Presence of epidermal allantoin further supports oxidative stress in vitiligo

Xanthine dehydrogenase/xanthine oxidase (XDH/XO) catalyses the hydroxylation of hypoxanthine to xanthine and finally to uric acid in purine degradation. These reactions generate H2O2 yielding allantoin from uric acid when reactive oxygen species accumulates. The presence of XO in the human epidermis has not been shown so far. As patients with vitiligo accumulate H2O2 up to mM levels in their epidermis, it was tempting to examine whether this enzyme and consequently allantoin contribute to the oxidative stress theory in this disease. To address this question, reverse transcription-polymerase chain reaction, immunoreactivity, western blot, enzyme kinetics, computer modelling and high performance liquid chromatography/mass spectrometry analysis were carried out. Our results identified the presence of XDH/XO in epidermal keratinocytes and melanocytes. The enzyme is regulated by H2O2 in a concentration-dependent manner, where concentrations of 10(-6) M upregulates the activity. Moreover, we demonstrate the presence of epidermal allantoin in acute vitiligo, while this metabolite is absent in healthy controls. H2O2-mediated oxidation of Trp and Met in XO yields only subtle alterations in the enzyme active site, which is in agreement with the enzyme kinetics in the presence of 10(-3) M H2O2. Systemic XO activities are not affected. Taken together, our results provide evidence that epidermal XO contributes to H2O2-mediated oxidative stress in vitiligo via H2O2-production and allantoin formation in the epidermal compartment.