Substrate specificity of duckling hepatic and renal D-amino acid oxidase

The urinary ratio of 3-hydroxykynurenine/3-hydroxyanthranilic acid is an index to predicting the adverse effects of D-tryptophan in rats

The adverse effects of D-tryptophan and the possibility of it being a surrogate index for predicting adverse effects in rats were investigated. Male rats were fed one of several test diets (20% casein diets with 0% (control), 0.1%, 0.2%, 0.3%, 0.4%, and 0.5% D-tryptophan) for 21 d, and 24-h urine samples on the final day of the experiment were collected. Analyses of food intake and body-weight changes revealed adverse effects to be observed in the group fed the 0.3% D-tryptophan diet. We propose urinary levels of 3-hydroxykynurenine/3-hydroxyanthranilic acid to be surrogate indicators for predicting the adverse effects of D-tryptophan from the break point of body-weight gains and urinary levels of D-tryptophan metabolites. The reaction 3-hydroxykynurenine→3-hydroxyanthranilic acid is catalyzed by the pyridoxal phosphate-dependent enzyme kynureninase. Increasing urinary 3-hydrokykynurenine indicates kynureninase deficiency. Intake of D-tryptophan in rats fed the 0.3% D-tryptophan diet was 0.21 g/kg body weight and feeding of the 0.3% D-tryptophan diet did not elicit adverse effects. Thus, the safe level of D-tryptophan was less than 0.2% in the diet, 0.15 g/kg body weight, in rats.

Contributions of tryptophan 2,3-dioxygenase and indoleamine 2,3-dioxygenase to the conversion of D-tryptophan to nicotinamide analyzed by using tryptophan 2,3-dioxygenase-knockout mice

We investigated the contribution percentage of tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO) to the conversion of D-tryptophan to nicotinamide in TDO-knockout mice. The calculated percentage conversions indicated that TDO and IDO oxidized 70 and 30%, respectively, of the dietary L-tryptophan. These results indicate that both TDO and IDO biosynthesize nicotinamide from D-tryptophan and L-tryptophan in mice.

Transamination of 2-oxo-4-[methylthio]butanoic acid in chicken tissues

The keto acid 2-oxo-4[methylthio]butanoic acid (OMTB) is an intermediate in the conversion of synthetic feed grade methionine sources to L-methionine in vivo in poultry and other animals. Because methionine sources are utilized by the chick with considerably less than 100% efficiency as sources of L-methionine, it is important to determine what metabolic process may limit the utilization of these sources. Because OMTB is converted to L-methionine by transamination, a study was conducted to determine which amino acids might serve as nitrogen donors in the conversion of OMTB to L-methionine in the chicken. Dialyzed tissue homogenates, mitochondria, and cytosol from liver, kidney, intestine, and skeletal muscle were incubated with OMTB and individual L-amino acids (isoleucine, leucine, valine, glutamic acid, aspartic acid, alanine, glutamine, asparagine, and phenylalanine) and the methionine that accumulated was determined by ion exchange chromatography. Tissues differed in the conversion of OMTB to methionine: kidney was most active, liver and intestinal mucosa were intermediate, and skeletal muscle had lowest activity. All amino acids supported methionine synthesis. Branched-chain amino acids and glutamic acid were the most effective substrates in tissue cytosols except in intestinal mucosa, in which asparagine was also effective. The preferred substrates in mitochondria were glutamate in liver mitochondria, isoleucine and alanine in kidney mitochondria, and branched-chain amino acids and glutamic acid in skeletal muscle mitochondria. All amino acids except alanine supported methionine synthesis from OMTB in mitochondria of intestinal mucosa. We conclude that a wide variety of amino acids can serve as substrates for transamination of OMTB in the chicken, and that the availability of nitrogen donors is unlikely to be a limiting factor in the conversion of OMTB to methionine.

D-amino-acid oxidase is not present in the mouse liver

Since there are conflict reports on the presence of D-amino-acid oxidase in the mouse liver, this problem was examined. D-Amino-acid oxidase activity was not detected in the homogenates of the mouse liver, lung, or heart, whereas it was detected in the homogenates of the mouse kidney and brain. Western blotting showed that a protein which reacted with the antiserum against pig D-amino-acid oxidase was present in the homogenates of the mouse kidney and brain but not in those of the liver or heart. Northern hybridization using a D-amino-acid oxidase cDNA probe detected a hybridizing signal in poly(A)+ RNAs extracted from the mouse kidney and brain but not in those from the liver, heart, or lung. Reverse transcription-polymerase chain reaction using three primer pairs always amplified D-amino-acid oxidase cDNA fragments of expected sizes in the mouse kidney and brain but very rarely did so in the liver, heart, or lung. The results indicate that D-amino-acid oxidase is not present in the mouse liver in a measurable amount.

The artificially perfused guinea-pig yolk sac placenta: transfer and uptake of water, glucose and amino acids

The development of a new technique to investigate maternal-fetal transfer across the near term guinea-pig yolk sac placenta by in-situ perfusion of the yolk sac vessels is described. The maternal-fetal transfer of labeled water, D- and L-glucose, O-methyl-D-glucose (oMDG), D- and L-alanine, D- and L-aspartate, L-lactate and alpha-amino-isobutyric acid (AIBA) was investigated after injection of these substances into the maternal circulation. After 15 min of perfusion at 0.5 ml/min the water clearance was 132 +/- 12 microliters/min (SEM, n = 30). The clearances for D- or L-glucose were less than 1.2 microliters/min. The activity of label in the venous yolk sac perfusate of all other substances was not different from background activity when 14C-label was used. The clearance of 3H-L-alanine approached the clearance value of water. The total uptake (as defined for single-injection double tracer dilution experiments) from the perfusate of D-glucose, oMDG, alanine and aspartate in comparison to L-glucose was also studied. Mean D-glucose uptake was 11.2 +/- 1.9 percent (n = 8), it was significantly reduced to 4.9 +/- 2 percent (n = 5) by cytochalasin B (1 X 10(-4) mmol/l), and by increasing concentrations of D-glucose (1 to 20 mmol/l, n = 4). The uptake of oMDG was 8.8 +/- 1.5 percent (n = 8). L-alanine uptake was 25 +/- 3.4 percent, D-alanine uptake was 8.3 +/- 1.5 percent (n = 12). Both uptake values were decreased significantly by 10 mmol/l L-alanine, but unaffected by [Na+] (less than 15 mequ/l). There was no uptake of AIBA. The uptakes of L-aspartate were 34.9 +/- 3.7 percent and of D-aspartate 40.4 +/- 4.8 percent (n = 11). Both uptake values were significantly and reversibly reduced by 1 mmol/l L-aspartate and D-aspartate, and by low [Na+] (less than 15 mequ/l). It is concluded that water can move by diffusion from maternal circulation into the yolk sac capillaries in considerable amounts whereas the contribution of the yolk sac placenta to fetal nutrition with D-glucose, L-alanine and L-aspartate is negligible. The membranes of yolk sac cells contain specific transport systems for D-glucose, D-/L-alanine and D-/L-aspartate transfer. The function of the vitelline placenta in the near-term guinea-pig is comparable more to the gut than to the chorio-allantoic placenta.