Aminotransferases in Peroxisomes from Spinach Leaves

Spectrophotometric Assays for Measuring Photorespiratory Glutamate:Glyoxylate and Serine:Glyoxylate Aminotransferase Reactions

Glutamate:glyoxylate aminotransferase (GGAT; EC 2.6.1.4) and serine:glyoxylate aminotransferase activities (SGAT; EC 2.6.1.45) are central photorespiratory reactions within plant peroxisomes. Both enzymatic reactions convert glyoxylate, a product of glycolate oxidase, to glycine, a substrate of the mitochondrial glycine decarboxylase complex. The GGAT reaction uses glutamate as an amino group donor and also produces α-ketoglutarate, which is recycled to glutamate in plastids by ferredoxin-dependent glutamate synthase. Using serine, a product of mitochondrial serine hydroxymethyltransferase, as an amino group donor, the SGAT reaction also produces hydroxypyruvate, a substrate of hydroxypyruvate reductase. The activities of these photorespiratory aminotransferases can be measured using indirect, coupled, spectrophotometric assays, detailed herein.

Low CO2 induces urea cycle intermediate accumulation in Arabidopsis thaliana

The non-proteinogenic amino acid ornithine links several stress response pathways. From a previous study we know that ornithine accumulates in response to low CO2. To investigate ornithine accumulation in plants, we shifted plants to either low CO2 or low light. Both conditions increased carbon limitation, but only low CO2 also increased the rate of photorespiration. Changes in metabolite profiles of light- and CO2-limited plants were quite similar. Several amino acids that are known markers of senescence accumulated strongly under both conditions. However, urea cycle intermediates respond differently between the two treatments. While the levels of both ornithine and citrulline were much higher in plants shifted to 100 ppm CO2 compared to those kept in 400 ppm CO2, their metabolite abundance did not significantly change in response to a light limitation. Furthermore, both ornithine and citrulline accumulation is independent from sugar starvation. Exogenous supplied sugar did not significantly change the accumulation of the two metabolites in low CO2-stressed plants, while the accumulation of other amino acids was reduced by about 50%. Gene expression measurements showed a reduction of the entire arginine biosynthetic pathway in response to low CO2. Genes in both proline biosynthesis and degradation were induced. Hence, proline did not accumulate in response to low CO2 like observed for many other stresses. We propose that excess of nitrogen re-fixed during photorespiration can be alternatively stored in ornithine and citrulline under low CO2 conditions. Furthermore, ornithine is converted to pyrroline-5-carboxylate by the action of δOAT.

High serine:glyoxylate aminotransferase activity lowers leaf daytime serine levels, inducing the phosphoserine pathway in Arabidopsis

Serine:glyoxylate aminotransferase (SGAT) converts glyoxylate and serine to glycine and hydroxypyruvate during photorespiration. Besides this, SGAT operates with several other substrates including asparagine. The impact of this enzymatic promiscuity on plant metabolism, particularly photorespiration and serine biosynthesis, is poorly understood. We found that elevated SGAT activity causes surprisingly clear changes in metabolism and interferes with photosynthetic CO2 uptake and biomass accumulation of Arabidopsis. The faster serine turnover during photorespiration progressively lowers day-time leaf serine contents and in turn induces the phosphoserine pathway. Transcriptional upregulation of this additional route of serine biosynthesis occurs already during the day but particularly at night, efficiently counteracting night-time serine depletion. Additionally, higher SGAT activity results in an increased use of asparagine as the external donor of amino groups to the photorespiratory pathway but does not alter leaf asparagine content at night. These results suggest leaf SGAT activity needs to be dynamically adjusted to ensure (i) variable flux through the photorespiratory pathway at a minimal consumption of asparagine and (ii) adequate serine levels for other cellular metabolism.

Peroxisomal alanine: glyoxylate aminotransferase AGT1 is indispensable for appressorium function of the rice blast pathogen, Magnaporthe oryzae

The role of β-oxidation and the glyoxylate cycle in fungal pathogenesis is well documented. However, an ambiguity still remains over their interaction in peroxisomes to facilitate fungal pathogenicity and virulence. In this report, we characterize a gene encoding an alanine, glyoxylate aminotransferase 1 (AGT1) in Magnaporthe oryzae, the causative agent of rice blast disease, and demonstrate that AGT1 is required for pathogenicity of M. oryzae. Targeted deletion of AGT1 resulted in the failure of penetration via appressoria; therefore, mutants lacking the gene were unable to induce blast symptoms on the hosts rice and barley. This penetration failure may be associated with a disruption in lipid mobilization during conidial germination as turgor generation in the appressorium requires mobilization of lipid reserves from the conidium. Analysis of enhanced green fluorescent protein expression using the transcriptional and translational fusion with the AGT1 promoter and open reading frame, respectively, revealed that AGT1 expressed constitutively in all in vitro grown cell types and during in planta colonization, and localized in peroxisomes. Peroxisomal localization was further confirmed by colocalization with red fluorescent protein fused with the peroxisomal targeting signal 1. Surprisingly, conidia produced by the Δagt1 mutant were unable to form appressoria on artificial inductive surfaces, even after prolonged incubation. When supplemented with nicotinamide adenine dinucleotide (NAD⁺)+pyruvate, appressorium formation was restored on an artificial inductive surface. Taken together, our data indicate that AGT1-dependent pyruvate formation by transferring an amino group of alanine to glyoxylate, an intermediate of the glyoxylate cycle is required for lipid mobilization and utilization. This pyruvate can be converted to non-fermentable carbon sources, which may require reoxidation of NADH generated by the β-oxidation of fatty acids to NAD⁺ in peroxisomes. Therefore, it may provide a means to maintain redox homeostasis in appressoria.

Properties of serine:glyoxylate aminotransferase purified from Arabidopsis thaliana leaves

The photorespiratory enzyme L-serine:glyoxylate aminotransferase (SGAT; EC 2.6.1.45) was purified from Arabidopsis thaliana leaves. The final enzyme was approximately 80% pure as revealed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis with silver staining. The identity of the enzyme was confirmed by LC/MS/MS analysis. The molecular mass estimated by gel filtration chromatography on Sephadex G-150 under non-denaturing conditions, mass spectrometry (matrix-assisted laser desorption/ionization/time of flight technique) and sodium dodecyl sulfate-polyacrylamide gel electrophoresis was 82.4 kDa, 42.0 kDa, and 39.8 kDa, respectively, indicating dimer as the active form. The optimum pH value was 9.2. The enzyme activity was inhibited by aminooxyacetate and beta-chloro-L-alanine both compounds reacting with the carbonyl group of pyridoxal phosphate. The enzyme's transaminating activity with L-alanine and glyoxylate as substrates was approximately 55% of that observed with L-serine and glyoxylate. The lower Km value (1.25 mM) for L-alanine, compared with that of other plant SGATs, and the kcat/Km(Ala) ratio being approximately 2-fold higher than kcat/Km(Ser) suggested that, during photorespiration, Ala and Ser are used by Arabidopsis SGAT with equal efficiency as amino group donors for glyoxylate. The equilibrium constant (Keq), derived from the Haldane relation, for the transamination reaction between L-serine and glyoxylate with the formation of hydroxypyruvate and glycine was 79.1, strongly favoring glycine synthesis. However, it was accompanied by a low Km value of 2.83 mM for glycine. A comparison of some kinetic properties of the studied enzymes with the recombinant Arabidopsis SGATs previously obtained revealed substantial differences. The ratio of the velocity of the transamination reaction with L-alanine and glyoxylate as substrates versus that with L-serine and glyoxylate was 1:1.8 for the native enzyme, whereas it was 1:7 for the recombinant SGAT. Native SGAT showed a much lower Km value for L-alanine compared to the recombinant enzyme.

The tryptophan oxidation pathway in mosquitoes with emphasis on xanthurenic acid biosynthesis

Oxidation of tryptophan to kynurenine and 3-hydroxykynurenine (3-HK) is the major catabolic pathway in mosquitoes. However, 3-HK is oxidized easily under physiological conditions, resulting in the production of reactive radical species. To overcome this problem, mosquitoes have developed an efficient mechanism to prevent 3-HK from accumulating by converting this chemically reactive compound to the chemically stable xanthurenic acid. Interestingly, 3-HK is a precursor for the production of compound eye pigments during the pupal and early adult stages; consequently, mosquitoes need to preserve and transport 3-HK for compound eye pigmentation in pupae and adults. This review summarizes the tryptophan oxidation pathway, compares and contrasts the mosquito tryptophan oxidation pathway with other model species, and discusses possible driving forces leading to the functional adaptation and evolution of enzymes involved in the mosquito tryptophan oxidation pathway.

Crystal structures of Aedes aegypti alanine glyoxylate aminotransferase

Mosquitoes are unique in having evolved two alanine glyoxylate aminotransferases (AGTs). One is 3-hydroxykynurenine transaminase (HKT), which is primarily responsible for catalyzing the transamination of 3-hydroxykynurenine (3-HK) to xanthurenic acid (XA). Interestingly, XA is used by malaria parasites as a chemical trigger for their development within the mosquito. This 3-HK to XA conversion is considered the major mechanism mosquitoes use to detoxify the chemically reactive and potentially toxic 3-HK. The other AGT is a typical dipteran insect AGT and is specific for converting glyoxylic acid to glycine. Here we report the 1.75A high-resolution three-dimensional crystal structure of AGT from the mosquito Aedes aegypti (AeAGT) and structures of its complexes with reactants glyoxylic acid and alanine at 1.75 and 2.1A resolution, respectively. This is the first time that the three-dimensional crystal structures of an AGT with its amino acceptor, glyoxylic acid, and amino donor, alanine, have been determined. The protein is dimeric and adopts the type I-fold of pyridoxal 5-phosphate (PLP)-dependent aminotransferases. The PLP co-factor is covalently bound to the active site in the crystal structure, and its binding site is similar to those of other AGTs. The comparison of the AeAGT-glyoxylic acid structure with other AGT structures revealed that these glyoxylic acid binding residues are conserved in most AGTs. Comparison of the AeAGT-alanine structure with that of the Anopheles HKT-inhibitor complex suggests that a Ser-Asn-Phe motif in the latter may be responsible for the substrate specificity of HKT enzymes for 3-HK.

Plant peroxisomes respire in the light: some gaps of the photorespiratory C2 cycle have become filled--others remain

The most prominent role of peroxisomes in photosynthetic plant tissues is their participation in photorespiration, a process also known as the oxidative C2 cycle or the oxidative photosynthetic carbon cycle. Photorespiration is an essential process in land plants, as evident from the conditionally lethal phenotype of mutants deficient in enzymes or transport proteins involved in this pathway. The oxidative C2 cycle is a salvage pathway for phosphoglycolate, the product of the oxygenase activity of ribulose 1,5-bisphosphate carboxylase/oxygenase (RubisCO), to the Calvin cycle intermediate phosphoglycerate. The pathway is highly compartmentalized and involves reactions in chloroplasts, peroxisomes, and mitochondria. The H2O2-producing enzyme glycolate oxidase, catalase, and several aminotransferases of the photorespiratory cycle are located in peroxisomes, with catalase representing the major constituent of the peroxisomal matrix in photosynthetic tissues. Although photorespiration is of major importance for photosynthesis, the identification of the enzymes involved in this process has only recently been completed. Only little is known about the metabolite transporters for the exchange of photorespiratory intermediates between peroxisomes and the other organelles involved, and about the regulation of the photorespiratory pathway. This review highlights recent developments in understanding photorespiration and identifies remaining gaps in our knowledge of this important metabolic pathway.

Evolution of two alanine glyoxylate aminotransferases in mosquito

In the mosquito, transamination of 3-HK (3-hydroxykynurenine) to XA (xanthurenic acid) is catalysed by an AGT (alanine glyoxylate aminotransferase) and is the major branch pathway of tryptophan metabolism. Interestingly, malaria parasites hijack this pathway to use XA as a chemical signal for development in the mosquito. Here, we report that the mosquito has two AGT isoenzymes. One is the previously cloned AeHKT [Aedes aegypti HKT (3-HK transaminase)] [Han, Fang and Li (2002) J. Biol. Chem. 277, 15781-15787], similar to hAGT (human AGT), which primarily catalyses 3-HK to XA in mosquitoes, and the other is a typical dipteran insect AGT. We cloned the second AGT from Ae. aegypti mosquitoes [AeAGT (Ae. aegypti AGT)], overexpressed the enzyme in baculovirus/insect cells and determined its biochemical characteristics. We also expressed hAGT for a comparative study. The new cloned AeAGT is highly substrate-specific when compared with hAGT and the previously reported AeHKT and Drosophila AGT, and is translated mainly in pupae and adults, which contrasts with AeHKT that is expressed primarily in larvae. Our results suggest that the physiological requirements of mosquitoes and the interaction between the mosquito and its host appear to be the driving force in mosquito AGT evolution.

Enzymatic and mechanistic studies on the formation of N-phenylglycolohydroxamic acid from nitrosobenzene and pyruvate in spinach leaf homogenate

The biotransformation mechanism of an unknown metabolite formed enzymatically from nitrosobenzene (NOB) and pyruvate in spinach (Spinacea oleracea L.) was investigated using spinach leaf homogenate. The unknown metabolite was identified as N-phenylglycolohydroxamic acid (PGA). The activity of PGA formation was decreased by l-alanine, increased by l-serine, and completely inhibited by aminooxyacetic acid, an inhibitor of transaminases. These results indicate that the transaminase participates in PGA formation. Indeed, hydroxypyruvate and alanine were produced in the transamination between pyruvate and serine. Hydroxypyruvate served as a direct-acting glycoloyl donor for PGA formation. A good correlation between the activities of the 200 g supernatant of spinach homogenate and commercial yeast transketolase for PGA formation from several glycoloyl donors was obtained. These results suggest the following mechanism for PGA formation from NOB and pyruvate: transamination of l-serine into hydroxypyruvate, which serves as a glycoloyl donor to NOB.

Identification of photorespiratory glutamate:glyoxylate aminotransferase (GGAT) gene in Arabidopsis

In the photorespiratory process, peroxisomal glutamate:glyoxylate aminotransferase (GGAT) catalyzes the reaction of glutamate and glyoxylate to 2-oxoglutarate and glycine. Although GGAT has been assumed to play important roles for the transamination in photorespiratory carbon cycles, the gene encoding GGAT has not been identified. Here, we report that an alanine:2-oxoglutarate aminotransferase (AOAT)-like protein functions as GGAT in peroxisomes. Arabidopsis has four genes encoding AOAT-like proteins and two of them (namely AOAT1 and AOAT2) contain peroxisomal targeting signal 1 (PTS1). The expression analysis of mRNA encoding AOATs and EST information suggested that AOAT1 was the major protein in green leaves. When AOAT1 fused to green fluorescent protein (GFP) was expressed in BY-2 cells, it was found to be localized to peroxisomes depending on PTS1. By screening of Arabidopsis T-DNA insertion lines, an AOAT1 knockout line (aoat1-1) was isolated. The activity of GGAT and alanine:glyoxylate aminotransferase (AGAT) in the above-ground tissues of aoat1-1 was reduced drastically and, AOAT and glutamate:pyruvate aminotransferase (GPAT) activity also decreased. Peroxisomal GGAT was detected in the wild type but not in aoat1-1. The growth rate was repressed in aoat1-1 grown under high irradiation or without sugar, though differences were slight in aoat1-1 grown under low irradiation, high-CO2 (0.3%) or high-sugar (3% sucrose) conditions. These phenotypes resembled those of photorespiration-deficient mutants. Glutamate levels increased and serine levels decreased in aoat1-1 grown in normal air conditions. Based on these results, it was concluded that AOAT1 is targeted to peroxisomes, functions as a photorespiratory GGAT, plays a markedly important role for plant growth and the metabolism of amino acids.

Alanine aminotransferase homologs catalyze the glutamate:glyoxylate aminotransferase reaction in peroxisomes of Arabidopsis

Plant peroxisomal glyoxylate aminotransferases play central roles within the photorespiratory pathway. Genes encoding glyoxylate aminotransferases have been isolated from several animals and microbes, but only recently have plant homologs been identified. Three Arabidopsis homologs of alanine (Ala):glyoxylate aminotransferase 2 (AGT2) contain a putative type 1 peroxisomal targeting signal (PTS1), but the metabolic significance of these AGT2 homologs is unknown. GGT1 and GGT2 are Ala aminotransferase (AlaAT) homologs from Arabidopsis that represent another type of glyoxylate aminotransferase. These proteins are class I aminotransferases, each containing a putative PTS1. GGT1 and GGT2 are members of a small family of AlaATs in Arabidopsis. When expressed as recombinant proteins in Escherichia coli, GGT1 and GGT2 displayed biochemical characteristics very similar to one another, and to the Arabidopsis protein purified from leaves. Four aminotransferase activities were specifically associated with GGT1 and GGT2, using the substrate pairs glutamate (Glu):glyoxylate, Ala:glyoxylate, Glu:pyruvate, and Ala:2-oxoglutarate. GGT1 and GGT2 may have partially redundant functions; transcripts of both genes were detected in many of the same tissues. Although Glu:glyoxylate aminotransferase (GGT) activity has been observed in several locations in different plants and algae, including the cytoplasm and mitochondria, our subcellular fractionation data indicate that GGT activity was exclusively peroxisomal in Arabidopsis. Thus, glyoxylate aminotransferase reactions in plant peroxisomes appear to be catalyzed by at least two distinct types of aminotransferases: an AGT1 homolog with serine:glyoxylate aminotransferase activity (A.H. Liepman, L.J. Olsen [2001] Plant J 25: 487-498), and a pair of closely related, potentially redundant AlaAT homologs with GGT activity.

Peroxisomal alanine : glyoxylate aminotransferase (AGT1) is a photorespiratory enzyme with multiple substrates in Arabidopsis thaliana

At least two glyoxylate aminotransferases are hypothesized to participate in the steps of photorespiration located in peroxisomes. Until recently, however, genes encoding these enzymes had not been identified. We describe the isolation and characterization of an alanine : glyoxylate aminotransferase (AGT1, formerly AGT) cDNA from Arabidopsis thaliana. Southern blot analysis confirmed that Arabidopsis AGT1 is encoded by a single gene. Homologs of this class IV aminotransferase are also known in other plants, animals, and methylotrophic bacteria, suggesting an ancient evolutionary origin of this enzyme. AGT1 transcripts were present in all tissues of Arabidopsis, but were most abundant in green, leafy tissues. Purified, recombinant Arabidopsis AGT1 expressed in Escherichia coli catalyzed three transamination reactions using the following amino donor : acceptor combinations: alanine : glyoxylate, serine : glyoxylate, and serine : pyruvate. AGT1 had the highest specific activity with the serine : glyoxylate transamination, and apparent Km measurements indicate that this is the preferred in vivo reaction. In vitro import experiments and subcellular fractionations localized AGT1 to peroxisomes. Sequence analysis of the photorespiratory sat mutants revealed a single nucleotide substitution in the AGT1 gene from these plants. This transition mutation is predicted to result in a proline-to-leucine substitution at residue 251 of AGT1. When this mutation was engineered into the recombinant AGT1 protein, enzymatic activity using all three donor : acceptor pairs was abolished. We conclude that Arabidopsis AGT1 is a peroxisomal photorespiratory enzyme that catalyzes transamination reactions with multiple substrates.

L-lactate dehydrogenase A4- and A3B isoforms are bona fide peroxisomal enzymes in rat liver. Evidence for involvement in intraperoxisomal NADH reoxidation

The subcellular localization of l-lactate dehydrogenase (LDH) in rat hepatocytes has been studied by analytical subcellular fractionation combined with the immunodetection of LDH in isolated subcellular fractions and liver sections by immunoblotting and immunoelectron microscopy. The results clearly demonstrate the presence of LDH in the matrix of peroxisomes in addition to the cytosol. Both cytosolic and peroxisomal LDH subunits have the same molecular mass (35.0 kDa) and show comparable cross-reactivity with an anti-cytosolic LDH antibody. As revealed by activity staining or immunoblotting after isoelectric focussing, both intracellular compartments contain the same liver-specific LDH-isoforms (LDH-A4 > LDH-A3B) with the peroxisomes comprising relatively more LDH-A3B than the cytosol. Selective KCl extraction as well as resistance to proteinase K and immunoelectron microscopy revealed that at least 80% of the LDH activity measured in highly purified peroxisomal fractions is due to LDH as a bona fide peroxisomal matrix enzyme. In combination with the data of cell fractionation, this implies that at least 0.5% of the total LDH activity in hepatocytes is present in peroxisomes. Since no other enzymes of the glycolytic pathway (such as phosphoglucomutase, phosphoglucoisomerase, and glyceraldehyde-3-phosphate dehydrogenase) were found in highly purified peroxisomal fractions, it does not seem that LDH in peroxisomes participates in glycolysis. Instead, the marked elevation of LDH in peroxisomes of rats treated with the hypolipidemic drug bezafibrate, concomitantly to the induction of the peroxisomal beta-oxidation enzymes, strongly suggests that intraperoxisomal LDH may be involved in the reoxidation of NADH generated by the beta-oxidation pathway. The interaction of LDH and the peroxisomal palmitoyl-CoA beta-oxidation system could be verified in a modified beta-oxidation assay by adding increasing amounts of pyruvate to the standard assay mixture and recording the change of NADH production rates. A dose-dependent decrease of NADH produced was simulated with the lowest NADH value found at maximal LDH activity. The addition of oxamic acid, a specific inhibitor of LDH, to the system or inhibition of LDH by high pyruvate levels (up to 20 mm) restored the NADH values to control levels. A direct effect of pyruvate on palmitoyl-CoA oxidase and enoyl-CoA hydratase was excluded by measuring those enzymes individually in separate assays. An LDH-based shuttle across the peroxisomal membrane should provide an efficient system to regulate intraperoxisomal NAD+/NADH levels and maintain the flux of fatty acids through the peroxisomal beta-oxidation spiral.

Malate dehydrogenase isoenzymes: cellular locations and role in the flow of metabolites between the cytoplasm and cell organelles

Malate dehydrogenases belong to the most active enzymes in glyoxysomes, mitochondria, peroxisomes, chloroplasts and the cytosol. In this review, the properties and the role of the isoenzymes in different compartments of the cell are compared, with emphasis on molecular biological aspects. Structure and function of malate dehydrogenase isoenzymes from plants, mammalian cells and ascomycetes (yeast, Neurospora) are considered. Significant information on evolutionary aspects and characterisation of functional domains of the enzymes emanates from bacterial malate and lactate dehydrogenases modified by protein engineering. The review endeavours to give up-to-date information on the biogenesis and intracellular targeting of malate dehydrogenase isoenzymes as well as enzymes cooperating with them in the flow of metabolites of a given pathway and organelle.

The value of mutants unable to carry out photorespiration

Manipulation of the CO2 concentration of the atmosphere allows the selection of photorespiratory mutants from populations of seeds treated with powerful mutagens such as sodium azide. So far, barley lines deficient in activity of phosphoglycolate phosphatase, catalase, the glycine to serine conversion, glutamine synthetase, glutamate synthase, 2-oxoglutarate uptake and serine: glyoxylate aminotransferase have been isolated. In addition one line of pea lacking glutamate synthase activity and one barley line containing reduced levels of Rubisco are available. The characteristics of these mutations are described and compared with similar mutants isolated from populations of Arabidopsis. As yet, no mutant lacking glutamine synthetase activity has been isolated from Arabidopsis and possible reasons for this difference between barley and Arabidopsis are discussed. The value of these mutant plants in the elucidation of the mechanism of photorespiration and its relationships with CO2 fixation and amino acid metabolism are highlighted.

Photorespiratory N donors, aminotransferase specificity and photosynthesis in a mutant of barley deficient in serine: glyoxylate aminotransferase activity

A mutant of Hordeum vulgare L. (LaPr 85/84) deficient in serine: glyoxylate aminotransferase (EC 2.6.1.45) activity has been isolated. The plant also lacks serine: pyruvate aminotransferase and asparagine: glyoxylate aminotransferase activities. Genetic analysis of the mutation strongly indicates that these three activities are all carried on the same enzyme protein. The mutant is incapable of normal rates of photosynthesis in air but can be maintained at 0.7% CO2. The rate of photosynthesis cannot be restored by supplying hydroxypyruvate, glycerate, glutamate or ammonium sulphate through the xylem stream. This photorespiratory mutant demonstrates convincingly that photorespiration still occurs under conditions in which photosynthesis becomes insensitive to oxygen levels. Two major peaks and one minor peak of serine: glyoxylate aminotransferase activity can be separated in extracts of leaves of wild-type barley by diethylaminoethyl-sephacel chromatography. All three peaks are missing from the mutant, LaPr 85/84. The mutant showed the expected rate (50%) of ammonia release during photorespiration but produced CO2 at twice the wild-type rate when it was fed [(14)C]glyoxylate. The large accumulation of serine detected in the mutant under photorespiratory conditions shows the importance of the enzyme activity in vivo. The effect of the mutation on transient changes in chlorophyll a fluorescence initiated by changing the atmospheric CO2 concentration are presented and the role of the enzyme activity under nonphotorespiratory conditions is discussed.

Light-stimulated accumulation of the peroxisomal enzymes hydroxypyruvate reductase and serine:glyoxylate aminotransferase and their translatable mRNAs in cotyledons of cucumber seedlings

The development of peroxisomal enzymes in cotyledons of cucumber seedlings is strongly dependent on light. In light-grown seedlings, activities of two peroxisomal enzymes, hydroxypyruvate reductase (HPR) and serine: glyoxylate aminotransferase (SGAT), were barely detectable until three days postimbibition, after which time both activities increased rapidly and linearly for at least three days. In the dark, the activities of these enzymes increased slightly over the same time period, but only to about 5% to 10% of 7-day light-induced levels. When 51/2-day dark-grown seedlings were transferred into white light, activities of HPR and SGAT began to increase after approximately 8 h. HPR protein was shown by an immunoprecipitation assay to increase concurrently with enzymatic activity in both light- and dark-grown cotyledons. Immunoblotting results suggested that the amounts of SGAT-A and SGAT-B, the two subunits of SGAT, also developed along with SGAT activity. The relative levels of translatable mRNAs encoding HPR, SGAT-A, and SGAT-B were also light-dependent, and increased with a developmental pattern similar to enzyme activity and protein levels in light- and dark-grown cotyledons. In 51/2-day dark-grown cotyledons that were transferred to the light, translatable mRNAs for SGAT-A and SGAT-B began to increase within 1 h of illumination and continued of increase rapidly and linearly for the next 24 h in the light to a new steady-state level that was 45 times that of dark controls. Translatable HPR mRNA exhibited a biphasic pattern of accumulation, with a three-fold increase during the first 6 h of illumination, followed by an additional six-fold increase between 8 and 24 h. The accumulation of translationally active mRNA for both enzymes preceded the accumulation of the corresponding protein and enzyme activity by about 8 h. Our data suggest that the rise in enzyme activity depends on an increase in translatable mRNA for these enzymes and is regulated at a pretranslational level, most likely involving transcription of new mRNA.

Reactivity of glyoxylate with hydrogen perioxide and simulation of the glycolate pathway of C3 plants and Euglena

The nonenzymatic reaction of glyoxylate and H2O2 was measured under physiological conditions of the pH and concentrations of reactants. The reaction of glyoxylate and H2O2 was secondorder, with a rate constant of 2.27 l mol(-1) s(-1) at pH 8.0 and 25° C. The rate constant increased by 4.4 times in the presence of Zn(2+) and doubled at 35°C. We propose a mechanism for the reaction between glyoxylate and H2O2. From a comparison of the rates of H2O2 decomposition by catalase and the reaction with glyoxylate, we conclude that H2O2 produced during glycolate oxidation in peroxisomes is decomposed by catalase but not by the reaction with glyoxylate, and that photorespiratory CO2 originates from glycine, but not from glyoxylate, in C3 plants. Simulation using the above rate constant and reported kinetic parameters leads to the same conclusion, and also makes it clear that alanine is a satisfactory amino donor in the conversion of glyoxylate to glycine. Some serine might be decomposed to give glycine and methylene-tetrahydrofolate; the latter is ultimately oxidized to CO2. In the simulation of the glycolate pathway of Euglena, the rate constant was high enough to ensure the decarboxylation of glyoxylate by H2O2 to produce photorespiratory CO2 during the glycolate metabolism of this organism.

Subcellular distribution of the enzymes of the malate-aspartate shuttle in rat heart and effect of experimental cardiac hypertrophy

The effect of experimental cardiac hypertrophy on the enzymes of the malate-aspartate shuttle aspartate aminotransferase (AAT) and malate dehydrogenase (MDH) was studied. Aortic constriction in adult rats resulted in 25% cardiac hypertrophy in 21/2-3 weeks. Total DNA (mg per heart) did not change. The proportions of mitochondrial and cytosolic isozymes of AAT and MDH did not change as a result of cardiac hypertrophy. About two-thirds of each enzyme occurred in the mitochondrial form and one-third in the cytosolic form. Total AAT in hypertrophic hearts, in enzyme units per mg DNA, increased by 24% compared to AAT content in the hearts of sham-operated animals. Total MDH did not change. Solubilized protein increased by 20%. Normal hearts contained 10 times more enzyme units of MDH than of AAT. Cardiac growth stimulation induced in newborn rats did not result in specific changes of either enzyme. It is suggested that true cardiac hypertrophy acts as a specific stimulus for the possibly rate-limiting enzyme AAT of the shuttle.

[Aspartate And Alanine Aminotransferase In Fasciola Hepatica]

The activity and distribution of aspartate aminotransferase (EC 2.6.1.1) and alanine aminotransferase (EC 2.6.1.2) in adult Fasciola hepatica have been studied. Fasciola hepatica was fractionated by differential centrifugation into nuclear, mitochondrial and cytosolic fractions. The activity of GOT and GPT was measured by the method of Reitman and Frankel. Isozyme patterns of those enzyme were also examined by DEAE-cellulose column chromatography. The results obtained were as follows: 1. The activity of aspartate and alanine aminotransferase was about 0.55 unit and 0.92 unit per 1 g of Fasciola hepatica, respectively. 2. The activity of those enzymes was relatively low compared with those in mammalian tissues. 3. The distribution of aspartate aminotransferase in the subcellular organelles showed that 71 % of the activity was in cytosolic, 24 % in mitochondrial and 5 % was in nuclear fraction. 4. About 22 % of the total alanine aminotransferase activity was found in the mitochondrial fraction, about 66 percent in the cytosolic fraction. 5. Aspartate aminotransferase from cytosolic fraction was separated into two types of isozymes, whereas alanine aminotransferase from cytosolic fraction gave only one active peak on DEAE-cellulose column chromatography.

Glyoxylate decarboxylation during glycollate oxidation by pea leaf extracts: significance of glyoxylate and extract concentrations

Hydrogen peroxide-dependent glyoxylate decarboxylation occurring during glycollate oxidation by pea leaf extracts (Pisum sativum L.) has been studied in relation to the effects of glyoxylate and extract concentration. With a saturating concentration of glycollate, decarboxylation was greatly stimulated by raising the glyoxylate concentration; at 30°C and with approx. 0.04 nkat of glycollate oxidase (as leaf extract) in the reaction mixture, CO2 release in the presence of 5 mM glycollate and 5 mM glyoxylate was equal to about 45% of glycollate oxidation. However, CO2 release at these substrate concentrations was not linearly proportional to the amount of extract supplied and was equal to a diminishing proportion of glycollate oxidation as the amount of extract was increased. This was shown to be due to the low affinity of catalase for H2O2, so that the endogenous catalase was able to destroy a larger proportion of the H2O2 generated at higher extract concentrations. It is argued that although at high glycoxylate concentrations (5-10 mM) in vitro, glyoxylate decarboxylation can be made to equal more than a third of the glycollate oxidised, less than 10% of the glyoxylate generated in vivo is likely to be decarboxylated in peroxisomes where high concentrations of glycollate oxidase and catalase are localised and where high concentrations of glyoxylate are unlikely to be maintained.

Purification of glycollate oxidase from greening cucumber cotyledons

Glycollate oxidase (glycollate: oxygen oxidoreductase, EC 1.1.3.1) was purified to apparent homogeneity from crude extracts of greening cucumber cotyledons (Cucumis sat vus). Molecular sieving and chromatofocusing resulted in 700-fold purification and specific activity of 1 μkat mg(-1) protein. The enzyme exhibited a Mr of 180,000, or 700,000, respectively, and is a tetramer or 16-mer made of identical subunits of Mr 43,000. Monospecific antibodies were raised against the homogeneous protein.

Metabolism and decarboxylation of glycollate and serine in leaf peroxisomes

The linked utilization of glycollate and L-serine has been studied in peroxisomal preparations from leaves of spinach beet (Beta vulgaris L.). The generation of glycine from glycollate was found to be balanced by the production of hydroxypyruvate from serine and similarly by 2-oxoglutarate when L-glutamate was substituted for L-serine. In the presence of L-malate and catalytic quantities of NAD(+), about 40% of the hydroxypyruvate was converted further to glycerate, whereas with substrate quantities of NADH, this conversion was almost quantitative. CO2 was released from the carboxyl groups of both glycollate and serine. Since the decarboxylation of both substrates was greatly in creased by the catalase inhibitor, 3-amino-1,2,4-triazole, and abolished by bovine liver catalase, it was attributed to the nonenzymic attack of H2O2, generated in glycollate oxidation, upon glyoxylate and hydroxypyruvate respectively. At 25-30° C, about 10% of the glyoxylate and hydroxypyruvate accumulated was decarboxylated, and the release of CO2 from each keto-acid was related to the amounts present. It is suggested that hydroxypyruvate decarboxylation might contribute significantly to photorespiration and provide a metabolic route for the complete oxidation of glycollate, the magnitude of this contribution depending upon the concentrations of glyoxylate and hydroxypyruvate in the peroxisomes.

Glutamate and serine as competing donors for amination of glyoxylate in leaf peroxisomes

When provided with glycollate, peroxisomal extracts of leaves of spinach beet (Beta vulgaris L. cv.) converted L-serine and L-glutamate to hydroxypyruvate and 2-oxoglutarate respectively. When approximately saturating concentrations of each of these amino acids were incubated separately with glycollate, the utilization of serine was greater than that of glutamate. The utilization of glutamate was substantially reduced by the presence of relatively low concentrations of serine in the reaction mixture, whereas even high concentrations of glutamate caused only small reductions in serine utilization. Over the entire range of concentrations of amino acids examined, serine was invariably the preferred amino-group donor, but this preference was abolished at higher concentrations of glyoxylate. Serine not only competed favourably for glyoxylate but also inhibited L-glutamate: glyoxylate aminotransferase (GGAT), the degree of inhibition depending upon the glyoxylate concentration. Studies of L-serine: glyoxylate aminotransferase (SGAT) and GGAT in partially purified extracts from spinach-beet leaves confirmed that serine competitively inhibited GGAT but glutamate did not affect SGAT. Both enzymes were inhibited by high glyoxylate concentrations, the inhibition being relieved by suitably high concentrations of the appropriate amino acid. It is concluded that at the low glyoxylate concentrations likely to occur in vivo, the preferential utilization of serine would ensure flux through the glycollate pathway to glycerate, but at higher concentrations of glyoxylate, both enzymes could be fully active in glyoxylate amination.