Exposure of proteins to oxygen and, especially to reduced oxygen compounds, such as hydrogen peroxide and hydroxyl radical, permits the oxidation of sensitive sites on the protein molecules. Of the sensitive sites, the sulfur of methionine is particularly subject to attack. An enzyme, methionine sulfoxide reductase, can reverse this oxidation and it has been shown to be a player in the continuing repair of oxidatively damaged proteins. Assays have shown that activity of this enzyme in Fisher 344 rats gradually, although not dramatically, declines with age. It has been shown, also, that the enzyme is widely distributed, being found in Escherichia coli, Saccharomyces cerevisiae, beef liver, and human cells (cell culture). The S. cerevisiae gene has been cloned and inserted in a plasmid which permits over-production of the enzyme. When the a mutant, lacking methionine sulfoxide reductase, is given the plasmid containing the gene, the phenotype resembles wild-type yeast. Like the enzyme activity in E. coli, the yeast activity is most vigorously expressed in late log and stationary phase cultures. This expression seems consistent with the idea that damaged methionines may be most prevalent and in need of repair when the cells have been exposed the longest to oxygen. The Km for protein-bound methionine sulfoxide (using the model compound, N-DABSYL-sulfoxide) is in the micromolar range: 15.3 for yeast, 31.8 for beef, and 120 for E. coli.