Vol. 27, Issue 1, 92-97, January 1999

A Unique Tertiary Amine N-Oxide Reduction System Composed of Quinone Reductase and Heme in Rat Liver Preparations

Shigeyuki Kitamura, Kazumi Sugihara, and Kiyoshi Tatsumi

Institute of Pharmaceutical Science, Hiroshima University School of Medicine, Kasumi 1-2-3, Minami-ku, Hiroshima 734-8551, Japan

The results of this study show the quinone-dependent reduction of tertiary amine N-oxides to the corresponding tertiary amines by rat liver preparations. The reduction of imipramine N-oxide to imipramine mediated by liver mitochondria, microsomes, and cytosol proceeded in the presence of both NAD(P)H and menadione under anaerobic conditions. When menadione was replaced with 1,4-naphthoquinone or 9,10-anthraquinone, similar results were obtained in the cytosolic reduction. The quinone-dependent reducing activity in liver cytosol was inhibited by dicumarol and carbon monoxide. This result suggested that the activity is caused by DT-diaphorase, a cytosolic quinone reductase, and hemoproteins in liver cytosol. In fact, catalase and hemoglobin showed the ability to reduce imipramine N-oxide when supplemented with DT-diaphorase. The hemoproteins also exhibited the N-oxide reductase activity with reduced menadione, menadiol. The N-oxide reductase activity of the hemoproteins was also exhibited with 1,4-dihydroxynaphthalene, 1,4,9,10-tetrahydroxyanthracene, or 1,4-dihydroxy-9,10-anthraquinone. Furthermore, hematin revealed a significant N-oxide-reducing activity in the presence of menadiol. The reduction appears to proceed in two steps. The first step is reduction of menadione to menadiol by a quinone reductase with NADPH or NADH. The second step is nonenzymatic reduction of tertiary amine N-oxides to tertiary amines by menadiol, catalyzed by the heme group of hemoproteins. Cyclobenzaprine N-oxide and brucine N-oxide were also transformed similarly to the corresponding amine by the quinone-dependent reducing system.