Singlet oxygen as a mediator in the hematoporphyrin-catalyzed photooxidation of NADPH to NADP+ in deuterium oxide.

The oxygen-dependent photooxidation of NADPH in the presence of hematoporphyrin in D2O results in the production of enzymatically active NADP+. The reaction is not inhibited by benzoate, mannitol, superoxide dismutase, or catalase. Moreover, addition of either potassium superoxide or H2O2 does not potentiate the reaction. This suggests OH-, H2O2, and O-2 are not likely to be the reactive oxygen species in this system. The oxidation is inhibited by various singlet oxygen quenchers and inhibitors such as 1,4-diazabicyclo[2.2.2]octane, 2,5-dimethylfuran plus methanol, histidine, and methionine. In addition, the rate of oxidation in H2O is less than one-fifth of that in D2O. The results suggest a singlet oxygen-mediated process. During the oxidation, no superoxide radical production could be detected with either ferricytochrome c or nitroblue tetrazolium. However, H2O2 has been found as one of the products. These observations are consistent with an oxidation-reduction reaction between singlet oxygen and NADPH to form H2O2 and NADP+, catalyzed by the light-activated photosensitizer hematoporphyrin.