Studies on the Circadian Rhythm of Phosphoenolpyruvate Carboxykinase Activity in Rats

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Abstract
Phosphoenolpyruvate carboxykinase [EC 4.1.1.32] activity in rat liver showed circadian variations with the highest activity in the evening (1600–1800 h) and the lowest activity in the morning (0700–1000 h). The circadian increase in enzyme activity was consistently observed through all seasons. The enzyme rhythm was found in rats of both sexes and with various body weights, though the amplitude varied with the season. The increase in enzyme activity in the evening was almost completely blocked by cycloheximide and was partially inhibited by actinomycin D. Neither adrenalectomy nor thyroidectomy affected the circadian rhythm. However, glucose was found to be an effective suppressor of the circadian increase in phosphoenolpyruvate carboxykinase activity, when given either by stomach tube or by intraperitioneal injection. Insulin was also effective in blocking the circadian increase in enzyme activity. The mechanism of the circadian rhythm of phosphoenolpyruvate carboxykinase activity is discussed with special reference to the relationships between glucose, insulin, and food intake.

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