Effect of supplementation of methionine and threonine to a protein free diet on urinary excretion of nitrogen and tissue free amino acids in rats

Abstract

It was previously reported that methionine and threonine supplementation to a protein free diet had a greater nitrogen sparing effect than methionine supplementation alone. The reduced urinary nitrogen excretion by methionine and threonine is due to the reduced excretion of urea. In the present study, the changes in tissue level of free amino acids and ammonia were investigated. Ornithine is one the urea cycle intermediates and may play an important role for urea formation; hence the liver concentration of ornithine was also determined. The contents of ammonia in plasma were the same in these two groups and the ammonia content in liver of rats fed the control diet was smaller than that of rats fed the protein free diet supplemented with methionine and threonine. The contents of some amino acids in tissues were significantly reduced. The concentrations of aspartic acid, glutamic acid, alanine and serine in plasma and liver of rats fed the protein free diet supplemented with methionine and threonine were, particularly, significantly reduced as compared with those of rats fed the protein free diet. In addition to the decrease of free amino acids which are considered to be nonspecific nitrogen carriers from the peripheral tissues, the concentration of ornithine in liver was also significantly reduced. The results suggest that the decreased migration of free amino acids from peripheral tissues to liver and the decreased level of liver ornithine would efficiently reduce the urea formation in rats fed the protein free diet supplemented with methionine and threonine.