Histidine endoperoxides

Figure 2.9 Chemical quenching of singlet oxygen, (a) The ene reaction — addition of singlet oxygen to an olefin with allylic hydrogen (b) the ene reaction of cholesterol (c) endoperoxide formation by singlet oxygen to imidazole residue as in histidine.

Oxidation of proteins. ROS can directly oxidize free or protein-bound amino acids, leading to deactivation of enzymes (Stadtman, 1990 Stadtman and Oliver, 1991 Stadtman and Berlett, 1991). Cysteine, methionine, histidine and tryptophan are preferentially oxidized, resulting in sulfenic, sulfinic or sulfonic acids from thio-containing amino acids and in histidine- and tryptophan-endoperoxides, which subsequently degrade (Sies,... 

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