The mouse methionine sulfoxide reductase A (MsrA) belongs to the subclass of MsrAs with one catalytic and two recycling Cys corresponding to Cys51, Cys198 and Cys206 in Escherichia coli MsrA, respectively. It was previously shown that in the absence of thioredoxin, the mouse and the E. coli MsrAs, which reduce two mol of methionine-O substrate per mol of enzyme, displays an in vitro S-stereospecific methionine oxidase activity. In the present study carried out with E. coli MsrA, kinetic evidence are presented which show that formation of the second mol of Ac-L-Met-NHMe is rate-limiting in the absence of thioredoxin. In the presence of thioredoxin, the overall rate-limiting step is associated with the thioredoxin-recycling process. Kinetic arguments are presented which support the accumulation of the E. coli MsrA under Cys51 sulfenic acid state in the presence of Trx. Thus, the methionine oxidase activity could be operative in vivo without the action of a regulatory protein in order to block the action of Trx as previously proposed. (C) 2014 Elsevier Inc. All rights reserved.