The metabolism of antitumor quinones by NRH:quinone oxidoreductase 2 (NQO2) Free

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NADPH:quinone oxidoreductase 1 (NQO1) is expressed at high levels in many solid tumors and has been shown to play a role in the bioactivation of a number of quinone-based chemotherapeutic drugs including mitomycin C, RH-1, EO9, streptonigrin, β-lapachone and 17-AAG. Recently, NRH:quinone oxidoreductase 2 (NQO2) has emerged as a an alternative to NQO1 in the bioactivation of many antitumor quinones. Both NQO1 and NQO2 are homodimeric flavoproteins that reduce quinone substrates by a direct two-electron reduction to their corresponding hydroquinones. A major difference between these two enzymes is their capacity to utilize reduced pyridine nucleotides as co-factors. In contrast to NQO1, which can utilize either NADH or NADPH as reducing co-factors, NQO2 utilizes reduced nicotinamide riboside (NRH, a precursor in NADH biosynthesis) as the reducing cofactor. In this study we examined the ability of quinones to undergo NRH-dependent bioreduction by recombinant human NQO2. Results from these studies clearly demonstrated that NQO2 can reduce many of the same quinone substrates as NQO1. The metabolism of β-lapachone and streptonigrin by NQO2 generated redox unstable hydroquinones that spontaneously underwent autooxidation back to the parent quinones. HPLC analysis of the metabolism of benzoquinone ansamysins (geldanamycin, 17-AAG, 17-DMAG) by NQO2 demonstrated the formation of stable Hsp-90 inhibitory hydroquinones similar to what was observed for NQO1. The diaziridinylbenzoquinone RH-1 was an efficient substrate for NQO2. A notable difference in the metabolism of mitomycin C was observed between NQO1 and NQO2. Unlike NQO1, which carries out pH-dependent metabolism of mitomycin C, the metabolism of mitomycin C by NQO2 was robust at both acidic and basic pH. Indolequinone mechanism-based inhibitors of NQO1 including ES936 and MAC220 did not inactivate NQO2 at concentrations that completed inhibited NQO1 in both recombinant protein and cell-based assays. In fact the reduction of both ES936 and MAC220 by NQO2 resulted in significant release of p-nitrophenol indicating that the indolequinones were efficiently reduced by NQO2 without enzyme inhibition. Because of high levels of expression of NQO2 in normal human tissues such as the liver and blood, NQO2 may have a marked influence on the metabolism and distribution of chemotherapeutic quinones. In addition, the high levels of expression of NQO2 in leukemias suggest the possibility of effective quinone-based therapies for the treatment of this disease (supported by CA51210).