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Effect of an Amino Acid Imbalance on the Metabolism of the Most-limiting Amino Acid in the Rat¹

Departments of Meat and Animal Science and Biochemistry, University of Wisconsin, Madison; Department of Physiological Sciences, University of California, Davis; and the Department of Nutrition and Food Science, Massachusetts Institute of Technology, Cambridge, Massachusetts

Rats trained to consume their entire daily ration in 1.5 hours were used to study the effect of an amino acid imbalance on the uptake of the limiting amino acid into the protein and protein-free portions of a number of tissues. They were fed 7.5 g of one of the following 3 diets containing L-histidine-U-¹⁴C: a 6% casein basal diet (B); and imbalanced diet (I) consisting of B plus an indispensable amino acid mixture devoid of histidine; or a corrected diet (C) consisting of diet I plus histidine. Eight and 12 hours after the animals were fed, samples of brain, liver, stomach, small intestine, large intestine plus cecum, muscle and portal and heart blood were obtained. After adjustment is made for dilution of histidine-U-¹⁴C by the additional histidine in the corrected diet, the amounts of radioactivity in acid extracts of tissues and plasma from the corrected group (C) were highest followed in order by those from the basal (B) and the imbalanced groups (I). The specific activity of liver protein from the corrected group was also highest followed in order by that from the imbalanced and the basal groups. The results (basal vs. imbalance) suggest that amino acids added to this diet to create the imbalance induced the liver to make more protein and thus increased the efficiency of utilization of histidine by this organ. The relative deficiency of histidine in the imbalanced diet (imbalance vs. corrected) limits the ability of the animal fed this diet to synthesize liver protein. Rats fed the imbalanced diet for 1.5 hours/day did not exhibit the typical growth depression observed in animals fed this type of diet ad libitum.

Manuscript received 30 December 1967.

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