Aspects of Amino Acid Metabolism in Phenylketonuria and Other Amino Acidopathies

Abstract

This chapter describes mutual interactions between amino acids and the effects of amino acids on unrelated enzymatic reactions with main emphasis on phenylketonuria to indicate common principles for the reaction patterns of amino acids in the primary amino acidopathies, which are accompanied by mental retardation or mental symptoms. Primary amino acidopathies accompanied by mental retardation are characterized by the elevation of one or several plasma amino acids in contrast to primary amino acidopathies not accompanied by mental defects. A number of aromatic and aliphatic amino acids are capable of inhibiting the intestinal absorption of amino acids creating an imbalance of the plasma amino acid patterns, one or several amino acids being elevated and others being depressed. There is evidence that the total plasma amino acid content (nitrogen) is maintained by a regulatory mechanism even in the presence of an excess of one or several plasma amino acids. The amino acid uptake by the brain is also inhibited by an elevation of aromatic and aliphatic amino acids in the plasma. The increase of an amino acid in the bloodstream is reflected by a similar increase in the brain, although to a somewhat lesser extent than in the blood. A disturbance of brain amino acids and amines similar to that in phenylketonuria is likely to exist in other conditions accompanied by the elevations of plasma amino acids.