Circadian rhythms in enzymatic activity of rat liver arginase and glucose 6-phosphate dehydrogenase

The circadian rhythms of glucose 6-phosphate dehydrogenase (G6PD) and of arginase activities and total protein content have been studied in the livers of 24-h fasted rats. Both G6PD and arginase activities reach a maximum at night and a minimum during the light period. On the other hand, the total protein level was maximal during the light period while it decreased to its lowest level during the dark period. These results are in agreement with the existence of a lipogenesis-lipolysis circadian rhythm in the rat, since the higher G6PD activity at night provides the necessary NADPH for lipid biosynthesis. The increase in arginase activity is also in agreement with an increase in amino-acid catabolism, probably as a source of energy and metabolic intermediates.