Figure 1.

Scheme of the antioxidant defense system created by cyclic oxidation and reduction of Met residues. Reduced forms of the proteins carry the subscript “red” and oxidized forms carry “ox.” Reading from left to right, ROS is intercepted by a Met residue, which is oxidized to MetO. MetO is reduced back to Met by msr, with the formation of a disulfide bond. The oxidized msr is reduced by thioredoxin, which now carries the disulfide bond. It is reduced by thioredoxin reductase, which, in mammals, contains a selenocysteine residue that is oxidized, forming a selenocysteine-cysteine bond. This disulfide analog is then reduced by NADPH. The net result is that ROS is reduced at the expense of NADPH.