The reductive metabolism of diaziquone (AZQ) in the S9 fraction of MCF-7 cells: Free radical formation and NAD(P)H: Quinone-acceptor oxidoreductase (DT-diaphorase) activity

The S9 fraction of MCF-7 human breast carcinoma cells has NAD(P)H (quinone-acceptor) oxidoreductase activity as measured by the reduction of dichlorophenol-indophenol (DCPIP). This reduction is dependent on the activators Tween-20 and bovine serum albumin and it is inhibitable by dicumarol. The S9 fraction also has cytochrome c reductase activity which is approximately 29 times less than the two-electron reduction activity of NAD(P)H (quinone-acceptor) oxidoreductase. Diaziquone (AZQ) is a substrate for this NAD(P)H oxidoreductase active S9 fraction as judged by its enzymatic reduction detected spectrophotometrically and by electron spin resonance spectroscopy. Two-electron mediated enzymatic reduction of AZQ was evidenced by the formation of the colorless dihydroquinone (AZQH2) which could be followed at 340 nm. The production of the dihydroquinone was inhibitable by dicumarol implicating NAD(P)H oxidoreductase in its formation. Under aerobic conditions, electron spin resonance spectroscopy showed evidence for the production of AZQ semiquinone (AZQH) and oxygen radicals. Under anaerobic conditions no oxygen radicals were observed, but the semiquinone was stable for hours. These results are also inhibitable by dicumarol and suggest a two-step one-electron oxidation process of the dihydroquinone. The production of semiquinone and oxygen radicals as detected by electron spin resonance spectroscopy was more sensitive to dicumarol when NADPH was used as cofactor (68% inhibition of OH and 65% inhibition of AZQH) than when NADH was used (28% inhibition of OH and 5% inhibition of AZQH). This suggests that NADH flavin reductases play a more important role in the one-electron reduction pathway of AZQ in MCF-7 S9 fraction than NADPH reductases. The reduction of AZQ by NAD(P)H (quinone-acceptor) oxidoreductase may play an important role in the bioreductive alkylating properties of AZQ.