Objective:To investigate the antioxidant function of Mycoplasma pneumoniae MPN662 and analyze the key active sites, and to explore the role of MPN662 in the regulation of the production of reactive oxygen species (ROS) and superoxide dismutase (SOD) in THP-1 cells. Methods:pET28a(+ )- mpn662, recombinant mutant plasmids pET28a(+ )- mpn662-Ser 66 (the 66 th Cys was mutated to Ser) and pET28a(+ )- mpn662-Ala 66 (the 66 th Cys was mutated to Ala) were constructed, recombinant proteins rMPN662, rMPN662-Ser 66 and rMPN662-Ala 66 were expressed, identified, and purified. DTNB method was employed to analyze the MetO reduction activity of rMPN662 and recombinant mutant protein. Quantitative reverse transcription polymerase chain reaction (qRT-PCR) and Western blot were applied to examine the transcription level of the mpn662 gene and the expression level of MPN662 protein after Mycoplasma pneumoniae were stimulated with different concentrations of hydrogen peroxide (H 2O 2) or tert-butyl hydroperoxide (t-BHP), respectively. Fluorescent probes (DCFH-DA) and the total SOD activity detection kit were used to test the levels of intracellular ROS and SOD in THP-1 cells, which were pretreated with rMPN662, and then stimulated by Mycoplasma pneumoniae lipid-associated membrane proteins (LAMPs). Results:Mycoplasma pneumoniae rMPN662 could reduce MetO to Met, and the enzyme activities of mutant protein were significantly lower than those of rMPN662 protein. mpn662 gene mRNA transcription level and MPN662 protein expression level were significantly increased in a dose-dependent manner when Mycoplasma pneumoniae was stimulated with H 2O 2 and t-BHP. Treatment with rMPN662 before THP-1 cells were exposed to LAMPs could decrease the level of ROS and increase the production of SOD. Conclusions:Mycoplasma pneumoniae MPN662 can reduce MetO to Met, and Cys66 is the key amino acid for this activity. MPN662 can decrease the release of ROS and increase the production of SOD in Mycoplasma pneumoniae LAMPs stimulated THP-1 cells.