Essential Nature of Aromatic Amino Acids for Mammals

Essential Nature of Aromatic Amino Acids for Mammals 

For higher organisms, e.g., mammals, phenylalanine and tryptophan are so-called essential amino acids, i.e., they cannot be synthesized by the animal and must be supplied in the diet (for man, see 728, 729). Tyrosine is derived, as we shall see later, from phenylalanine, and is not therefore itself an essential amino acid as long as the phenylalanine intake is adequate (726, 950) if the conversion of phenylalanine to tyrosine is inhibited, as in phenylketonuria, tyrosine can become essential (65). The essential nature of the aromatic amino acids is a reflection of the general inability of higher organisms to synthesize the benzene ring. 

D-Amino acids vary in availability with the species. For example d-phenylalanine is used by rat, mouse, and man (15, 35, 727, 730, 962), whereas D-tryptophan is used by the rat (53, 54, 759, 895), is partially used by the mouse and pig (139, 867), and is not used by man (7, 29). The utilization of the D-amino acids is probably determined by the relative rates of absorption of the D-amino acid from the intestine, and of conversion of d- to L-amino acid in the liver (288). The conversion of d- to L-phenylalanine is reduced in vitamin-Be deficiency (52), as is to be expected for a transformation involving transamination to phenylpyruvic acid. Phenylpyruvic and indolepyruvic acids, the a-keto acids corresponding to phenylalanine and tryptophan, may also, to an extent varying with the species, satisfy growTh requirements (e.g., 55, 109, 436, 725, 911). 

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