Stoichiometric studies on the oxidation of tetrahydropterin with ferri-cytochrome c.

The oxidation of tetrahydropterin with ferri-cytochrome c was studied using a tetrahydropterin-generating system composed of dihydropteridine reductase [EC 1.6.99.7] and NADH. Under aerobic conditions, 1.5 to 1.8 mol of cytochrome c was reduced per mol of NADH, whereas 2 mol of cytochrome c was reduced under anaerobic conditions. When superoxide dismutase [EC 1.15.1.1] was added to the system under aerobic conditions, only 1 mol of cytochrome c was reduced per mol of NADH, while the pterin oxidation was scarcely affected. Based on these results, we propose that the oxidation of tetrahydropterin to quinonoid dihydropterin proceeds via two steps: tetrahydropterin is first oxidized by ferri-cytochrome c to give a pterin intermediate, which has lost one electron, then in turn this reduces O2 to form O2-.