Human glutathione transferase M2-2 prevents the formation of neurotoxic aminochrome and dopachrome by catalyzing the conjugation of dopamine and dopa o-quinone with glutathione. NMR analysis of dopamine and dopa o-quinone-glutathione conjugates revealed that the addition of glutathione was at C-5 to form 5-S-glutathionyl-dopamine and 5-S-glutathionyldopa, respectively. Both conjugates were found to be resistant to oxidation by biological oxidizing agents such as O-2, H2O2, and O-2(.-), and the glutathione transferase-catalyzed reaction can therefore serve a neuroprotective antioxidant function.