Oxidation of Hydroquinone by Copper: Chemical Mechanism and Biological Effects

Abstract

Exposure of humans and experimental animals to benzene has been shown to result in hematotoxicity such as pancytopenia, aplastic anemia, and leukemia. The oxidative activation of the benzene metabolite, hydroquinone (HQ), in the bone marrow to the electrophilic benzoquinone (BQ) has been suggested to play an important role in benzene-induced hematotoxicity. Since the interaction of several xenobiotics with copper has been shown to result in their metabolism, in this study we have investigated the role of copper in the oxidation of HQ and HQ-induced toxicity to mice hone marrow stromal cells, target cells of HQ in the bone marrow. In phosphate-buffered saline, HQ underwent autoxidation slowly to BQ, while the presence of Cu(II) ions (1, 2.5, 5, 10, 50 μM) strongly accelerated the oxidation of HQ to BQ in a concentration-dependent manner. Reaction of HQ with Cu(II) was also accompanied by the reduction of Cu(II) to Cu(I), the utilization of O₂, and the concomitant generation of H₂O₂. The oxidation of HQ by Cu(II) could be blocked by the Cu(I)-specific chelator bathocuproinedisulfonic acid (ECS), particularly when the ratio of BCS to Cu(II) was 4:1. By observing the kinetics of the reactions derived from mixing 100 μM HQ and 100 μM Cu(II), it was found that all of the Cu(II) was reduced to Cu(I) within 5 s, followed by consumption of O₂ and the generation of BQ, which reached maximum levels at 4 min after mixing HQ and Cu(II). In addition, oxidation of HQ by Cu(II) also generated chemiluminescence. In the presence of myeloperoxidase, Cu(II)-mediated oxidation of HQ was increased. Addition of Cu(II) to primary bone marrow stromal cell cultures significantly enhanced HQ-induced cytotoxicity. The enhanced cytotoxicity of HQ by Cu(II) could be completely prevented by adding BCS, glutathione (GSH), or dithiothreitol but not by catalase. Supplementation of stromal cells with 20 μM BCS in the absence of exogenously added Cu(II) significantly abated HQ-induced cellular GSH depletion and cytotoxicity, suggesting a possible involvement of endogenous copper in the activation of HQ. The above results indicate that Cu(II) strongly induces the oxidation of HQ and as such may be a factor involved in the oxidative activation and toxicity of HQ in target cells.