Effects of superoxide dismutase and catalase on catalysis of 6-hydroxydopamine and 6-aminodopamine autoxidation by iron and ascorbate

Abstract

The effects of superoxide dismutase and catalase on autoxidation of 6-hydroxydopamine and 6-aminodopamine in several chemical environments were studied. Inhibition by superoxide dismutase of autoxidation of 6-hydroxydopamine to its p-quinone required the presence of metal chelators, EDTA or diethylenetriamine pentaacetic acid (DETAPAC). A “lag” period in 6-hydroxydopamine autoxidation in the presence of superoxide dismutase couid be prevented by carrying out autoxidation in a mixture of 6-hydroxydopamine and its p-quinone, conditions in which adequate levels of the semiquinone are available for reaction with O 2. Catalase potentiated the inhibitory effect of superoxide dismutase in the presence of EDTA but had no effect in the presence of DETAPAC. Superoxide dismutase and catalase had no effect on the initial rate of 6-aminodopamine autoxidation to the p-quinone imine or on later intracyclization and polymer formation. Iron chelated by EDTA functioned as a catalyst in 6-hydroxydopamine autoxidation, making the reaction independent of superoxide as shown by the lack of effect of superoxide dismutase. the presence of iron-EDTA resulted in bleaching of the p-quinone product and the consumption of the H 2O 2 formed during autoxidation. Catalase had no effect on the rate of 6-hydroxydopamine autoxidation but completely prevented bleaching of the p-quinone, probably by preventing formation of hydroxyl radical by a Fenton reaction between iron-EDTA and H 20 2. Ethanol, which scavenges hydroxyl radical but not superoxide of H 2O 2, similarly prevented bleaching of the p-quinone with no other effects on autoxidation. Iron-EDTA also catalyzed 6-aminodopamine autoxidation with associated consumption of H 2O 2. Superoxide dismutase, catalase and ethanol had no effect on 6-aminodopamine autoxidation in the presence or absence of iron-EDTA, showing the independence of the kinetics of 6-aminodopamine autoxidation and polymerization from its products, superoxide, H 20 2 and hydroxyl radical. Chelation by DETAPAC prevented the effects of iron on 6-hydroxydopamine and 6-aminodopamine autoxidation. Autoxidation of 6-hydroxydopamine in the presence of ascorbate exhibited a lag phase followed by a linear phase of autoxidation. Superoxide dismutase, catalase and ethanol had no effect on 6-hydroxydopamine or 6-aminodopamine autoxidation in the presence of ascorbate. Autoxidation of 6-hydroxydopamine or 6-aminodopamine in the combined presence of iron-EDTA and ascorbate showed the full effects of both additions, except that 6-hydroxydopamine autoxidation exhibited no lag phase and the iron-catalyzed autoxidation of ascorbate occurred simultaneously with the 6-hydroxydopamine or 6-aminodopamine reactions.