The protective effect of superoxide dismutase and catalase against formation of reactive oxygen species during reduction of cyclized norepinephrine ortho-quinone by DT-diaphorase.

Norepinephrine was oxidized by the Mn(3+)-pyrophosphate complex to the corresponding o-quinone at pH 6.5. Cyclized norepinephrine ortho-quinone showed an absorption maximum at 289 and 483 nm. No oxygen consumption was observed during oxidation of norepinephrine to o-quinone by Mn3+ and subsequent cyclization. The reduction of cyclized norepinephrine ortho-quinone to the corresponding hydroquinone was catalyzed by DT-diaphorase. However, the hydroquinone formed proved to be unstable in the presence of oxygen, since reduction of cyclized norepinephrine o-quinone by DT-diaphorase was accompanied by continuous oxidation of NADH and oxygen consumption. Addition of the chelator DETAPAC or SOD to the incubation mixture during reduction of cyclized norepinephrine ortho-quinone by DT-diaphorase strongly inhibited NADPH oxidation and oxygen consumption, suggesting that manganese and superoxide radicals were involved in hydroquinone autoxidation. Elimination of the effects of superoxide radicals, manganese and H2O2 on autoxidation of hydroquinone by addition of SOD, catalase and DETAPAC to the incubation mixture resulted in a 79% inhibition of NADH oxidation, suggesting that 21% of the autoxidation is oxygen-dependent. However, the effect of these additions on oxygen consumption was even more pronounced (93% inhibition).