Effect of quality and quantity of dietary protein on free amino acids in plasma and tissues of adult rats.

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Abstract
The nutritional significance of plasma free amino acids was examined by studying the effects of the quality and quantity of dietary protein on free amino acids in plasma and tissues of adult rats. The animals were given a diet containing 3, 6 or 10% egg protein or 3, 13 or 20% wheat gluten or rice protein for 4 wk. At the end of the 4 wk period, the rats were fasted for 12 h and the samples were prepared for the measurements of free amino acids in plasma and tissues. In rats on low-protein diets, the plasma levels of essential amino acids (EAA) decreased significantly, the reduction unexpectedly being greatest in rats on egg protein and least in those on wheat gluten. The levels of threonine, leucine and phenylalanine showed significant correlations with the dietary protein level, irrespective of the protein source. The plasma lysine levels in rats on rice and wheat gluten diets were maintained at almost the normal levels, irrespective of the protein level, although lysine is a limiting factor in these proteins. Of the nonessential amino acids (NEAA), serine and glycine in the plasma increased greatly in rats on low-protein diets, and then the ratio of EAA to NEAA (E/N) decreased significantly to about half (0.44-0.47) the values in rats on the highest protein diets, irrespective of the protein source. Findings in muscle were comparable to those in plasma. In liver, the ingestion of low-protein diets resulted in higher levels of serine and threonine, especially in groups on poor quality protein, while total EAA were not not affected by the dietary protein level. Unlike in the plasma and other tissues, in the small intestine tissue, free amino acids showed no consistent change in response to the dietary protein. This tissue evidently contributes to correcting the amino acid pattern of the ingested protein.