1. The pathogenesis of the mental retardation in phenylketonuria remains obscure. Leucocytes have proved of value in the study of other inborn errors of metabolism. The lymphocyte is a suitable model cell for the study of mammalian metabolism, because of its ability to divide in vitro in response to various stimuli.

2. We have examined the effects of phenylalanine, phenylpyruvate, phenyl-lactate and phenylacetate on the human leucocyte and the resting and phyto-haemagglutinin-stimulated rabbit lymphocyte.

3. Phenylpyruvate and phenyl-lactate reduced acetate incorporation into leucocyte lipid by 38% and 48% respectively. Only phenyl-lactate reduced acetate incorporation into the resting and stimulated lymphocyte, by 20% and 34% respectively.

4. Glucose incorporation into leucocyte lipid was unaffected by phenylalanine, phenylpyruvate and phenyl-lactate. Only phenyl-lactate inhibited (46%) the production of CO2 from glucose.

5. Phenylalanine and leucine incorporation into trichloroacetic acid-insoluble material of resting and stimulated lymphocytes was inhibited by phenyl-lactate (10–42%), phenylpyruvate (27–57%) and phenylacetate (19–39%).

6. Uridine incorporation into resting and stimulated cells was inhibited by phenyl-lactate (22–26%), phenylpyruvate (42–52%) and phenylacetate (20%).

7. Thymidine incorporation into resting lymphocytes was reduced by phenyl-lactate, phenylpyruvate, phenylacetate and phenylalanine by 12–26%. Incorporation into the stimulated cell was inhibited by phenylpyruvate and phenyl-lactate (90%) and phenylacetate (66%).

8. Phenylalanine inhibited lymphocyte pyruvate kinase and phenylpyruvate inhibited citrate synthetase.

9. These results are compared with published data relating to experimental hyperphenylalaninaemia and the effects of these metabolites on nervous tissue in vitro.

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