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The activity of amino acid-activating enzymes in tissues from protein-depleted rats was tested by the PPi- P32⇄ ATP exchange reaction. Previously we had found that following protein depletion and in concomitance with a greater incorporation of amino acids, the activity of the liver amino acid-activating enzymes significantly increased. Presently the aim was to test the possible generalization of the above by investigating whether the activity of the amino acid-activating enzymes in various tissues of protein-depleted rats would vary in a manner analogous to the differences reported for amino acid incorporation in these tissues. The results, besides confirming that in protein depletion the liver amino acid-activating enzymes increase when tested in the presence of the complete mixture of amino acids or in the presence of individual amino acids, have also shown that in heart and gastrocnemius muscle the level of these enzymes is less than normal, in agreement with the decreased amino acid incorporation in these organs. A general hypothesis was thus set up that, in tissues from protein-depleted animals, the amount of protein synthesis is regulated through modifications of the level of amino acid-activating enzymes.