Absorption and metabolism of amino acids with special emphasis on phenylalanine

The protein and essential amino acid content of the average American diet is reviewed; in the case of phenylalanine, average daily intake is 4.7 g, nearly five times greater than daily requirements, including those for tyrosine synthesis. Dietary protein is mostly absorbed as free amino acids and part as small peptides. Proteins from the digestive juices and from shed intestinal cells provide more protein for absorption, to give a total of some 150 g amino acids absorbed per day (probable phenylalanine content, 7.5 g). After a meal containing protein, the liver monitors access of the incoming amino acids to the systemic blood. For most essential amino acids, hepatic degradation is regulated in relation to adequacy of intake and rises sharply when intake exceeds requirements. In spite of this protection, the level of a given essential amino acid in the systemic plasma increases progressively when its intake exceeds requirements. The point at which plasma amino acids start to rise has been used to determine dietary requirements. Phenylalanine absorbed from the diet Is distributed to the tissues for synthesis to body proteins. The excess is catabolized by hydroxylation to tyrosine and is then further metabolized. This pathway of phenylalanine catabolism is dependent upon a hydroxylase located in the liver. In the disease phenylketonuria (PKU), this enzyme is inactive; as a consequence, elevated levels of phenylalanine can be observed shortly after birth. The main pathway of catabolism in such patients is through formation of phenylpyruvic acid. However, because of the ease of the conversion of phenylalanine to tyrosine in normal subjects, it is difficult to elevate plasma phenylalanine levels by administration of phenylalanine.