Circadian variations in glutathione-S-transferase and glutathione peroxidase activities in the mouse

Circadian variations in hepatic glutathione-S-transferase and glutathione peroxidase activities were found in male Swiss-Webster mice maintained under a 12/12 lighting schedule (L: 06.00-18.00 h) for 3 weeks prior to use. Hepatic glutathione-S-transferase activity was significantly elevated during the dark phase as compared to the light phase. The glutathione peroxidase activity, using two different substrates, was biphasic in nature. Using cumene hydroperoxide, the enzyme activity was significantly elevated at 18.00 and 06.00 h; nadirs occurred at 10.00, 14.00 and 22.00 h. Using hydrogen peroxide as the substrate, enzyme activity was greatest at 14.00 and 06.00 h; nadirs occurred at 10.00 and 22.00 h. Glutathione concentration also varied in a circadian manner with peak levels occurring between 06.00 and 10.00 h and the nadir occurring at 18.00 h. These studies have shown that glutathione levels and two enzymes utilizing glutathione as a cosubstrate vary in a similar circadian manner, with the highest values primarily occurring during the dark phase. Thus, these results may have considerable toxicological importance.