Effect of Excessive Levels of Lysine and Threonine on the Metabolism of These Amino Acids in Rats

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Abstract
The metabolic fate of lysine and threonine in rats fed either excessive lysine or threonine was studied. Excessive feeding of the two amino acids exhibited little effect on the growth of rats. However, the content of free threonine in plasma, liver and muscle of rats was found to increase markedly due to the feeding of the excess threonine diet, whereas the lysine content in the tissues remained relatively constant when the excess lysine diet was fed. The higher level of free threonine in rat tissues did not affect the protein synthesis in rat body. The catabolic breakdown of threonine-U-14C to respiratory CO2 in rats fed the excess threonine diet did not increase as much as that of lysine-U-14C in rats fed the excess lysine diet. The excessive feeding of each respective amino acid resulted in a lowered radioactivity recovery in the protein fraction in contrast to the elevated recovery in the acid-soluble fraction, the effect of the excess threonine diet being more marked than that of the excess lysine diet. Results of the radioactivity measurement of the metabolic transformation of intraperitoneally administered lysine-U-14C and threonine-U-14C to a) blood sugar, b) liver glycogen, c) liver lipids and d) carcass lipids showed that the conversion of threonine-U-14C to carbohydrate greatly increased in rats fed the excess threonine diet. The overall findings in the present investigation further strengthen our previous view concerning a possible homeostatic mechanism operative in rats which maintains a constant body level of lysine and the absence of such a mechanism to maintain threonine levels which fluctuate depending upon the different feeding conditions.

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