ESR study on free radical scavenging and/or generating activity of dopamine-4-O-sulfate

In human, dopamine (DA) is mostly present as a sulfate conjugated form in the circulation, and very little of the free form. Recently, it was found that DA react with O_2 to produce reactive oxygen species (ROS) and semiquinone DA radicals, which are cytotoxic species. However, it is not known whether DA sulfate conjugates act like DA. Therefore, we compared DA and DA-4-O-sulfate (DA-4-OS) for susceptibility to the oxidization in vitro. The free radical metabolites of DA and DA-4-OS were initiated by UV, ROS, and alkaline solution. ESR spectrometer was adopted for the detection of radical species. Absorption spectra (250-600 nm) were measured to detect the oxidative products of DA derivatives by NaIO_4 oxidation. In this study, DA reacted with ROS, UV, and spontaneously oxidized at alkaline pH, then formed o-semiquinone DA radical as previously reported. However, DA-4-OS showed no such radicals even in the presence of ROS at neutral pH (7.4). In addition, DA-4-OS showed no absorption changes on addition of NaIO_4 , whereas DA did. DA-4-OS showed o-semiquinone radical only by UV. These results suggest that DA-4-OS has resistance against autoxidation, and seems to be a stable metabolite of DA under physiological conditions.