Inhibition of xanthine oxidase by the aldehyde oxidase inhibitor raloxifene: implications for identifying molybdopterin nitrite reductases.

Sources of nitric oxide alternative to nitric oxide synthases are gaining significant traction as crucial mediators of vessel function under hypoxic inflammatory conditions. For example, capacity to catalyze the one electron reduction of nitrite (NO2-) to ·NO has been reported for hemoglobin, myoglobin and molybdopterin-containing enzymes including xanthine oxidoreductase (XOR) and aldehyde oxidase (AO). For XOR and AO, use of selective inhibition strategies is therefore crucial when attempting to assign relative contributions to nitrite-mediated ·NO formation in cells and tissue. To this end, XOR inhibition has been accomplished with application of classic pyrazolopyrimidine-based inhibitors allo/oxypurinol or the newly FDA-approved XOR-specific inhibitor, Uloric® (febuxostat). Likewise, raloxifene, an estrogen receptor antagonist, has been identified as a potent (Ki=1.0 nM) inhibitor of AO. Herein, we characterize the inhibition kinetics of raloxifene for XOR and describe the resultant effects on inhibiting XO-catalyzed ·NO formation. Exposure of purified XO to raloxifene (PBS, pH 7.4) resulted in a dose-dependent (12.5-100 μM) inhibition of xanthine oxidation to uric acid. Dixon plot analysis revealed a competitive inhibition process with a Ki=13 μM. This inhibitory process was more effective under acidic pH; similar to values encountered under hypoxic/inflammatory conditions. In addition, raloxifene also inhibited anoxic XO-catalyzed reduction of NO2- to NO (EC50=64 μM). In contrast to having no effect on XO-catalyzed uric acid production, the AO inhibitor menadione demonstrated potent inhibition of XO-catalyzed NO2- reduction (EC50=60 nM); somewhat similar to the XO-specific inhibitor, febuxostat (EC50=4 nM). Importantly, febuxostat was found to be a very poor inhibitor of human AO (EC50=613 μM) suggesting its usefulness for validating XO-dependent contributions to NO2- reduction in biological systems. Combined, these data indicate care should be taken when choosing inhibition strategies as well as inhibitor concentrations when assigning relative NO2- reductase activity of AO and XOR.