Lysine Metabolism in Rats Fed Lysine-free Diet

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Abstract
The specificity in the metabolic fate of lysine and threonine was studied in rats fed either a lysine-free or a threonine-free diet. It was found that rats fed the lysine-free diet ad libitum lost less body weight than those fed the threonine-free diet. The intake of the lysine-free diet was considerably greater than of the threonine-free diet. By determining the content of free lysine and threonine in plasma, liver and muscle, the level of threonine in all of these tissues in rats fed the threonine-free diet was found to decline markedly. The reduction of free-lysine content in rats fed the lysine-free diet, however, was not so great. By examining the metabolic transformation of lysine-U-14C in rats fed the lysine-deficient diet, and of threonine-U-14C in rats fed the threonine-deficient diet, a notable difference was observed between the two rat groups. The percentage recovery of 14C from lysine-U-14C in a) respiratory CO2, b) blood sugar, c) liver glycogen, d) liver lipids and e) carcass lipids in rats fed the lysine-free diet decreased considerably, whereas that of threonine-U-14C in these fractions of rats fed the threonine-free group did not alter much compared with the controls. The ratio of the percentage recovery of 14C in the protein fraction to that in the respiratory CO2 was higher in the lysine-free diet-fed groups than in the threonine-free diet groups. Overall results indicate that in rats fed the lysine-free diet there may operate a certain homeostatic mechanism to maintain the level of lysine in the body compared with that of threonine in the animal body on the threonine-free diet.