Quinones and Glutathione Metabolism

This chapter reviews relevant methods used for the measurement of quinone redox cycling, glutathione (GSH) content, glutamate cysteine ligase (GCL) mRNAcontent, and γ-glutamyl transpeptidase (GGT), a second GSH-metabolizing enzyme that is also induced by H2O2. The metabolism of quinones and of GSH is remarkably intertwined. The chapter also describes how to measure several of the relevant parameters of those interactions by using recent studies. Nonetheless, the assays described in the chapter and the precautions in method and interpretation are generally applicable, regardless of these differences. Most quinones can be conjugated to GSH as their major route to elimination. When quinones are redox cycled, H2O2 is produced, the elimination of which depends on the use of GSH by glutathione peroxidase (GSHPx). Quinones are excellent inducers of the enzyme GCL, which catalyzes the rate-limiting step in GSH synthesis. Quinone reductases are identified in the plasma membrane, although their precise identities remain unresolved. Two-electron reduction of a quinone yields the corresponding hydroquinone, QH2. Several two-electron quinone reductases, including NQO2, are identified later in the chapter. Among them, NQO1 (NAD(P)H quinone oxidoreductase 1; DT-diaphorase), the prime cytosolic quinone reductase, is well-characterized.