Liver Polysome Profiles and Protein Synthesis in Rats Fed a Threonine-imbalanced Diet

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Abstract
Liver polysome aggregation and protein synthesis in vivo were studied in rats fed a threonine-imbalanced diet. In one experiment, rats trained to eat their food in 2 hours daily were offered one meal of either a basal, a threonine-imbalanced, or a corrected diet. The basal (B) diet contained 6% casein supplemented with 0.3% of L-methionine; the imbalanced (I) diet was the same as B but with 10% of an indispensable amino acid mixture devoid of threonine substituted for part of the carbohydrate; the composition of the corrected (C) diet was the same as I with the addition of 0.5% L-threonine also at the expense of carbohydrate. When the tissue threonine pool had previously been depleted by feeding rats a diet deficient in threonine, polysome profiles and rates of 14C-leucine incorporation into liver protein were similar for rats fed either the basal or the imbalanced diet. However, when the tissue threonine pool had not been depleted prior to feeding of the experimental diets, ingestion of the imbalanced diet stimulated polysome aggregation and 14C-leucine incorporation into protein more than did ingestion of the basal diet. In both conditions, rats fed the corrected diet had the highest polysome aggregation and 14C-leucine incorporation into protein. In another experiment, rats were fed ad libitum for 1 week either the basal, the imbalanced, or the corrected diet. Plasma and liver amino acid analyses showed that in the imbalanced group, concentration of free threonine was much depressed, while concentrations of other indispensable amino acids were elevated compared with that of the basal group. Hepatic ribosomal preparations from the imbalanced group showed a highly disaggregated pattern. However, if tissue threonine concentration was normalized by increasing food intake through force-feeding, liver polysome pattern of rats receiving the imbalanced diet was more aggregated than that of the group receiving the same diet ad libitum. These experiments indicate that polysome aggregation and protein synthesis are influenced by the concentration of threonine when it is the most limiting amino acid in the diet and that changes in the responses of these variables to ingestion of a meal of an imbalanced diet can be influenced by the prior nutritional state of the animal.

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