ROLE OF CU ZN-SUPEROXIDE DISMUTASE IN XENOBIOTIC ACTIVATION .1. CHEMICAL-REACTIONS INVOLVED IN THE CU/ZN-SUPEROXIDE DISMUTASE-ACCELERATED OXIDATION OF THE BENZENE METABOLITE 1,4-HYDROQUINONE/

Cu/Zn-superoxide dismutase (Cu/Zn-SOD) has been shown to modulate the autoxidation of a variety of phenolic compounds, including 1,4-hydroqu inone (HQ), a benzene-derived metabolite. The acceleration of autoxida tion of HQ by Cu/Zn-SOD results in the production of 1,4-benzoquinone (BQ). It has been proposed that the chemical mechanism involved in the Cu/Zn-SOD-catalyzed autoxidation of HQ may be occur through either it s conventional activity as a superoxide:superoxide oxidoreductase or a s a semiquinone:superoxide oxidoreductase. However, which of the above two mechanisms is responsible for the Cu/Zn-SOD-accelerated oxidation of HQ has not been resolved experimentally. In this study, with ESR s pectroscopy we investigated further the chemical reactions involved in the SOD-accelerated oxidation of HQ. In phosphate-buffered sa line (P BS), HQ underwent a slow autoxidation to BQ, which was accelerated by Cu/Zn-SOD, Mn-SOD, or Fe-SOD with a similar efficiency. In contrast, a mong free metals, only Cu(ll) strongly mediated the oxidation of HQ to BQ. Mn(ll) exhibited a slight capacity to oxidize HQ, whereas neither Fe(ll) nor Fe(lll) was capable of modulating the autoxidation of HQ. The presence of either form of SOD also dramatically enhanced the form ation of semiquinone anion radicals (SQ(.-)) from HQ. The SOD-accelera ted oxidation of HQ was also accompanied by the generation of H2O2. In PBS containing bovine serum albumin (BSA) (PBS/ BSA), HQ did not unde rgo autoxidation to SQ(.-), and as such the presence of SOD was unable to induce the formation of either SQ(.-) or BQ or the consumption of O-2. The addition of 10 mu M BQ to HQ (100 or 1000 mu M) in PBS/BSA re sulted in the formation of SQ(.-) and initiated a slow rate of oxidati on of HQ to BQ. In this case, the presence of Cu/Zn-SOD strongly accel erated the oxidation of HQ to SQ(.-) and BQ and the utilization of O-2 . Furthermore, the enhancement by Cu/Zn-SOD of the generation of SQ(.- ) or BQ from HQ in PBS/BSA was extensively inhibited under anaerobic c onditions. The enhancement of SQ(.-) generation from HQ by all three f orms of SOD does not support the possibility that Cu/Zn-SOD can oxidiz e SQ(.-) to BQ. Taken together, this study demonstrates that unlike fr ee copper, Cu/Zn-SOD does not directly interact with HQ to cause its o xidation to BQ. Rather, the autoxidation of HQ to SQ(.-) is a prerequi site for the enhancing capacity of Cu/Zn-SOD, and the dismutation of s uperoxide anion radicals generated from the SQ(.-) in the presence of O-2 appears to be the underlying mechanism responsible for the enhance ment by Cu/Zn-SOD of the oxidation of HQ.