The Role of Ergothioneine in the Oxidation of Reduced Nicotinamide Adenine Dinucleotide by Metmyoglobin or Methemoglobin

In phosphate buffer (pH 6.8), reduced nicotinamide adenine dinucleotide (NADH) was oxidized by metmyoglobin or methemoglobin in the presence of Mn (II) ion and ergothioneine. The NADH oxidation was dependent on the concentrations of Mn (II) ion, ergothioneine and metmyoglobin (or methemoglobin). Various phosphorus compounds also influenced the oxidation. Orthophosphate was the most effective at concentrations or more than 50 mM. In contrast, nucleotides such as adenosine-5'-triphosphate (ATP), adenosine 5'-diphosphate (ADP) and adenosine-5'-monophosphate (AMP) were less effective. Superoxide dismutase or catalase inhibited the present NADH oxidation, suggesting the participation of superoxide anion (O-2) and H2O2 in this metmyoglobin system as well as the peroxidase system. Metmyoglobin pretreated with H2O2, i.e., ferrylmyoglobin, promoted the NADH oxidation reaction, but oxymyoglobin inhibited it. A mechanism is proposed for the oxidation of NADH by metmyoglobin or methemoglobin on the basis of the redox potential of ergothioneine, the formation of Mn-phosphate complex and the heme state of metmyoglobin or methemoglobin.