Amino Acid Activation in the Liver of Growing Rats Maintained with Normal and with Protein-deficient Diets

Abstract

The activity of the amino acid-activating enzymes was measured in the liver of growing rats maintained with a normal and with a protein-deficient diet. The determinations were based on the isotope exchange between ATP and PP labeled with 32P. The changes in the DNA content of the cell nucleus were also determined. Studies of changes in DNA content, and hence changes in ploidy during growth, indicated that in normal rats the activity of the amino acid-activating enzymes increased when expressed per average nucleus, whereas it remained constant and unaffected by growth when expressed per unit weight of DNA. On the contrary, in rats maintained with a protein-deficient diet, in which the arrest of growth causes an arrest of ploidy, the activity of the amino acid-activating enzymes increased both when expressed per average nucleus and per unit weight of DNA. We suggest that under these conditions a control mechanism is brought into action by changes in the amount of amino acids in circulation. This mechanism is independent of the increase in the DNA content of cell nucleus, and results, for growing rats maintained with a protein-deficient diet, in a preferential utilization of the amino acids for the synthesis of proteins.