Systematic Oscillations in Tyrosine Transaminase and Other Metabolic Functions in Liver of Normal and Adrenalectomized Rats on Controlled Feeding Schedules

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Abstract
In an attempt to clarify the regulatory mechanism of synthesis and degradation of enzymes in mammalian cells in vivo, rats were fed 12, 30 and 60% protein diets and were adapted to 8 hours' feeding in either a 24-hour cycle or a 48-hour cycle under controlled lighting conditions. The rats were killed at various times in the cycle to determine any oscillatory changes in the activity of tyrosine transaminase, serine dehydratase, glucose 6-phosphate dehydrogenase and citrate cleavage enzyme in liver, compared with changes of glycogen deposition in liver and corticosterone level in plasma. Tyrosine transaminase shows increased activity 6 hours after the onset of food intake, and the increased activities are in proportion to the protein content of the diets. Direct proportionality of activity to protein level in the diet was also found for serine dehydratase and inverse proportionality for glucose 6-phosphate dehydrogenase and citrate cleavage enzyme. A secondary rise in tyrosine transaminase activity observed 16 hours into the fasting periods in rats adapted to 8 hours' feeding in 48-hour cycle is clearly dependent upon the protein content of the diet previously ingested, and is also dependent upon the availability of adrenal hormone. Glucose 6-phosphate dehydrogenase shows higher activity in rats adapted to 8 hours' feeding in a 48-hour cycle than in rats in a 24-hour cycle, and, on the contrary, serine dehydratase presents higher activity in the latter group of rats.