Ubiquinol is considered to serve as an endogenous antioxidant. However, the mechanism by which the redox state of intracellular ubiquinone (UQ) is maintained is not well established. The effect of dicumarol, an inhibitor of NAD(P)H: quinone acceptor oxidoreductase 1 (NQO1 = DT-diaphorase, EC 1.6.99.2), on the reduction of UQ in cultured rat hepatocytes was investigated in order to clarify whether or not NQO1 is involved in reducing intracellular UQ. A concentration of 5 microM dicumarol, which does not inhibit cytosolic NADPH-dependent UQ reductase in vitro, was observed to almost completely inhibit NQO1 and thereby to stimulate cytotoxicity of 2-methyl-1,4-naphthoquinone (menadione) in cultured rat hepatocytes. However, 5 microM dicumarol did not inhibit reduction of endogenous UQ-9, as well as exogenous UQ-10 added to the hepatocytes. In addition, it did not stimulate the formation of thiobarbituric acid reactive substances (TBARS) in the hepatocytes. These results suggested that NQO1 is not involved in maintaining UQ in the reduced state in the intact liver cells.

Protective effect of the cellular ubiquinone (UQ) reducing system linked to cytosolic NADPH-dependent ubiquinone reductase (NADPH-UQ reductase) against hydrogen peroxide (H2O2)-induced lipid peroxidation was investigated using UQ and control hepatocytes freshly isolated from rats injected with UQ-10 and the vehicles 14 d in advance, respectively. The UQ hepatocytes had higher levels of ubiquinol (UQH2)-10 content and NADPH-UQ reductase activity than the control hepatocytes but did not differ in other antioxidant factors from the latter cells. The UQ hepatocytes exhibited higher cell viability and lower release of lactate dehydrogenase than the control hepatocytes when they were exposed to H2O2 of up to 100 mM for 1 h at 37 degrees C. Furthermore, the formation of thiobarbituric acid reactive substances (TBARS) by H2O2 was almost completely inhibited in the UQ hepatocytes. Decreases in UQH2 and alpha-tocopherol contents and NADPH-UQ reductase activity by H2O2 exposure were observed in both types of the hepatocytes, but those levels in the UQ hepatocytes after the exposure were still higher than in the control hepatocytes. The decreases in ascorbic acid, reduced glutathione and protein thiol contents and DT-diaphorase activity by H2O2 were not different between in the two types of hepatocytes. Antioxidant enzyme activities of catalase, superoxide dismutase, glutathione peroxidase, glutathione S-transferase and glutathione reductase in the hepatocytes were not inhibited by H2O2. From these results, it was concluded that the cellular UQ reducing system linked to cytosolic NADPH-UQ reductase functions mainly as an antioxidant defense for cellular membranes.

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