Physiological Regulation of the Circadian Rhythm in Hepatic Tyrosine Transaminase: Schedule of Protein Ingestion as a Determinant Factor

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Abstract
The rhythm in hepatic tyrosine α-ketoglutarate aminotransferase was followed in rats fed protein as a separate meal at various times in the day, while a protein-free diet was provided ad libitum. It was shown that the ingestion of the protein meal was, in all cases, immediately followed by a rise in tyrosine transaminase (TAT) independent of the time of administration. This rise was not mediated by an increase in circulating levels of insulin. Feeding the protein meal at 09:00 hours or 23:00 hours resulted in a biphasic rhythm indicative of a second factor of induction acting at dusk distinct from protein feeding, and probably related to the change of day and night. The absence of a biphasic rhythm when the protein was fed at 05:00 hours was interpreted as resulting from a certain degree of inhibition following induction by protein. However, the enzyme was still inducible by protein during this period. It was concluded that under normal physiological conditions of mixed diet feeding, dietary proteins act to sustain an elevation in TAT, initially triggered by factors related to the cycle of light and dark. Under conditions of separate feeding, the two types of action can be discriminated.