Pro-Oxidant Effect of Uric Acid Metabolites in Inflammatory Cells

Uric acid is the final product of purine metabolism in humans and accumulates in plasma in the order of hundred micromolar. The one electron oxidation of uric acid by peroxidases generates urate free radical and urate hydroperoxide. Urate hydroperoxide is a strong oxidant and might contribute to oxidative imbalance in inflammatory states. Urate hydroperoxide efficiently reacts with glutathione and thiol proteins. In this study, we investigated the oxidative metabolism of uric acid in inflammatory cells and whether it contributes to an oxidative environment. Human leukemic (HL-60) were differentiated in neutrophils (dHL-60) and activated with phorbol myristate acetate (PMA, 100 ng/mL). The activation of cells greatly increased oxygen consumption and superoxide production. The pre-incubation with uric acid (0.05 – 0.5 mM) increased even further the oxygen consumption and superoxide production (51%). Conversely, uric acid (0.2 – 2 mM) significantly decreased hypochlorous acid (HOCl) production. The overall oxidative status of the cell was evaluated by GSH/GSSG ratio using LC-MS/MS. Uric acid significantly decreased GSH/GSSG ratio in activated dHL-60 cells, showing its pro-oxidant capability. Data by LC-MS/MS revealed that activated dHL-60 cells efficiently oxidized uric acid to urate hydroperoxide. This oxidation was dependent on the activity of myeloperoxidase and on the addition of superoxide to the urate free radical. These results show that uric acid increases superoxide but decreases HOCl formation in activated dHL-60 cells. The decrease in HOCl might be due to a competition between chloride and uric acid by myeloperoxidase catalysis. In agreement, uric acid was largely oxidized to urate hydroperoxide and significantly increased the oxidation of GSH to GSSG in these cells. Therefore, the oxidation of uric acid in inflammatory cells can lead to an oxidative environment and exacerbate tissue damage. Acknowledgment FAPESP, INCT/NAP/CEPID-Redoxoma