Radioprotective Effect of Ergothioneine on γ-Irradiation of Metmyoglobin : Comparison with Cysteine on Sulfmyoglobin-Formation

Abstract

The radioprotective effect of ergothioneine, 2-mercaptoimidazole, and 2-mercapto-1-methylimidazole, containing mercaptoimidazole ring as thione form, has been compared with that of the aminothiols such as cysteine and cysteamine. Irradiation of the brown metmyoglobin only causes the formation of the bright red myoglobin which has ferryl structure. The conversion is inhibited by the addition of a 10- and 30-fold molar excess of ergothioneine and cysteine to metmyoglobin, respectively. The addition of cysteine exceeding the radioprotective concentration forms the green color sulfmyoglobin, which is an abnormal blood pigment. Ergothioneine allows the spectral change of metmyoglobin only by γ-irradiation in the addition of a 1000-fold molar excess, but no sulfmyoglobin forms. The spectrum in the presence of ergothioneine shows simply the formation of oxymyoglobin. The mechanism for the formation of ferrylmyoglobin, oxymyoglobin and sulfmyoglobin is also discussed.