The metabolism of 14C-labelled essential amino acids given by intragastric or intravenous infusion to rats on normal and protein-free diets

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Abstract
1. Leucine, lysine, valine and phenylalanine uniformly labelled with 14C were given by intragastric or intravenous infusion for 4 h, after an overnight fast, to rats which had been fed on normal or protein-free diets for 20 d. Expired CO2 was collected during the infusion. At the end of the infusion the rats were killed and radioactivity was measured in liver, muscle, skin, gastrointestinal tract and the remaining carcass. 2. There were no significant differences between the results obtained with different amino acids. 3. When the two routes of infusion were compared, there was no difference in the output of 14CO2, expressed as a proportion of the administered radioactivity. With intragastric infusion a slightly larger proportion of radioactivity was recovered from liver and gastrointestinal tract and significantly less from the skin. 4. When the rats adapted to a protein-free diet were compared with those on the control diet, there was again no difference in the proportion of dose oxidized to CO2. In the rats on the protein-free diet significantly more of the dose was recovered in the liver and gastrointestinal tract, and less in the skin. 5. The rats on the protein-free diet excreted more CO2 per kg body-weight, with a lower specific radioactivity. 6. The results show no indication of adaptive changes in the oxidation of essential amino acids, but the evidence is not conclusive. Under these experimental conditions the output of 14CO2 is not a valid measure of the extent of amino acid oxidation. The difficulties of interpretation are discussed.