Does the reduced form of neopterin serve as an antioxidant?

The oxygen free radical scavenging activity of the reduced form of neopterin, 5,6,7,8-tetrahydroneopterin (NPH-4), was investigated in vitro using electron paramagnetic resonance spectroscopy. In the hypoxanthine-xanthine oxidase (HPX-XOD) reaction system, NPH-4 suppressed the production of the superoxide spin adduct, but enhanced that of the hydroxyl radical spin adduct. The hydroxyl radical spin adduct was also observed in the solution containing NPH-4 and XOD but excluding HPX. These results indicate that NPH-4 is readily oxidized to some intermediates before the reaction with oxygen free radicals and that the substance which actually serves as an antioxidant in vivo is not NPH-4 but its metabolites.