Twenty-one monkeys continuously fed one of rive diets high in tyrosine, histidine, alanine, glycine, or tryptophan between the ages of 1 and 12 months were compared with 20 controls. Social behavior either (a) while on the diet and tested in familiar groups of four or (b) while on a normal diet and paked with unfamiliar monkeys showed no effects of the high amino acid diets, confmning a previous analysis of learning ability.
Altered metabolism of several of the amino acids has been implicated in human pathology. In most enzymatic disorders of amino acid metabolism, the amino acid and its metabolites build up in the blood and urine. It has been shown that it is possible to experimentally increase these levels by feeding high levels of the amino acid, and that in some cases, such as with phenylalanine and related compounds (Meier, 1973), it is possible to mimic some of the inborn errors of metabolism. Some (Bessman, 1979) have suggested that even the nonessential amino acids, such as tyrosine, have their essential roles and one might predict effects resulting from alteration of their levels. This has not been tried before using the monkey.
Tyrosinosis is associated with abnormalities in tyrosine metabolism but has not been shown to be concomitant with mental retardation or other behavioral symptoms. Nevertheless, high levels of urinary tyrosine are seen in phenylketonuria, and there is conflicting opinion as to its importance (Bessman, 1979;Vorhees, Butcher, & Berry, 1981). Children with abnormal urinary glycine levels, hyperglycinuria, are severely retarded and characteristically self-destructive. High levels of dietary glycine can also lead to neurologic changes in adults (Dodge, Prensky, & Feigin, 1975). Similar disorders have not been associated with alterations in the structurally small amino acid, alanine (Frazier, Summer, & Chamberlin, 1978; Waisman & Gerritsen, 1964).
Perhaps the best known of the amino acidurias, aside from phenylketonuria, is Hartnup's disease. A low circulating level of plasma tryptophan, although not associated with mental retardation, is associated with emotional instability extending to delirium (Dodge et al., 1975). Children with histidine abnormalities, histidinemia, are sometimes mentally retarded and grow normally (Menkes