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The Nutritive Value and Safety of D-Phenylalanine and D-Tyrosine in Mice

Western Regional Research Center, Agricultural Research Service, U.S. Department of Agriculture, 800 Buchanan Street, Berkeley, CA 94710

Growth studies in mice fed synthetic amino acid diets showed that: 1) the biological utilization of D-phenylalanine relative to its L-isomer ranged from 28–81%, depending on the respective concentrations of these amino acids in the diet; 2) L-tyrosine can replace about one-half of the L-phenylalanine needed to achieve maximum weight gain and 3) D-tyrosine has no similar sparing effect on L-phenylalanine. In fact, addition of D-tyrosine to amino acid or casein diets depressed weight gain to 10% that of the control. This inhibition was significantly reduced by increasing the L-phenylalanine content of the amino acid diets and the protein content of the casein diets. Growth inhibition, therefore, appears to be related to the ratio of D-tyrosine to L-phenylalanine and possibly other amino acids in the diet, rather than to the absolute levels of D-tyrosine. Mechanistic rationalizations are offered to account for the toxic or antinutritional manifestation of D-tyrosine.

KEY WORDS: • D-phenylalanine • D-tyrosine • nutritive value • growth inhibition

Manuscript received 12 December 1983.