d-Amino-Acid oxidase and its physiological function

D-amino-acid oxidase is confined to the lower brain stem and cerebellum in rat brain: regional differentiation of astrocytes

Based on enzymatic activity, the localization and the identification of D-amino-acid oxidase-containing cells in rat whole brain was systematically studied in serial fixed sections. The oxidase activity was absent or scarce in the forebrain, was confined to the brain stem (midbrain, pons and medulla oblongata) and cerebellum, and its localization was extended to the spinal cord. In the brain stem the oxidase was mainly localized in the tegmentum, particularly in the reticular formation. The intense oxidase reactions were present in the red nucleus, oculomotor nucleus, trochlear nucleus, ventral nucleus of the lateral lemniscus, dorsal and ventral cochlear nuclei, vestibular nuclei, nuclei of posterior funiculus, nucleus of the spinal tract of the trigeminal nerve, lateral reticular nucleus, inferior olivary nucleus, and hypoglossal nucleus. In the cerebellum the activity in the cortex was much more intense than that in the medulla. In all the fields described above, the oxidase-containing cells were exclusively astrocytes including Bergmann glial cells, and neither neuronal components, endothelial cells, oligodendrocytes nor ependymal cells showed oxidase activity. These results indicated that the astrocytes regionally differentiated into two distinct types, one of which expressed oxidase in the midbrain, rhombencephalon and spinal cord, and the other which did not in the forebrain. The localization of the oxidase was inversely correlated with the distribution of free D-serine in mammalian brains (Nagata, Y., Horiike, K. and Maeda, T., Brain Res., 634 (1994) 291-295). Based on the characteristic localization of the oxidase-containing astrocytes, we discussed the physiological role of the oxidase.

Gas chromatographic characterization of free D-amino acids in the blood serum of patients with renal disorders and of healthy volunteers

A capillary gas chromatographic method, using the chiral stationary phase Chirasil-L-Val, after treatment and isolation with Dowex 50W X8 cation exchanger and conversion into trifluoroacetyl-1-propyl esters or pentafluoropropionyl-1 (or 2)-propyl esters, has been developed for the determination of the relative amounts of free D-amino acids in the blood serum of eighteen patients with renal failure (continuous ambulatory peritoneal dialysis (CAPD), n = 11; hemodialysis, n = 5; nephrotic syndrome, n = 2) and compared with data obtained from healthy volunteers (n = 5). Significant amounts of D-Ala (0.5-13%) and D-Asx (1.5-7.7%; Asx = Asp + Asn) were found in all serum samples. D-Ser was detected in the serum of all patients with renal disorders and, in addition, D-Pro (0.6-2.5%) was found in the serum of all patients undergoing hemodialysis and with nephrotic syndrome. D-Ser (2.9-3.1%) and D-Pro (0.6-0.9%) were also found in the samples of three volunteers. D-Leu (1.2-1.7%) was present in three patients with CAPD, and D-Glx (0.3-1.3%; Glx = Glu + Gln) was present in eight of eighteen patients with renal malfunction. Linear regression analysis of the relative amounts of D-amino acids and the serum creatinine levels of all donors revealed positive correlation factors for D-Asx (r = 0.748) and D-Ser-(r = 0.667), but not for D-Pro and D-Ala. Remarkably high amounts of D-Ser (12.1 and 19.8%) were found in two hemodialysates investigated. Participation of intestinal bacteria and nutrition are discussed as possible sources of serum D-amino acids. An increase of some D-amino acids in the serum of patients with renal diseases might be explained, in part, by decreased activity of renal D-amino acid oxidase.

Chirality of amino acids of microorganisms used in food biotechnology

Bacteria of the genera Acetobacter, Bifidobacterium, Brevibacterium, Lactobacillus, Micrococcus, Propionibacterium, and Streptococcus, which are used as so-called starter cultures for the large-scale production of fermented foods and beverages in food biotechnology, have been investigated for the chirality of their amino acids (AA) by gas chromatography (GC). Bacteria were grown in complex media, centrifuged, and washed with 0.85% aqueous NaCl. Aliquots were totally hydrolyzed (6 M HCl, 110 degrees C, 18 h), or extracted with 70% aqueous ethanol in order to isolated free AA. The AA were adsorbed on Dowex WX 8 cation-exchanger, eluted with 4 M ammonia and converted into their N(O)-trifluoro-acetyl(TFA) 2-propyl esters or TFA methyl esters. The AA derivatives were investigated by capillary GC using the chiral stationary phases Chirasil-L-Val, Chirasil-D-Val, and Lipodex E. Besides L-AA, in all bacteria D-amino acids (D-AA) were detected; those in the highest relative amounts were D-Ala and D-Asp (occurring in all bacteria) and, in several cases, D-Glu. Lower, but significant amounts of other D-AA such as D-Ser, D-Pro, D-Val, D-Thr. D-Ile, D-Leu, D-Met, D-Phe, D-Tyr, D-Orn, and D-Lys were also detected in certain bacteria. These findings explain the origin of D-AA found in all fermented foods and drinks produced with the aid of bacterial starter cultures.

A single-base-pair substitution abolishes D-amino-acid oxidase activity in the mouse

Mutant ddY/DAO- mice lacking D-amino-acid oxidase (DAO) activity were examined for the cause of their lack of enzyme activity. Total RNA was extracted from the kidney of the ddY/DAO- mice and cDNA was synthesized. After cDNA encoding DAO was amplified by the polymerase chain reaction it was cloned into a plasmid and sequenced. Comparison of the DAO cDNA sequence with that of normal BALB/c mice revealed the presence of a single-base substitution (G----A) which causes a Gly-181----Arg substitution in the middle of the enzyme molecule. The mutant DAO cDNA was inserted into an expression vector and was expressed in transfected COS-1 cells. The transfected cells synthesized the DAO mutant protein, but they did not show DAO activity. In contrast, when cells were transfected with an expression vector carrying wild-type DAO cDNA, where the substituted base-pair was replaced by a normal base-pair, they showed DAO activity. These results indicate that the single base-pair substitution is the cause of the loss of DAO activity in the ddY/DAO- mice.