Nitric oxide-induced oxidation of 5,6-dihydroxyindole and 5,6-dihydroxyindole-2-carboxylic acid under aerobic conditions: non-enzymic route to melanin pigments of potential relevance to skin (photo)protection.

Diffusible melanin-related metabolites have recently been suggested to subserve a variety of functions that are critical for protection of skin against inflammatory stimuli and oxidative tissue injury. We report here the results of in vitro studies showing that 5,6-dihydroxyindole (DHI) and its 2-carboxylic acid (DHICA) exhibit a marked reactivity toward potentially cytotoxic nitrogen oxides produced by autoxidation of nitric oxide (NO) under physiologically relevant conditions. Exposure of DHI or DHICA to NO in air-equilibrated 0.1 M phosphate buffer, pH 7.4, resulted in a fast, concentration-dependent consumption of the substrates and the concomitant deposition of dark melanin-like pigments. All NO-induced oxidations were completely inhibited in the absence of oxygen. Addition of 10 mu M DHI and DHICA completely prevented the oxidation of 10 mu M alpha-tocopherol in 0.1 M phosphate buffer, pH 7.4 in the presence of 300 mu M NO. Overall, these results shed light on novel oxidative pathways of melanin-related metabolites of possible relevance to the mechanisms of skin hyperpigmentation under oxidative stress conditions.