Methionine (Met) residues in proteins are susceptible to oxidation. The resulting methionine sulfoxide can be reduced back to methionine by methionine sulfoxide-S-reductase (MsrA). The MsrA gene, isolated from Caenorhabditis elegans, was cloned and expressed in Escherichia coli. The resultant enzyme was able to revert both free Met and Met in proteins in the presence of either NADPH or dithiothreitol (DTT). However, approximately seven times higher enzyme activity was observed in the presence of DTT than of NADPH. The enzyme had an absolute specificity for the reduction of l-methionine-S-sulfoxide but no specificity for the R isomer. K(m) and k(cat) values for the enzyme were approximately 1.18 mM and 3.64 min(-1), respectively. Other kinetics properties of the enzyme were also evaluated.