Formation, nutritional value, and safety of D-amino acids

The extent of racemization of L-amino acid residues to D-isomers in food proteins increases with pH, time, and temperature. The nutritional utilization of different D-amino acids vary widely, both in animals and humans. In addition, some D-amino acids may be deleterious. For example, although D-phenylalanine is nutritionally available as a source of L-phenylalanine, high concentrations of D-tyrosine inhibit the growth of mice. The antimetabolic effect of D-tyrosine can be minimized by increasing the L-phenylalanine content of the diet. Similarly, L-cysteine has a sparing effect on L-methionine when fed to mice; however, D-cysteine does not. The wide variation in the utilization of D-amino acids is exemplified by the fact that D-lysine is not utilized as a source of L-lysine, whereas the utilization of D-methionine as a source of the L-isomer for growth is dose-dependent, reaching 76% of the value obtained with L-methionine. Both D-serine and the mixture of L-L and L-D isomers of lysinoalanine induce histological changes in the rat kidneys. D-tyrosine, D-serine, and lysinoalanine are produced in significant amounts under the influence of even short periods of alkaline treatment. Unresolved is whether the biological effects of D-amino acids vary, depending on whether they are consumed in the free state or as part of a food protein. Possible, metabolic interaction, antagonism, or synergism among D-amino acids in vivo also merits further study. The described results with mice complement related studies with other species and contribute to the understanding of nutritional and toxicological consequences of ingesting D-amino acids. Such an understanding will make it possible to devise food processing conditions to minimize or prevent the formation of undesirable D-amino acids in food proteins and to prepare better and safer foods.