1. No category

WHAT IS THE PHYSIOLOGICAL FUNCTION OF D-AMINO-ACID OXIDASE?
Ryuichi Konno
Department of Microbiology, Dokkyo University School of Medicine, Mibu, Tochigi 321-0293, Japan
FAX: 0282-86-5616,
E-mail: konno@dokkyomed.ac.jp
(Received 26 July 2002 Accepted 30 August 2002)
Abstract
D-Amino acid oxidase (DAO) catalyzes oxidative deamination of D-amino acids (steroisomers of naturally
occurring L-amino acids), producing 2-oxo acids, hydrogen peroxide and ammonia. In higher animals DAO is mainly
present in the kidney, liver and brain. However, since its substrates (D-amino acids) are considered very rare in higher
animals, the physiological role of DAO has been enigmatic since its discovery in 1935. We are interested in why we
have such a curious enzyme in our body.
We believed that mutant animals having dysfunctions in DAO would be useful for the elucidation of the
physiological function of this enzyme. We chose mice as our experimental animals due to their small size, short
generation time, clear genetic background, and their close relationship to humans. A screening method for mutant
mice possessing reduced levels of DAO activity was devised. Mutant mice lacking DAO activity were isolated and
their strain was established.
Genetic cross experiments between the mutant mice and normal mice indicated that mice have one gene
encoding DAO and that it is expressed in the kidney and brain. The DAO gene was transmitted through the Mendelian
manner. The cross experiments revealed a gene dosage effect on DAO activity: mice carrying two normal DAO
alleles have twice the DAO activity than mice carrying one normal and one mutant DAO allele. The DAO gene was
mapped at 65 cM on the chromosome 5 by the linkage analysis. Fluorescence in situ hybridization mapped this gene
at E3-F on chromosome 5.
The nucleotide sequence analysis of the cloned DAO cDNA revealed the presence of a nucleotide
substitution in the middle of the coding region of mutant DAO. This nucleotide substitution caused a substitution of
amino acid residue of DAO. The transfection and expression of the mutant and normalized DAO cDNA in cells in
culture verified that the nucleotide substitution was the real cause of the loss of DAO activity.
From the nutritional experiments using the mutant and normal mice, DAO was shown to be the most
important and indispensable enzyme in the utilization of D-amino acids in the body.
Large amounts of amino acids were detected in the urine of the mutant mice. Most abundant alanine was
mainly D-isomer and was determined to originate from the cell walls of intestinal bacteria. Abundant methionine was
also mostly D-isomer and determined to derive from DL-methionine supplemented in the diet of the mice. Serine was
mostly D-isomer and a portion of the D-serine was indicated to be synthesized in the body. These D-amino acids were
not observed in the urine of normal mice. That means that they are constantly metabolized by DAO in normal mice.
Therefore, we conclude that the metabolism of endogenous and exogenous D-amino acids is the physiological role of
DAO.
Keywords: D-Amino-acid oxidase, physiological function, mutant mice, genetics, point mutation, aminoaciduria, urine
Viva Origino 30 (2002) 216 - 220
© 2002 by SSOEL Japan
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Viva Origino 30 (2002) 216 - 220
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TEL 0282-87-2131 FAX 0282-86-5616
E-mail: konno@dokkyomed.ac.jp
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Viva Origino 30 (2002) 216 - 220
8.
9.
10.
Fig.1. D- Aminoaciduria in mutanat mice lacking D-amino-acid
oxidase. A large amounts of D-methionine, D-methionine
sulfoxide/sulfone, D-alanine, D-serine and D-proline are present
in the urine of mutant mice lacking D-amino-acid oxidase.
Urinary D-methione and D-methionine sulfoxide/sulfone
originate from D-methionine supplemented in the mouse diet.
Urinary D-alanine derives from D-alanine in the cell walls of
intestinal bacteria. A portion of urinary D-serine is synthesized
in the body.
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11.
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