Effect of Threonine-Induced Amino Acid Imbalance on the Distribution of Isotope from DL-Tryptophan-5-C14

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Abstract
After three weeks of ingestion of amino acid diets devoid of niacin, limiting in tryptophan and containing 0.6 or 0.8% of DL-threonine, rats were given by force-feeding two meals of the same diets in which the tryptophan was labeled with C14 in position -5 of the indole nucleus. During the 12-hour period following the first meal containing tryptophan-C14, the expired CO2 was collected and its radio-activity estimated. Urine and feces were also collected during this period. At the end of the 12-hour experimental period, the animals were decapitated and the liver, niacin and tryptophan were isolated with the aid of unlabeled carrier. The isotope analyses showed that the animals receiving 0.8% of DL-threonine from the beginning of the 21-day period excreted a much smaller percentage of the C14 in the urine than those animals that had received 0.6% of threonine during the three-week depletion period or those animals that received test meals containing 0.6% of threonine (non-growth-depressing). The percentage of isotope from the test meal accounted for in the fractions mentioned above was 6 to 7% in the deficient animals and 9 to 13% in the growing animals. The specific activities of the liver tryptophan and niacin were not significantly altered by the dietary threonine level. These results demonstrate that the animals, not growing because of a high threonine level, were capable of incorporating tryptophan into liver tryptophan and niacin. Their lack of growth is not the result of excessive oxidation of the tryptophan to CO2 or of wasting this amino acid by excretion as urinary products. The results were discussed in terms of hypotheses that have been presented to account for threonine imbalance.