Tryptophan Metabolism of Rats Fed a Threonine-free Amino Acid Diet

Abstract

This study was conducted to clarify metabolic changes in rats given an amino acid diet devoid of one essential amino acid from the complete amino acid diet. At the beginning of the study, rats were force-fed the threonine- or tryptophan-free diet and the complete amino acid diet to observe the body weight changes, the urinary nitrogen and N1-methylnicotinamide (MNA), and changes in the liver enzyme activities, tryptophan pyrrolase (TPase) and tyrosine-α-ketoglutarate transaminase (TKase), and the like. It was observed that the excretion of the urinary MNA of rats force-fed a threonine-free diet was higher than that of animals fed the complete amino acid diet, and that increase of the liver TPase activity three hours after feeding was larger in the latter. It was presumed from the result that tryptophan in the diet, which is normally used for protein synthesis, was converted to nicotinic acid when protein formation was limited by the omission of threonine, and excreted in urine as MNA. There was also a remarkable increase of TKase activity 3 hours after feeding either the threonine-free diet or the complete amino acid diet, but a greater increase was observed with the deficient than with the complete diet. The mechanism of the induction of TPase and TKase activities after force-feeding of the different patterns of amino acid diets is discussed.