Elsevier

Journal of Chromatography B

Volume 879, Issue 29, 1 November 2011, Pages 3162-3168
Journal of Chromatography B

Review
d-Amino acid metabolism in mammals: Biosynthesis, degradation and analytical aspects of the metabolic study

https://doi.org/10.1016/j.jchromb.2011.06.028Get rights and content

Abstract

It was believed for long time that d-amino acids are not present in mammals. However, current technological advances and improvements in analytical instruments have enabled studies that now indicate that significant amounts of d-amino acids are present in mammals. The most abundant d-amino acids are d-serine and d-aspartate. d-Serine, which is synthesized by serine racemase and is degraded by d-amino-acid oxidase, is present in the brain and modulates neurotransmission. d-Aspartate, which is synthesized by aspartate racemase and degraded by d-aspartate oxidase, is present in the neuroendocrine and endocrine tissues and testis. It regulates the synthesis and secretion of hormones and spermatogenesis. d-Serine and d-aspartate bind to the N-methyl-d-aspartate (NMDA) subtype of glutamate receptors and function as a coagonist and agonist, respectively. The enzymes that are involved in the synthesis and degradation of these d-amino acids are associated with neural diseases where the NMDA receptors are involved. Knockout mice for serine racemase and d-aspartate oxidase have been generated, and natural mutations in the d-amino-acid oxidase gene are present in mice and rats. These mutant animals display altered behaviors caused by enhanced or decreased NMDA receptor activity. In this article, we review currently available studies on d-amino acid metabolism in mammals and discuss analytical methods used to assay activity of amino acid racemases and d-amino-acid oxidases.

Introduction

d-Amino acids have been historically considered unnatural amino acids. Even after their discovery in bacteria, researchers still believed that d-amino acids were not present in eukaryotes, especially in higher animals. However, recent technological advances and improvements in analytical instruments have enabled studies that have shown that d-amino acids are present in mammals [1], [2], [3]. The most abundant d-amino acids in mammals are d-serine and d-aspartate. The enzymes that synthesize and degrade these d-amino acids have also been discovered.

In a landmark study in 1992, Hashimoto et al. [4] found a large quantity of d-serine in rat brains. A second study indicated that d-serine was abundant, specifically in the cerebral cortex, hippocampus, anterior olfactory nucleus, olfactory tubercle, and the amygdala of rats [5]. d-Serine was originally detected in the glial cells, but is also present in the neurons [6], [7]. d-Serine, which is synthesized by serine racemase and degraded by d-amino-acid oxidase, binds to the coagonist binding site of the N-methyl-d-aspartate (NMDA)-subtype of glutamate receptors and modulates NMDA receptor activity. The NMDA receptors are involved in numerous physiological and pathological processes, including synaptic plasticity, learning, memory, neuronal cell migration, and neural disease [8], [9].

In 1986, Dunlop et al. [10] reported that d-aspartate is present in mammals. Since then, a large quantity of d-aspartate has been detected in the brain, pineal gland, pituitary gland, adrenal gland, and testis [11]. d-Aspartate, which is synthesized by aspartate racemase and degraded by d-aspartate oxidase, regulates the synthesis and secretion of hormones and spermatogenesis. Similar to d-serine, d-aspartate also regulates NMDA receptor activity. In this review, we summarize recent findings on the enzymes involved in d-amino acid metabolism and analytical methods utilized in these studies.

Section snippets

Analytical methods for the assay of amino acid racemase activity

In mammals, only two amino acid racemases have been reported; serine racemase (EC 5.1.1.18) and aspartate racemase (EC 5.1.1.13). Various assay methods have been reported to measure serine racemase activity. In the first report, luminol reaction was used [12]. Substrate l-serine was converted to d-isomer by serine racemase. The resultant d-serine was converted to 2-oxo acid (α-keto acid), ammonia and hydrogen peroxide by d-amino-acid oxidase. The generated hydrogen peroxide was reacted with

Analytical methods for the assay of oxidase activity toward d-amino acids

As the oxidation enzyme against d-amino acids, d-amino-acid oxidase (EC 1.4.3.3) and d-aspartate oxidase (EC 1.4.3.1) have been reported in mammals. d-Amino-acid oxidase catalyzes the oxidation of neutral and basic d-amino acids, and d-aspartate oxidase catalyzes the oxidation of acidic amino acids.

A number of assay methods have been developed to measure d-amino-acid oxidase activity. In the past, it was measured by the consumption of oxygen dissolved in the reaction mixture using a manometer.

Serine racemase (EC 5.1.1.18)

It was historically believed that amino acid racemases were only present in prokaryotes and lower eukaryotes. Strikingly, in 1999, the mammalian serine racemase was purified from rat brains [12]. Shortly thereafter, cDNAs encoding mouse and human serine racemase were cloned [25], [26].

Similar to other racemases, serine racemase has pyridoxal 5′-phosphate (PLP) as a prosthetic group and catalyzes the conversion between l-serine and d-serine (Fig. 1). It was originally thought that serine

d-Amino-acid oxidase (EC 1.4.3.3)

Among the enzymes that are involved in d-amino acid metabolism, d-amino-acid oxidase (DAO or DAAO) was the first enzyme discovered in mammals [46]. d-Amino-acid oxidase contains flavin adenine dinucleotide (FAD) as the prosthetic group and catalyzes the oxidative deamination of d-amino acids. This enzyme has a wide range of substrate specificity and metabolizes a number of neutral and basic d-amino acids. During the initial step of catalysis, the d-amino acid is oxidized to an imino acid and

Aspartate racemase (EC 5.1.1.13)

Although it was known that a large amount of d-aspartate is present in the tissues and organs of mammals, it was unknown for a long period of time how this d-amino acid is produced. Several studies have shown that the enzyme aspartate racemase is present in bacteria, mollusks, and amphibians. Wolosker et al. [76] showed that embryonic neuronal primary culture cells from rats can produce [14C]d-aspartate from the [14C]l-isomer. Topo et al. [17] developed a method to measure aspartate racemase

d-Aspartate oxidase (EC 1.4.3.1)

d-Aspartate oxidase, which is an enzyme that is similar to d-amino-acid oxidase, was discovered a number of years ago [77]. d-Aspartate oxidase (DDO or DAspO) has FAD as a prosthetic group, and catalyzes the oxidative deamination of acidic d-amino acids, such as d-aspartate and d-glutamate, to produce the corresponding 2-oxo acid, hydrogen peroxide and ammonia. The reaction mechanism is similar to the mechanism that is shown in Fig. 1.

d-Aspartate oxidase is present in the kidneys, liver, and

Future directions

In addition to d-serine and d-aspartate, d-alanine is also present in mammals at moderate levels [75], [84]. Although most d-alanine derives from food and intestinal bacteria, it may also be synthesized in the body. Alanine racemase has been purified from crayfish [85], and a mammalian ortholog may be soon discovered in a manner similar to the discovery of aspartate racemase.

Currently, four enzymes that metabolize d-amino acids have been discovered in mammals, and the literature regarding these

References (85)

  • K. Hamase et al.

    J. Chromatogr. B

    (2002)
  • A. Hashimoto et al.

    FEBS Lett.

    (1992)
  • E. Kartvelishvily et al.

    J. Biol. Chem.

    (2006)
  • D.S. Dunlop et al.

    Biochem. Biophys. Res. Commun.

    (1986)
  • S.P. Cook et al.

    J. Biol. Chem.

    (2002)
  • M.A. Smith et al.

    J. Biol. Chem.

    (2010)
  • K. Stříšovský et al.

    FEBS Lett.

    (2003)
  • A. D’Aniello et al.

    J. Biol. Chem.

    (1993)
  • T. Watanabe et al.

    Anal. Biochem.

    (1978)
  • K. Hamase et al.

    J. Chromatogr. A

    (2006)
  • V.N. Foltyn et al.

    J. Biol. Chem.

    (2005)
  • R. Konno

    Neurosci. Lett.

    (2003)
  • E. Dumin et al.

    J. Biol. Chem.

    (2006)
  • V.N. Foltyn et al.

    FEBS Lett.

    (2010)
  • H. Sarras et al.

    Schizophr. Res.

    (2010)
  • I. Bendikov et al.

    Schizophr. Res.

    (2007)
  • R. Konno et al.

    Biochim. Biophys. Acta

    (1997)
  • K. Horiike et al.

    Brain Res.

    (1994)
  • S. Sacchi et al.

    J. Biol. Chem.

    (2008)
  • T. Ohnuma et al.

    Prog. Neuropsychopharmacol. Biol. Psychiatry

    (2009)
  • K. Hashimoto et al.

    Prog. Neuropsychopharmacol. Biol. Psychiatry

    (2005)
  • R. Kapoor et al.

    Brain Res.

    (2006)
  • C. Madeira et al.

    Schizophr. Res.

    (2008)
  • K. Wake et al.

    Neurosci. Lett.

    (2001)
  • M. Maekawa et al.

    Neurosci. Res.

    (2005)
  • S.L. Almond et al.

    Mol. Cell. Neurosci.

    (2006)
  • A. Hashimoto et al.

    Brain Res.

    (2005)
  • Y. Miyoshi et al.

    J. Chromatogr. B

    (2009)
  • H. Wolosker et al.

    Neuroscience

    (2000)
  • J.L. Still et al.

    J. Biol. Chem.

    (1949)
  • F. Errico et al.

    Gene

    (2006)
  • F. Errico et al.

    Mol. Cell. Neurosci.

    (2008)
  • Z.M. Weil et al.

    Behav. Brain Res.

    (2006)
  • A. Morikawa et al.

    Anal. Biochem.

    (2003)
  • K. Shibata et al.

    Comp. Biochem. Physiol. B: Biochem. Mol. Biol.

    (2000)
  • D.L. Kirschner et al.

    J. Sep. Sci.

    (2009)
  • M.J. Schell et al.

    Proc. Natl. Acad. Sci. U.S.A.

    (1995)
  • K. Miya et al.

    J. Comp. Neurol.

    (2008)
  • H. Mori et al.

    Chem. Biodivers.

    (2010)
  • H. Wolosker et al.

    FEBS J.

    (2008)
  • M. Katane et al.

    Chem. Biodivers.

    (2010)
  • Cited by (0)

    This paper is part of the special issue “Analysis and Biological Relevance of d-Amino Acids and Related Compounds”, Kenji Hamase (Guest Editor).

    View full text