Induction of Tyrosine Aminotransferase and Depression of Protein Synthesis in Rat Liver by a Tryptophan-free Mixture of Amino Acids

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Abstract
Hepatic tyrosine aminotransferase activity was followed for 8 hours in protein-deprived rats force-fed one single meal of imbalanced mixtures of amino acids devoid either of tryptophan or methionine. It was compared with the levels obtained after intubation with starch, casein or a balanced mixture of amino acids. TAT activity was rapidly stimulated by all the aminated meals: within 4 hours stimulation was six-fold with the tryptophan-free mixture, fourfold with the methionine free, threefold with the complete mixture or with casein. No direct correlation was found between the levels of free tryptophan or leucine in the liver and the extent of TAT stimulation. Contrasting with the response of TAT, tryptophan pyrrolase was not affected by any of the treatments, except for some degree of inhibition after the tryptophan-free meal. Incorporation studies and polysome profiles showed that stimulation of enzyme activity was not dependent upon stimulation of protein synthesis. The tryptophan-free mixture in particular, although the most potent stimulator of TAT, induced a considerable drop in protein synthesis and in polysomal aggregation.

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