Abstract—The conversion of methionine to methionine sulfoxide (MetO) is one of the most common oxidative modifications in proteins due to the special susceptibility of methionine to oxidative conditions. Methionine oxidation can affect the protein structure and function, while the level of MetO increases with the development of oxidative stress. Most cells contain methionine sulfoxide reductases (MSRs), which catalyze a thioredoxin-dependent reduction of methionine sulfoxide to the original methionine. It was demonstrated that mutations leading to a decrease in MSR activity are associated with a decrease in the resistance of some cells to oxidative stress, while mutations leading to an overproduction of MSR activity result in an increase in resistance to oxidative stress. The redox reactions of methionines in the functional regulation of some intracellular proteins, actin, and calmodulin, are analyzed in the work, and the presence of antioxidant methionines in intracellular proteins, such as glutamine synthetase, 15-lipoxygenase, recombinant proteins, interferon α-2b, tissue plasminogen activator, and human stem cell factor, is discussed. The absence of MSR in the blood plasma makes the oxidation of methionines in the proteins irreversible; therefore, the ability of methionines to serve as interceptors of oxidant molecules without impairment of the function of plasma proteins is quite controversial. Antioxidant methionines were found in a number of proteins, such as macroglobulin, antithrombin III, and blood coagulation factor XIII. However, no antioxidant methionines were detected for most blood plasma proteins. There is a correlation between the oxidation of methionines and the development of pathological conditions in the organism.