A New Member of Human TSA/AhpC as Thioredoxin-dependent Thiol Peroxidase

A new type of the human TSA homologous gene was cloned from a HeLa cell cDNA and characterized. The gene product consists of 161 amino acids with a molecular mass of 16,900. The TSA homologous protein, as a new 6th member of the human TSA (hTSA VI), exerted a thiol-dependent peroxidase activity with the use of thioredoxin system as a physiological electron donor. The values of V_max/ K_m of hTSA VI for H₂O₂ and t-butyl hydroperoxide (t-BOOH) were calculated as 5.53×10^-2 and 3.70×10^-2, respectively. This implies that hTSA VI is a peroxidase, which reduces H₂O₂ and t-BOOH. The mutation of Cys⁴⁷ to serine resulted in a complete loss of the peroxidase activity. This suggests that Cys⁴⁷ acts as a primary site of catalysis. The analysis of the tryptic digest derived from hTSA VI revealed that the Cys⁴⁷ exists as a free thiol form. Taken together, these results suggest that the TSA homologous protein is a new type of the human family, which exerts thioredoxin-linked peroxidase activity toward H₂O₂ and alkyl hydroperoxide.