Photochemical and enzymatic redox transformations of reduced forms of coenzyme NADP+

The two reduced forms of NADP+, NADPH and its dimer (NADP)2, on irradiation in aqueous medium at 365 nm, are converted to enzymatically active NADP+, with accompanying formation of H2O2. The rate photooxidation of NADPH is strongly dependent on the presence of oxygen, but that of (NADP)2 is similar under aerobic and anaerobic conditions. In the presence of oxygen, but not in its absence, O2-. is an intermediate in the reaction. Generation of H2O2 under anaerobic conditions, confirmed by the fact that presence of peroxidase in irradiated solutions of (NADP)2 enhances photooxidation of the latter, is ascribed to attack on water of the excited dimer. Under anaerobic conditions at pH 9.5, Fe(EDTA)2+ and Fe(CN)4-(6) increase the rate of photooxidation of NADP dimer two-fold. gamma-Irradiation of (NADP)2 at pH 9.5 in the presence of N2O results in 80% conversion to enzymatically active NADP+. A mechanism for photooxidation of (NADP)2 under anaerobic conditions is suggested, and some relevant biological implications are presented.