Shifts in Thioredoxin Reductase Activity and Oxidant Status in Mononuclear Cells Obtained from Transition Dairy Cattle

Measures of oxidative status were examined in 14 dairy cows during the transition period. Blood samples were obtained approximately 21 d before expected calving, at calving, and again at 21 d in milk (DIM). Plasma samples were used to determine lipid hydroperoxide concentrations. Total white blood cells were used to determine the oxidative status of glutathione. Peripheral blood mononuclear cell (PBMC) lysates were used to determine the total antioxidant potential and enzymatic activities of glutathione peroxidase (GPX) and thioredoxin reductase (TrxR1). Both plasma lipid hydroperoxide concentrations and GPX activity in PBMC increased at calving and during the first 21 DIM when compared with prepartum samples. Conversely, the total antioxidant potential and TrxR activity declined in PBMC during the first 21 DIM, even though both GPX activity and the glutathione-to-GSSG ratio remained elevated during this time period. Results from this study support previous findings that report increased GPX activity when reactive oxygen metabolites, including lipid hydroperoxides, increase in transition dairy cows. The significant decrease in TrxR activity with a concomitant decrease in total antioxidant potential in PBMC during this same stage of lactation, however, would suggest that this selenoprotein is not able to rebound during periods of oxidative stress to the same extent as GPX1. This study shows for the first time that TrxR may be an important antioxidant defense mechanism in PBMC that is compromised during the periparturient period.