Effects of Fasting and Subsequent Feeding of a Complete or Tryptophan-Free Diet on the Activity of DNA-Synthesizing Enzymes and Protein Synthesis in Gastric Mucosa of Rats

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Abstract
Effects of fasting for 1-4 days, and subsequent feeding of 4-day fasted rats with a complete or trp-free liquid diet (40 kcal/day) for 1-4 days, on the activity of thymidine kinase and DNA polymerase and protein synthesis were investigated in gastric mucosa. Rats maintained on ad lib chow pellets served as control. Starvation for 1-4 days diminished gastric mucosal thymidine kinase and DNA polymerase activities, which attained their respective lowest levels of 53 and 65% of the fed control (chow-fed) after 2 days. Refeeding the complete diet for 1 day increased thymidine kinase activity to 87% of the fed control; it increased slightly thereafter. The trp-free diet was ineffective in stimulating gastric mucosal thymidine kinase for up to 3 days but, after 4 days on the diet, the enzyme activity was increased to 78% of the fed control level. DNA polymerase activity began to increase after 1 day on the typ-free diet and reached the fed control level after 4 days. The complete diet caused an initial reduction (for up to 2 days) in DNA polymerase activity, but after 3 days it rose to the level of the fed control. The ability of gastric mucosal polyribosomes to synthesize endogenous mRNA-directed protein in a cell-free system was decreased by 44% after 4 days of fasting. While refeeding of the complete diet for 4 days enhanced protein synthesis to 75% of the fed control level, the typ-free diet was ineffective. The protein to polyphenylalanine ratio, which represents a ratio of polysomes to monosomes, was decreased by 42% after 4 days of fasting, but could be returned to the fed control level by the complete diet and not by the trp-free diet. The presence of typ in the dietary mixture is important for initiation of cellular proliferative activity and for resumption of protein synthesis in the gastric mucosa of fasted rats.