The circadian variation in glutathione S-transferase (GST) activity was studied in the hepatic cytosolic fraction of the male and female mouse. A circadian variation in GST activity towards 1-chloro-2,4-dinitrobenzene (CDNB) was observed in the male, the activity being higher in the light phase (07:00-19:00 h) than in the dark phase (19:00-07:00 h) during a day under normal lighting conditions. 2. The circadian variation was only existed from June to October. The difference between the lowest activity (at 01:00 h) and the highest activity (at 13:00 h) was maximum in August. 3. In both the normal and reversed light/dark cycle (lights on 07:00 and 19:00 h, respectively), reduced glutathione (GSH) content was lowest in the middle of the light period and highest in the middle of the dark period and GST activity toward 1,2-epoxy-3-(p-nitrophenoxy)propane (EPNP) exhibited opposite peaks and troughs. GST activities toward CDNB and 1,2-dichloro-4-nitrobenzene (DCNB) during the normal lighting schedule was higher at 13:00 h than at 01:00 h, but no differences were observed under reversed lighting conditions. 4. A circadian variation in GST activity for CDNB and DCNB was also observed in the female in a similar manner to the male, but the variation in the activity for EPNP was not observed in the female. 5. Thus, the circadian variation of hepatic GST activities in mouse were dependent on the enzyme substrates used, and seemed to be reflected by the difference in each isozyme levels. The daily change in the hepatic GSH levels is also thought involved, at least in part, in the regulation of GST activity.
