Lack of D-amino-acid oxidase activity causes a specific renal aminoaciduria in the mouse

Why is d-serine nephrotoxic and α-aminoisobutyric acid protective?

d-Serine selectively causes necrosis of S3 segments of proximal tubules in rats. This leads to aminoaciduria and glucosuria. Coinjection of the nonmetabolizable amino acid α-aminoisobutyric acid (AIB) prevents the tubulopathy. d-serine is selectively reabsorbed in S3, thereby gaining access to peroxisomal d-amino acid oxidase (d-AAO). d-AAO-mediated metabolism produces reactive oxygen species. We determined the fractional excretion of amino acids and glucose in rats after intraperitoneal injection of d-serine alone or together with reduced glutathione (GSH) or AIB. Both compounds prevented the hyperaminoaciduria. We measured GSH concentrations in renal tissue before (control) and after d-serine injection and found that GSH levels decreased to ∼30% of control. This decrease was prevented when equimolar GSH was coinjected with d-serine. To find out why AIB protected the tubule from d-serine toxicity, we microinfused d-[14C]serine or [14C]AIB (0.36 mmol/l) together with [3H]inulin in late proximal tubules in vivo and measured the radioactivity in the final urine. Fractional reabsorption of d-[14C]serine and [14C]AIB amounted to 55 and 70%, respectively, and 80 mmol/l of AIB or d-serine mutually prevented reabsorption to a great extent. d-AAO activity measured in vitro (using d-serine as substrate) was not influenced by a 10-fold higher AIB concentration. We conclude from these results that 1) d-AAO-mediated d-serine metabolism lowers renal GSH concentrations and thereby provokes tubular damage because reduction of reactive oxygen species by GSH is diminished and 2) AIB prevents d-serine-induced tubulopathy by inhibition of d-serine uptake in S3 segments rather than by interfering with intracellular d-AAO-mediated d-serine metabolism.

In vitro study of the metabolic effects of D-amino acids

While the L-configuration of amino acids predominates in all known living systems, D-enantiomers of amino acids have been detected with highly sensitive chromatographic techniques in human physiological fluids. In the present study, the survival of Chinese hamster ovary cells (CHO) and HeLa cells was inhibited by exposure to high concentrations of some D- or L-amino acids. Inhibition of colony formation, though, was not necessarily observed to be chiral-dependent. Some L-amino acids (LAAs) were found to be toxic while other D-amino acids (DAAs) were innocuous in both cultures. This is contradictory to the previous observations that DAAs were generally considered to be harmful. Frequently it was implied, although not experimentally proven, that the LAAs were not toxic. One of the metabolites produced by oxidative deamination of D- or LAAs is hydrogen peroxide (H2O2), a reactive oxygen species (ROS) that is decomposed by catalase. Increased intracellular H2O2 can result in peroxidation of lipids. We measured catalase activity and the lipid peroxide levels (LPO) after incubating cells in either D- or LAAs. The amino acids (AAs) that were found to inhibit colony formation were found to be associated with higher levels of catalase activity and LPO. Therefore, we hypothesize that enhanced ROS generation may be, in part, responsible for the observed toxicity of some amino acids.

Free D-serine, D-aspartate and D-alanine in central nervous system and serum in mutant mice lacking D-amino acid oxidase

We have examined whether D-amino acid oxidase (DAO) regulates free D-serine content using mutant ddY/DAO- mice lacking DAO activity. We find that the content of D-serine in the serum and cerebellum of mutant mice is much higher than that of normal mice, whereas a slight but significant difference in the cerebral D-serine level is observed between the two strains. These results suggest that, although DAO may participate in the catabolism of D-serine in the cerebellum and periphery, there appears to be other mechanisms for catabolism of endogenous D-serine in the brain.

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.

Presence of D-amino-acid oxidase protein in mutant mice lacking D-amino-acid oxidase activity

1. Mutant mice lacking D-amino-acid oxidase activity were examined as to whether they possessed the enzyme protein. 2. Immunoblotting using an antibody against hog kidney D-amino-acid oxidase showed that kidney homogenates of the mutant mice as well as that of the normal mice had proteins reactive to the antibody. 3. Peroxisomal proteins of the kidney cells of the mutant mice were not different from those of the normal mice. 4. The peroxisomes of the mutant mice possessed a protein reactive to the antibody in the immunoblotting whose size was the same as the D-amino-acid oxidase protein present in the peroxisomes of the normal mice. 5. These results suggest that the mutant mice synthesize the D-amino-acid oxidase protein and integrate it into peroxisomes, though it is a nonfunctional enzyme.

Differential effect of sunflower seed oil on hepatic and renal D-amino acid oxidase in the rat

1. The specific activity of hepatic and renal peroxisomal D-amino acid oxidase (D-AAOX) was measured in rats fed diets containing various quantities of vegetable oil. 2. Increasing the amount of dietary sunflower seed oil (SSO) from 10 to 25% (w/w) reduced the specific activity of hepatic D-AAOX by up to 30% after 10 days. 3. In both tissues, the enzyme activity was moderately decreased during the first two-day period after administration of the 25% SSO diet was begun. Unlike hepatic D-AAOX, renal D-AAOX returned to its baseline level in the kidney after the third day. 4. In contrast to SSO, hydrogenated coconut oil (HCO) did not evoke alterations of D-AAOX activity. 5. The activity levels of another peroxisomal enzyme, L-2-hydroxy acid oxidase (L-HAOX), in the liver of rats fed the high-SSO diet vs those fed the control diet were similar. 6. The subcellular distribution of D-AAOX and L-HAOX was not altered in the liver of rats fed the 25% SSO diet during the 10-day period.

Intestinal bacterial origin of D-alanine in urine of mutant mice lacking D-amino-acid oxidase

Urine from mutant mice which lack D-amino-acid oxidase contained a large amount of D-alanine. The alanine content was not changed by changes in the dietary composition or by starvation. However, oral administration of an antibiotic, amoxicillin, decreased the urinary alanine to the normal level. These results suggest that the D-alanine is not of dietary origin but of intestinal bacterial origin.

D-aspartate oxidase activity and D-aspartate content in a mutant mouse strain lacking D-amino acid oxidase

A mutant mouse strain ddY/DAO- lacks D-amino acid oxidase activity and accumulates free neutral D-amino acids in its tissues. In this study, D-aspartate oxidase activity and D-aspartate content in the tissues of these mutant mice were compared with those of normal mice. No significant difference was observed, indicating that the metabolism of acidic D-amino acids was unaffected in the mutant.

Spontaneous excretion of D-alanine in urine in mutant mice lacking D-amino-acid oxidase

Urine from mutant mice lacking D-amino-acid oxidase contained a large amount of alanine compared with that from normal mice. Urinary alanine of the mutant mice was sensitive to D-amino-acid oxidase. H.p.l.c. showed that about 94% of the urinary alanine had the D-configuration. These results suggest that D-amino-acid oxidase functions to decompose D-amino acid(s) in normal mice.