Enhancement of platelet function by superoxide anion

Abstract

During the aerobic conversion of xanthine to uric acid by xanthine oxidase, superoxide anion and hydrogen peroxide are produced along with the hydroxyl radical. The studies demonstrate that washed human platelets incubated with xanthine and xanthine oxidase aggregated and released [C14] serotonin. Aggregation and release were dependent on the duration of exposure to xanthine oxidase as well as the concentration of enzyme. Both reactions were inhibited by the superoxide scavenger enzyme superoxide dismutase but not by catalase, or the free radical scavenger mannitol, suggesting that they were induced by superoxide anion. Superoxide dependent release was inhibited by prior incubation of platelets with 1 mM EDTA, 1 μM prostaglandin E1, or 1 mM dibutyryl cyclic AMP, but was unaffected by 1 mM acetylsalicylic acid or 1 μM indomethacin. After prolonged incubation with xanthine and xanthine oxidase there was also efflux of up to 15% of intraplatelet Cr51, a cytosol marker. This leakage was prevented by the addition of catalase to the media but not by superoxide dismutase. Incubation with xanthine and xanthine oxidase did not produce malonyldialdehyde, the three carbon fatty acid fragment produced during prostaglandin endoperoxide synthesis and lipid peroxidation. Prior exposure of platelets to low fluxes of superoxide anion lowered the threshold for release by subsequent addition of thrombin, suggesting a synergistic effect. The authors conclude that superoxide dependent aggregation and release may be a physiologically important method of modulating hemostatic reactions particularly in areas of inflammation or vessel injury which could have high local concentrations of superoxide anion.