Knockout of SOD1 promotes conversion of selenocysteine to dehydroalanine in murine hepatic GPX1 protein

Se-dependent glutathione peroxidase-1 (GPX1) and Cu,Zn-superoxide dismutase (SOD1) are two major intracellular antioxidant enzymes. The purpose of this study was to elucidate the biochemical mechanisms for the 40% loss of hepatic GPX1 activity in SOD1−/− mice. Compared with the wild type (WT), the SOD1−/− mice showed no change in the total amount of GPX1 protein. However, their total enzyme protein exhibited 31 and 38% decreases (P < 0.05) in the apparent kcat for hydrogen peroxide and tert-butylperoxide (at 2 mM GSH), respectively. Most striking, mass spectrometry revealed two chemical forms of the 47th residue of GPX1: the projected native selenocysteine (Sec) and the Se-lacking dehydroalanine (DHA). The hepatic GPX1 protein of the SOD1−/− mice contained 38% less Sec and 77% more DHA than that of WT and showed aggravated dissociation of the tetramer structure. In conclusion, knockout of SOD1 elevated the conversion of Sec to DHA in the active site of hepatic GPX1, leading to proportional decreases in the apparent kcat and activity of the enzyme protein as a whole. Our data reveal a structural and kinetic mechanism for the in vivo functional dependence of GPX1 on SOD1 in mammals and provide a novel mass spectrometric method for the assay of oxidative modification of the GPX1 protein.