Glyoxylate aminotransferase in peroxisomes from rat liver and kidney

The metabolic importance of the overlooked asparaginase II pathway

The asparaginase II pathway consists of an asparagine transaminase [l-asparagine + α-keto acid ⇆ α-ketosuccinamate + l-amino acid] coupled to ω-amidase [α-ketosuccinamate + H₂O → oxaloacetate + NH₄⁺]. The net reaction is: l-asparagine + α-keto acid + H₂O → oxaloacetate + l-amino acid + NH₄⁺. Thus, in the presence of a suitable α-keto acid substrate, the asparaginase II pathway generates anaplerotic oxaloacetate at the expense of readily dispensable asparagine. Several studies have shown that the asparaginase II pathway is important in photorespiration in plants. However, since its discovery in rat tissues in the 1950s, this pathway has been almost completely ignored as a conduit for asparagine metabolism in mammals. Several mammalian transaminases can catalyze transamination of asparagine, one of which - alanine-glyoxylate aminotransferase type 1 (AGT1) - is important in glyoxylate metabolism. Glyoxylate is a precursor of oxalate which, in the form of its calcium salt, is a major contributor to the formation of kidney stones. Thus, transamination of glyoxylate with asparagine may be physiologically important for the removal of potentially toxic glyoxylate. Asparaginase has been the mainstay treatment for certain childhood leukemias. We suggest that an inhibitor of ω-amidase may potentiate the therapeutic benefits of asparaginase treatment.

Distribution and characteristics of D-amino acid and D-aspartate oxidases in fish tissues

The distributions of D-amino acid oxidase (D-AAO, EC 1.4.3.3) and D-aspartate oxidase (D-AspO, EC 1.4.3.1) activities were examined on several tissues of various fish species. Both enzyme activities were commonly high in kidney and liver and low in intestine with some exceptions. After oral administration of D-alanine at 5 micromol /g body weight(-1)day(-1) to carp for 30 days, D-AAO activity increased by about 8-, 3-, and 1.5-fold in intestine, hepatopancreas, and kidney, respectively, whereas no increase was found in brain. In contrast, oral administration of D-glutamate or D-aspartate did not show any increase of D-AspO activity in any tissues. D-AAO and D-AspO of common carp kidney and hepatopancreas were subcellularly localized in peroxisomes, as clarified in mammals. D-proline was the best substrate for D-AAO in rainbow trout kidney, common carp kidney, and hepatopancreas, followed by D-alanine and D-phenylalanine. N-methyl-D-aspartate was the best substrate for D-AspO in rainbow trout kidney and common carp hepatopancreas. The optimal pH for D-AAO in rainbow trout kidney was broad, from 7.4 to 8.2, and that for D-AspO was around 10. D-AAO was inhibited by benzoate known as D-AAO inhibitor and D-AspO was strongly inhibited by meso-tartarate as D-AspO inhibitor. From these results, at least D-AAO in fish is considered to work as a metabolizing agent of exogenous and endogenous free D-alanine that is abundant in aquatic invertebrates such as crustaceans and bivalve mollusks, which are potential food sources of these fishes.

Alcohol:NAD+ oxidoreductase is present in rat liver peroxisomes

Alcohol:NAD+ oxidoreductase was found in the peroxisomes of animal liver for the first time as follows. The distribution of alcohol:NAD+ oxidoreductase activity with nonanol as substrate in the light mitochondrial fraction (peroxisome-enriched fraction) of rat liver was examined by centrifugation in a sucrose density gradient. Most of the enzyme activity was localized in the mitochondria, with some activity in the peroxisomes. The administration of clofibrate, a peroxisome proliferator, to rats resulted in a marked increase of the enzyme activity in the peroxisomes, but not in the mitochondria. The enzyme was found to be located in the matrix of the peroxisomes. The evidence was obtained that the enzyme differed from alcohol dehydrogenases and alcohol oxidizing systems found previously. The enzyme activity was not affected by pyrazole, an inhibitor of alcohol dehydrogenase and sodium azide, an inhibitor of catalase. The enzyme was NAD(+)-dependent and oxidized straight chain aliphatic alcohols with a variety of carbon chains (C2-C18), showing the maximum on nonanol. Km values toward these aliphatic alcohols decreased with increasing chain length. The major reaction product was identified as the carboxylic acid by using high performance liquid chromatography.

Cetaben is an exceptional type of peroxisome proliferator

1. Cetaben in contrast to fibrates affect differently peroxisomal constituents. 2. Changes in large scale of liver non-peroxisomal parameters were compared after 10 days administration of equal doses (200 mg/kg/day) of cetaben and clofibric acid to male Wistar rats. 3. Clofibric acid treatment increased markedly the activities of FAD-glycerol-3-P dehydrogenase, beta-hydroxyacyl-CoA dehydrogenase, cytochrome-c oxidase, malic enzyme, NAD-glycerol-3-P dehydrogenase, ethoxycoumarin deethylase, p-nitroanisole demethylase and amounts of cytochrome P-450 and b5. 4. However no analogical changes were observed after cetaben treatment in the livers of experimental animals. 5. Both drugs increased the activities of alanine-glyoxylate aminotransferase-1 and acetylcarnitine transferase--enzymes with proven mitochondrial and peroxisomal location. 6. Cetaben contrary to clofibric acid does not increase solubilization of peroxisomal enzymes. 7. Enhanced acetylcarnitine transferase and alanine-glyoxylate aminotransferase-1 activities were distributed in mitochondria as well as in peroxisomes after clofibric acid treatment, however, only peroxisomes were enriched after cetaben administration. 8. The results obtained suggest that cetaben represents an exceptional type of peroxisome proliferator, specifically affecting peroxisomes, without having a negative influence on the processes of peroxisome biogenesis.

Peroxisomes and peroxisomal enzymes along the crypt-villus axis of the rat intestine

The development of peroxisomes and expression of their enzymes were investigated in differentiating intestinal epithelial cells during their migration along the crypt-villus axis. Sequential cell populations harvested by a low-temperature method were identified by microscopy, determination of alkaline phosphatase and sucrase activities and incorporation of [3H]-thymidine into DNA. Ultrastructural cytochemistry after staining for catalase activity, revealed the presence of peroxisomes in undifferentiated stem cells located in the crypt region. Morphometry indicated that the number of these organelles increased as intestinal epithelial cells differentiate. Catalase activity was higher in the crypt cells than in the mature enterocytes harvested from villus tips. On the other hand, an increasing gradient of activity was observed from crypts to villus tips for peroxisomal oxidases, i.e. fatty acyl coA oxidase, D-amino acid oxidase and polyamine oxidase. These findings indicate that biogenesis of peroxisomes occurs during migration of intestinal epithelial cells along the crypt-villus axis and that peroxisomal oxidases contribute substantially to the biochemical maturation of enterocytes.

Peroxisomes in human colon carcinomas. A cytochemical and biochemical study

The presence of peroxisomes and their enzymic content were investigated and compared in healthy and neoplastic human colon epithelial cells using cytochemical studies at the ultrastructural level as well as biochemical analyses. Catalase-positive organelles were found to be more numerous in normal than in colonic neoplastic cells. Biochemical assays revealed that no D-aminoacid oxidase or L-alpha-hydroxyacid oxidase activity was detected in normal or tumor tissues. The specific activities of catalase, fatty-acyl CoA oxidase and enoyl-CoA hydratase/3 hydroxyacyl-CoA dehydrogenase (the so-called peroxisomal bifunctional enzyme of the beta-oxidation system) were found to be diminished in carcinoma cells compared with the control tissue. The fall in catalase activity correlated well with tumor stage according to Dukes, suggesting that this peroxisomal enzyme could be used as a potential prognostic marker.

Coenzyme specificity of mammalian liver D-glycerate dehydrogenase

D-Glycerate dehydrogenase (glyoxylate reductase) was partially purified from rat liver by anion- and cation-exchange chromatography. When assayed in the direction of D-glycerate or glycolate formation, the enzyme was inhibited by high (greater than or equal to 0.5 mM), unphysiological concentrations of hydroxypyruvate or glyoxylate much more potently in the presence of NADPH than in the presence of NADH. However, the dehydrogenase displayed a much greater affinity for NADPH (Km less than 1 microM) than for NADH (Km = 48-153 microM). Furthermore, NADP was over 1000-fold more potent than NAD in inhibiting the enzyme competitively with respect to NADH. NADP also inhibited the reaction competitively with respect to NADPH whereas NAD, at concentrations of up to 10 mM had no inhibitory effect. When measured by the formation of hydroxypyruvate from D-glycerate, the enzyme also displayed a much greater affinity for NADP than for NAD. These properties indicate that liver D-glycerate dehydrogenase functions physiologically as an NADPH-specific reductase. In agreement with this conclusion, the addition of hydroxypyruvate or glyoxylate to suspensions of rat hepatocytes stimulated the pentose-phosphate pathway. The coenzyme specificity of D-glycerate dehydrogenase is discussed in relation to the biochemical findings made in D-glyceric aciduria and in primary hyperoxaluria type II (L-glyceric aciduria).

Fluorometric determination of alpha-ketosuccinamic acid in rat tissues

A method for the fluorometric determination of alpha-ketosuccinamic acid, the alpha-keto acid analog of asparagine, is described. The procedure involves the hydrolysis of alpha-ketosuccinamate to oxaloacetate by omega-amidase followed by NADH-dependent reduction of oxaloacetate to malate by malate dehydrogenase. A correction for endogenous oxaloacetate is made by using control samples lacking omega-amidase. Of the rat tissues investigated, liver contained the highest concentration, followed by kidney (53 +/- 6 (n = 11) and 18 +/- 3 (n = 3) mumol/kg wet wt, respectively). alpha-Ketosuccinamate was not detected in brain (less than 8 mumol/kg wet wt). Some chemical properties of alpha-ketosuccinamate were investigated. Concentrated solutions of sodium alpha-ketosuccinamate frozen for extended periods and the solid sodium salt of alpha-ketosuccinamate dimer heated to 130 degrees C are converted to at least 10 products by processes involving dimerization, dehydration, and decarboxylation. Isobutane chemical ionization mass spectral analysis (170-230 degrees C) of the free acid monomer yielded similar products. Many of the breakdown products were identified as di- and monoheterocyclic compounds, some of which are known to be of biological importance.

Insoluble uricase in liver peroxisomes of Old World monkeys

1. Subcellular localization form and properties of liver uricase of Macaca fascicularis were examined. 2. Uricase was present as the insoluble form in the peroxisomal core. 3. Evidence was obtained to show that the peroxisomal core is uricase itself. 4. The number and mol. wts of the subunits of the enzyme were identical to those of rat liver uricase. 5. The same results were also obtained for liver uricase of Macaca fuscata.

The role of peroxisomes in mammalian cellular metabolism

Peroxisomes, which are widely distributed in mammalian tissues, carry out several important functions in cellular metabolism. Production of alkylglycerol-3-phosphate, a key intermediate in the synthesis of plasmalogens and other ether lipids, occurs in the peroxisome. A fatty acid beta-oxidation system with significant differences from mitochondrial beta-oxidation is also found in the peroxisomes; the acetyl-CoA produced is used for synthetic reactions. This pathway has a particularly important physiological role in the oxidation of very long chain fatty acids and the side chain of cholesterol. Peroxisomes also possess a number of oxidases that produce H2O2 which is decomposed by peroxisomal catalase. The function of this peroxisomal respiratory pathway is disposal of excess reducing equivalents, protection of the cell against H2O2 and possibly a role in thermogenesis in brown adipose tissue. Other peroxisomal functions include a role in gluconeogenesis and in purine and polyamine catabolism. Some enzymes of peroxisomes can be induced by dietary, hormonal and other physiological changes. The entire organelle proliferates under certain of these conditions.

Hydroxyproline metabolism by the rat kidney: distribution of renal enzymes of hydroxyproline catabolism and renal conversion of hydroxyproline to glycine and serine

The metabolism of hydroxyproline by the rat kidney leads to the production of significant quantities of both glycine and serine. This process was observed in both the isolated perfused kidney and in isolated cortical tubule suspensions. The rate of hydroxyproline metabolism was increased in both preparations by the addition of alanine. The distribution of hydroxyproline oxidase, hydroxyoxoglutarate aldolase and alanine-glyoxalate transaminase were determined in detail. All three enzymes were found exclusively in the renal cortex where they were restricted to the mitochondria. Cortical tubule fractionation studies indicated that the enzymes are located in the proximal convoluted and proximal straight segments at the nephron. The results suggest that hydroxyproline degradation could contribute significantly to the renal synthesis of serine.

Kinetic properties of rat kidney D-amino acid oxidase associated with peroxisomes

In contrast to hog kidney D-amino acid oxidase, the v vs s plots of D-amino acid oxidase in homogenized rat kidney did not have the form of a rectangular hyperbola, and showed an apparent negative cooperativity. After subcellular fractionation of rat kidney, both of the oxidases in the supernatant fraction and the peroxisomal fraction showed Michaelis-Menten type kinetics. The Km values for D-alanine and D-proline of the peroxisomal fraction were significantly lower than those of the supernatant fraction. The partially purified enzyme from the peroxisomal fraction showed the same kinetic properties as the supernatant fraction. These facts suggest that the two types of rat kidney D-amino acid oxidase were originally identical and that some interaction between the enzyme and peroxisomes is physiologically important for the function of the enzyme.

Transaminative metabolism of alpha-amino-n-butyrate in rats

alpha-Amino-n-butyrate metabolism was studied in a rt tissue homogenate system using L-[1-14C] alpha-amino-n-butyrate. Transamination was found to be the major route in the liver for the metabolism of L-alpha-amino-n-butyrate based on 44.8 mumoles/g/h of [1-14-C] alpha-ketobutyrate formed in the presence of pyruvate versus 1.2 mumoles/g/h without pyruvate. The pH optimum for the reaction was 9.2. The abilities of other alpha-keto acids to act as a cosubstrate relative to pyruvate were (%): pyruvate, 100; alpha-ketobutyrate, 80; alpha-keto-gamma-methiolbutyrate, 15; phenylpyruvate, 14; alpha-ketoglutarate, p-hydroxyphenylpyruvate and the alpha-keto analogs of the branched-chain amino acids, all less than 10. The apparent Km for alpha-amino-n-butyrate in the liver homogenate was approximately 38 mM and the Km for pyruvate was 5 mM. Kidney was found to have about twice the activity as liver. Activities in brain, heart, diaphragm, muscle and small intestine were negligible. With the exception of serine, no other added amino acids could compete effectively with alpha-amino-n-butyrate for transamination in the rat liver homogenate system. Activity in rat liver was inhibited by aminooxyacetic acid and cycloserine. These results indicate that alpha-amino-n-butyrate is metabolized primarily by a transaminase reaction with pyruvate which occurs almost exclusively in liver and kidney.

Intraperoxisomal and intramitochondrial localization, and assay of pyruvate (glyoxylate) aminotransferase from rat liver

Pyruvate (glyoxylate) aminotransferase is found in the peroxisomal and mitochondrial matrices, and in soluble fractions of rat liver homogenates. Soluble activity is from broken peroxisomes. Differential solubility of the mitochondrial and peroxisomal enzymes in digitonin can be used to assay the two activities separately.