin, the redox product of 6BHR, which is increased in the epidermis in patients with vitiligo (reviewed in [13,61]). Since beta2AR activation by catecholamines also induces calcium influx into keratinocytes [14], one might have anticipated that intracellular calcium levels are increased in keratinocytes in vitiliginous epidermis.…”

Maintenance of cellular tetrahydrobiopterin homeostasis

“…An interesting finding was that the DHPR protein decreases progressively in aortic smooth muscles of spontaneously hypertensive rats (81). DHPR activities in the white-blood cells are low in patients with vitiligo compared to normal subjects (82). DHPR activities in erythrocytes from patients with insulin-resistant diabetes are lower than normal subjects (83).…”

Photoinduced Generation of H2O2 and O2•- by 6-formylpterin in Aqueous Solutions

“…Moreover, formylpterin is one of the oxidized pterins that accumulate in the white skin patches of the patients affected by vitiligo ( 19). In addition, the oxidative stress generated by the accumulation of H 2 0 2 has been reported to be involved in the pathogenesis ofthat disease (18). Therefore photoinduced generation of 0 2 · by formylpterin, and very likely by other pterin derivatives present in skin, could contribute to that oxidative stress.…”

“…mediated oxidative stress is involved in the pathogenesis of vitiligo. High levels of this reactive oxygen species deactivate enzymes of the melanin metabolism (17,18). Several perturbed pathways leading to the accumulation of H 2 0 2 have been identified.…”