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-tyrosin...

Chemistry, nutrition, and microbiology of D-amino acids.

Chiral separation of amino acids and peptides by capillary electrophoresis.

More than half a century ago researchers thought that D-amino acids had a minor function compared to L-enantiomers in biological processes. Many evidences have shown that D-amino acids are present in high concentration in microorganisms, plants, mammals and humans and fulfil specific biological functions. In the brain of mammals, D-serine (D-Ser) acts as a co-agonist of the N-methyl-D-aspartate (NMDA)-type glutamate receptors, responsible for learning, memory and behaviour. D-Ser metabolism is relevant for disorders associated with an altered function of the NMDA receptor, such as schizophrenia, ischemia, epilepsy and neurodegenerative disorders. On the other hand, D-aspartate (D-Asp) is one of the major regulators of adult neurogenesis and plays an important role in the development of endocrine function. D-Asp is present in the neuroendocrine and endocrine tissues and testes, and regulates the synthesis and secretion of hormones and spermatogenesis. Also food proteins contain D-amino acids that are naturally originated or processing-induced under conditions such as high temperatures, acid and alkali treatments and fermentation processes. The presence of D-amino acids in dairy products denotes thermal and alkaline treatments and microbial contamination. Two enzymes are involved in the metabolism of D-amino acids: amino acid racemase in the synthesis and D-amino acid oxidase in the degradation.

An overview on d-amino acids

In several organisms d-cysteine desulfhydrase (d-CDes) activity (EC 4.1.99.4) was measured; this enzyme decomposes d-cysteine into pyruvate, H2S, and NH3. A gene encoding a putative d-CDes protein was identified in Arabidopsis thaliana (L) Heynh. based on high homology to an Escherichia coli protein called YedO that has d-CDes activity. The deduced Arabidopsis protein consists of 401 amino acids and has a molecular mass of 43.9 kDa. It contains a pyridoxal-5′-phosphate binding site. The purified recombinant mature protein had a Km for d-cysteine of 0.25 mm. Only d-cysteine but not l-cysteine was converted by d-CDes to pyruvate, H2S, and NH3. The activity was inhibited by aminooxy acetic acid and hydroxylamine, inhibitors specific for pyridoxal-5′-phosphate dependent proteins, at low micromolar concentrations. The protein did not exhibit 1-aminocyclopropane-1-carboxylate deaminase activity (EC 3.5.99.7) as homologous bacterial proteins. Western blot analysis of isolated organelles and localization studies using fusion constructs with the green fluorescent protein indicated an intracellular localization of the nuclear encoded d-CDes protein in the mitochondria. d-CDes RNA levels increased with proceeding development of Arabidopsis but decreased in senescent plants; d-CDes protein levels remained almost unchanged in the same plants whereas specific d-CDes activity was highest in senescent plants. In plants grown in a 12-h light/12-h dark rhythm d-CDes RNA levels were highest in the dark, whereas protein levels and enzyme activity were lower in the dark period than in the light indicating post-translational regulation. Plants grown under low sulfate concentration showed an accumulation of d-CDes RNA and increased protein levels, the d-CDes activity was almost unchanged. Putative in vivo functions of the Arabidopsisd-CDes protein are discussed.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1742-4658.2005.04567.x

Isolation and characterization of a D-cysteine desulfhydrase protein from Arabidopsis thaliana.

Summary Quantities of free L- and D-amino acids (L- and D-AAs) in plants (leaves of coniferous and decidious trees, fleshy fruits, leaf blades of fodder grasses, and seeds and seedlings of edible legumes) were determined Amino acid (AA) enantiomers were converted into diastereomers using pre-column derivatization with ophthaldialdehyde together with N-isobutyryl-L(or D)-cysteine followed by separation of the resulting fluorescent isoindol derivatives on an octadecylsilyl stationary phase using high-performance liquid chromatography Relative amounts of D-AAs were also determined by enantioselective gas chromatography-mass spectrometry on Chirasil-L-Val ®  Free D-AAs acids in the range of about 02% up to 8% relative to the corresponding L-AAs acids were found in plants D-Asp, D-Asn, D-Glu, D-Gln, D-Ser and D-Ala could be detected in most of the plants, and D-Pro, D-Val, D-Leu and D-Lys in certain plants As D-AAs were detected in gymnosperms as well as monoand dicotyledonous angiosperms of major plant families it is concluded that free D-AAs in the low percentage range are principle constituents of plants

/pdf/chromatographic-determination-of-l-and-d-amino-acids-in-23nre2p34t.pdf

Chromatographic determination of L- and D-amino acids in plants.

Free D-amino acids (D-AA) were detected as native constituents in juices of vegetables (cultivars of cabbage, tomato, carrot, garlic) and fruits (organes, clementine, grapefruit, lemon, apples, pear, grapes) using gas chromatography (GC) or high-performance liquid chromatography (LC). For investigation by GC, AA enantiomers were converted into theirN(O)-pentafluoropropionyl 2-propyl esters and resolved on a Chirasil-L-Val capillary column. For determination by LC, precolumn derivatization of AA enantiomers usingo-phthaldialdehyde together with the chiral thiolsN-isobutyryl-L-cysteine orN-isobutyryl-D-cysteine and fluorescence detection of the diastereomeric isoindole derivatives, resolvable on an octadecylsilyl stationary phase, were used. D-Ala (0.6–3.8%) was detected in all freshly pressed plant juices usually in the highest relative amounts. Other D-AA detected were D-Asx (0.1–1.9%), D-Glx (0–1.3%), D-Ser (0–1.7%), D-Arg (0.4–1.2%, in grapes, orange, grapefruit, and clementine) and D-Leu and D-Val (1% in cabbage). Absolute amounts of native D-AA were totally 28–57 μmol L−1 in fruit juices, 14.5 μmol L−1 in a tomato juice and 8.5 μmol L−1 in a carrot juice.

T. Westhauser

Papers

Chromatographic determination of D-amino acids as native constituents of vegetables and fruits