At physiological pH values, oxidation of the neurotransmitter dopamine (DA) by the peroxidase/HzOz system leads to, besides dopaminochrome and 5, 6-dihydroxyindole resulting from oxidative cyclization of dopaminequinone (DQ), significant amounts of the neurotoxin 6-hydroxydopamine (6-OHDA) in the oxidized quinonoid form (topaminequinone, TQ). Formation of TQ was shown to depend critically on the presence of hydrogen ...