Inhibition of platelet aggregation with glyceryl trinitrate and xanthine oxidoreductase

Xanthine oxidoreductase (XOR) is a mammalian enzyme that possesses a series of redox centers, which use either NAD(+) or molecular oxygen for oxidatio n of the purines xanthine and hypoxanthine to uric acid. The ability of XOR to act as an NADH oxidase is a less well recognized function of the enzyme , and it is this function that we used to explore the metabolism of glycery l trinitrate. The antiplatelet effect of nitric oxide (NO) on platelet aggr egation was used as a bioassay to assess the bioconversion of glyceryl trin itrate to NO by XOR. The thromboxane mimetic U46619, 2 mu M, was used to st imulate platelet aggregation in platelet-rich plasma prepared from healthy drug-free human volunteers. All incubations were carried out at 37 degrees C for 2 min after the addition of U46619. XOR produced a dose-dependent ant iaggregant effect when incubated with glyceryl trinitrate (GTN), 220 mu M. This did not occur when GTN or XOR was incubated with platelet-rich plasma independently. The antiaggregant effect of XOR plus GTN was dose dependentl y inhibited by allopurinol, with an IC50 of 100 mu M. The addition of super oxide dismutase (SOD), 100 U/ml produced a shift to the left in the antiagg regant dose-response curve for XOR. The IC50 for XOR at 200 U/l without SOD was decreased to 80 U/l with SOD. Oxyhemoglobin, an extracellular NO scave nger, produced a dose-dependent, noncompetitive inhibition of the antiaggre gant effect of XOR plus GTN. These findings suggest that GTN may be reduced to NO in vitro by the enzyme XOR in sufficient amounts to inhibit platelet aggregation.