The role of dietary protein in generating daily rhythms in rat liver tryptophan pyrrolase and tyrosine transaminase

Abstract

Groups of rats exposed to light for 12 hr daily were given access to an 18% casein diet or to a protein-free diet at different times of day or night, and killed 24 hr later. Tryptophan pyrrolase activity rose significantly during the dark period (by 34%–67%) in all animals. Tyrosine transaminase activity, however, rose during the dark period (by 163%) only among those eating protein. In rats given access to a protein-containing chow diet in the afternoon following an overnight fast and killed 2 or 4 hr later, tryptophan pyrrolase activity did not increase after food consumption, but tyrosine transaminase activity rose by 120%–140%. If animals consumed chow for 12 hr/day, starting either at the onset of darkness or 6 hr later, the daily increases in tryptophan pyrrolase began several hours before animals were given access to food rather than after food ingestion, while the daily increases in tyrosine transaminase were always noted immediately after eating. These observations confirm findings which indicate that the cyclic ingestion of dietary protein is of major importance in generating the tyrosine transaminase rhythm, but is of little significance in producing the tryptophan pyrrolase rhythm. Variations in plasma tryptophan concentration did not correlate well with tryptophan pyrrolase activity in any group fed a diet containing protein; plasma tryptophan concentrations did not rise prior to the increases in tryptophan pyrrolase activity, nor did they fall after these increases. Hence, the normal daily rhythm in tryptophan pyrrolase activity apparently neither results from nor elicits the plasma tryptophan rhythm.