Chemistry, Nutrition, and Microbiology ofd-Amino Acids

Abstract

Exposure of food proteins to certain processing conditions induces two major chemical changes: racemization of all l-amino acids to d-isomers and concurrent formation of cross-linked amino acids such as lysinoalanine. Racemization of l-amino acids residues to their d-isomers in food and other proteins is pH-, time-, and temperature-dependent. Although racemization rates of the 18 different l-amino acid residues in a protein vary, the relative rates in different proteins are similar. The diet contains both processing-induced and naturally formed d-amino acids. The latter include those found in microorganisms, plants, and marine invertebrates. Racemization impairs digestibility and nutritional quality. The nutritional utilization of different d-amino acids varies widely in animals and humans. In addition, some d-amino acids may be both beneficial and deleterious. Thus, although d-phenylalanine in an all-amino-acid diet is utilized as a nutritional source of l-phenylalanine, high concentrations of d-tyrosine in such diets inhibit the growth of mice. Both d-serine and lysinoalanine induce histological changes in the rat kidney. The wide variation in the utilization of d-amino acids is illustrated by the fact that whereas d-methionine is largely utilized as a nutritional source of the l-isomer, d-lysine is totally devoid of any nutritional value. Similarly, although l-cysteine has a sparing effect on l-methionine when fed to mice, d-cysteine does not. Because d-amino acids are consumed by animals and humans as part of their normal diets, a need exists to develop a better understanding of their roles in nutrition, food safety, microbiology, physiology, and medicine. To contribute to this effort, this multidiscipline-oriented overview surveys our present knowledge of the chemistry, nutrition, safety, microbiology, and pharmacology of d-amino acids. Also covered are the origin and distribution of d-amino acids in the food chain and in body fluids and tissues and recommendations for future research in each of these areas. Understanding of the integrated, beneficial effects of d-amino acids against cancer, schizophrenia, and infection, and overlapping aspects of the formation, occurrence, and biological functions of d-amino should lead to better foods and improved human health. Keywords: d-Amino acids; food processing; formation; microbiology; nutrition; safety