One-electron reduction of excited NMN and NADP in the presence of amino acids

Excitation of oxidized forms of nicotinamide coenzymes (NADP⁺, NMN⁺) at 254 nm under anaerobic conditions in the presence of EDTA, lysine, serine, glycine leads, in the initial stage of irradiation, to photoreduction of coenzyme. Formation of the photoreduction products was observed by polarographic, spectrophotometric and enzymatic methods. Quantum yields for NMN⁺ and NADP⁺ photoreduction have been calculated and a mechanism proposed. No photoreduction was observed with histidine. Long-term irradiation leads to further reduction of the nicotinamide ring to tetrahydroderivatives absorbing at 280–290 nm. The photochemically generated dimers undergo phototransformation to the parent monomers on irradiation at 365 nm either in the presence or absence of oxygen. The biological significance of the results is discussed.