Activity Changes in Selected Enzymes from Soybean Leaves Following Ozone Exposure

Soybeans [Glycine max(L.) Merr.] were harvested at various time periods after a 2-h exposure to either 0 or 0.5 μ1/1 ozone to determine the effects of ozone on selected enzymes. Carbohydrate metabolism was modified by a depression of glyceraldehyde 3-phaosphate dehydrogenase and an activation of glucose 6-phosphate dehydrogenase. Ozone did not alter the levels of RNase, protease, acid phosphatase or esterase as might be expected if ozone enhanced leaf senescence. The activities of phenylalanine ammonia lyase, polyphenol oxidase and peroxidase were initially depressed and then stimulated following the ozone exposure. The reactions of soybeans to an acute ozone stress were more nearly akin to those elicited in response to other stresses than to the process of senescence.     

Enzymatic Activity in Mitochondria from Orobanche

Mitochondria from Orobanche were analysed for the activities of aconitate hydratase, isocitrate dehydrogenase, succinate dehydro-genase, fumarate hydratase, malate dehydrogenase, NADH oxidase, substrate-cytochrome c oxidoreductases, glutamate dehydrogenase, aminotransferases, ATPase and “malic” enzyme. The specific activities of isocitrate dehydrogenase, NADH oxidase, substrate-cytochrome c oxidoreductases and glutamate dehydrogenase in the mitochondria) fraction from parasite tissue compared favourably with those reported for most of the mitochondria from growing and storage tissues. Succinate dehydrogenase, fumarate hydratase and aspartate aminotransferase were of intermediate activity, while aconitate hydratase and malate dehydrogenase had rather low activity, and “malic” enzyme had very low activity in comparison with other preparations. The relevance of these findings in relation to mitochondrial metabolism in the parasite is discussed. No evidence was obtained to suggest any basic abnormality in the biochemical properties of the mitochondria from Orobanche centua which may be correlated with its obligatorily parasitic existence.     

Growth and Metabolism of Soybean as Affected by Paclobutrazol

Paclobutrazol, an experimental growth retardant, was soil-appliedat the rate of 125 or 250 µg active ingredient per 10cm pot to 19 day-old soybean plants. This compound considerablyreduced plant height, leaf area, and stem dry weight. In addition,paclobutrazol-treated plants had numerous thickened lateralroots at the soil surface and had increased chlorophyll andsoluble protein contents compared to controls. During the first14 days after treatment, paclobutrazol increased the activitiesof NAD- and NADP-glyceraldehyde-3-phosphate dehydrogenase andaminotransferases but decreased the activity of nitrate reductase.Net photosynthesis (P_n) of the first and second trifoliatesof treated plants remained fairly constant throughout the studywhile control P_n declined during the latter portion of the experimentalperiod presumably due to leaf senescence. This decline of P_nin controls was accompanied by a decrease in the activitiesof NAD- and NADP-glyceraldehyde-3-phosphate dehydrogenase, nitratereductase, aminotransferases, and NAD malate dehydrogenase.Activities of these enzymes also tended to decline in paclobutrazol-treatedplants, but were still considerably higher than in controlsat the end of the experiment. The activities of RNase, protease,and glutamic dehydrogenase were higher in controls than in treatedplants. Our results suggest that paclobutrazol not only modifiesthe activity of a number of soybean enzymes but also delaysthe onset of senescence, thereby prolonging the period of normalmetabolic activity in a given leaf. ¹ Permanent address: Department of Botany, University of Jodhpur,India.     

Intermediary carbohydrate metabolism in the adult filarial worm Setaria digitata.

Several key enzymes related to carbohydrate metabolism were assayed in Setaria digitata. In the cytosolic fraction pyruvate kinase, phosphoenolpyruvate carboxykinase, malate dehydrogenase, malic enzyme, aspartate transaminase and alanine transaminase were found. Among the TCA cycle enzymes succinate dehydrogenase, fumarate reductase, fumarase (malate dehydration), malate dehydrogenase (malate oxidation and oxaloacetate reduction) and malic enzyme (malate decarboxylation) were detected in the mitochondrial fraction. Only reduced nicotinamide adenine dinucleotide (NADH) dehydrogenase, NADH oxidase and NADH-cytochrome c reductase were found in the mitochondrial fraction. The significance of these results with respect to the metabolic capabilities of the worm are discussed.     

马尾松毛虫蛋白质、核酸酶和羧酸酯酶与耐药性的关系

马尾松毛虫(Dendrolimus punctatus Walker)幼虫蛋白质含量、羧酸酯酶和多酚氧化酶活力与虫龄大小成正相关;氰戊菊酯处理后,兴奋期的蛋白质含量和羧酸酯酶活力上升,到抑制期均降低到正常虫体的水平,而多酚氧化酶的变动不大。正常虫体中脱氧核糖核酸酶(DNase)和核糖核酸酶(RNase)的活力存在差异:DNase从3—5龄幼虫随虫体增大而上升,到6龄时明显降低;RNase与虫龄大小成负相关;氰戊菊酯处理后,DNase便开始下降并低于正常虫体的水平,而RNase在兴奋期上升,到抑制期下降亦低于正常虫体水平。结果说明,除多酚氧化酶外,蛋白质、核酸酶和羧酸酯酶均与耐药性存在一定的相关性,研制酶的抑制剂具有实用意义。根据毒力测定结果,马尾松毛虫幼虫随虫龄增大而耐药力增加,氰戊菊醋对5龄幼虫是3龄的2.8倍,6龄是4龄的2.3倍。因此,掌握在4龄前进行药物防治是合理的。     

Changes in the electron transport chain of pea leaf mitochondria metabolizing malate

Pea leaf mitochondria showed complex kinetics for malate metabolism. O₂ uptake increased as malate concentration increased from 0 to 10 mm, reached a plateau between 10 and 20 mm malate, and then increased again up to 40 mm malate. Analysis of the products of malate oxidation by high-performance liquid chromatography revealed that the first phase of O₂ uptake coincided with the synthesis of both pyruvate and oxalacetate (OAA) while the second phase of O₂ uptake at higher malate levels usually occurred with a large increase in OAA formation. The biphasic response in O₂ uptake and the changing ratios of pyruvate and OAA synthesis did not appear to be the direct result of the differing K_m values of malate dehydrogenase and malic enzyme. Rather, they resulted from thermodynamic properties of these two malate oxidases and the kinetics of the two NADH dehydrogenases found in plant mitochondria. At low malate concentrations the rotenone-sensitive NADH dehydrogenase was active and could accept electrons from both malate oxidases. This NADH dehydrogenase became saturated at about 10 mm malate. At higher malate concentrations the rotenone-insensitive NADH dehydrogenase was increasingly important and its increased electron transport capacity was best exploited by malate dehydrogenase. At the higher malate concentrations an increasing portion of the electrons from malate reduce O₂ through the alternative oxidase. Although this coincided with the second phase of malate-dependent O₂ uptake it was not required for this phase to be seen.     

Organic acid accumulation may inhibit N2 fixation in phosphorus-stressed lupin nodules.

Nodulated lupins (Lupinus angustifolius cv. Wonga) were hydroponically grown under conditions of low phosphate (LP) or adequate phosphate (HP) to assess the effect of phosphoenolpyruvate carboxylase (PEPC)-derived organic acids on nitrogen assimilation in LP nodules. LP conditions are linked to altered organic acid metabolism, by the engagement of PEP metabolism via PEPC. In LP nodules, the enhanced organic acid synthesis may reduce the available organic carbon for nitrogen assimilation. The diversion of carbon between the organic acid- and amino acid pools was assessed through key nodular enzymes and (14)CO(2) metabolism. Under LP conditions, increased rates of organic acid synthesis via PEPC and malate dehydrogenase (MDH), coincided with reduced nitrogen assimilation via aspartate aminotransferase (AAT), aspartate synthetase (AS) and glutamine synthetase (GS)/glutamate synthase (GOGAT) activities. There was a preferential metabolism of nodular (14)CO(2) into organic acids and particularly into malate. High malate levels were associated with reduced N(2) fixation and synthesis of amino acids. These results indicate that phosphorus deficiency can enhance malate synthesis in nodules, but that excessive malate accumulation may inhibit N(2) fixation and nitrogen assimilation.     

Cell organelles from crassulacean-acid-metabolism (CAM) plants

Cell organelles were isolated from the CAM plants Crassula lycopodioides Lam., Bryophyllum calycinum Salisb. and Sedum rubrotinctum R.T. Clausen by isopycnic centrifugation in sucrose gradients. The inclusion of 2.5% Ficoll in the grinding medium proved to be essential for a satisfactory separation of cell organelles during the subsequent centrifugation. Peroxisomes, mitochondria, and whole and broken chloroplasts were at least partially resolved as judged by marker-enzyme-activity profiles. The isolated peroxisomes contained activities of glycollate oxidase, catalase, hydroxypyruvate reductase, glycine aminotransferase, serine-glyoxylate aminotransferase, and aspartate aminotransferase, comparable to activities found in spinach (Spinacia oleracea L.) leaf peroxisomes. In contrast to spinach, however, only little, if any, particulate malate dehydrogenase activity could be attributed to isolated peroxisomes of the three CAM plants.     

Behaviour of some mitochondrial enzymes in ripening tomato fruit

The activities of four mitochondrial enzymes were studied in four stages of ripening tomato fruit. The highest enzyme activity was recorded for malate dehydrogenase followed by cytochrome c oxidase. Succinate dehydrogenase and NADH oxidase levels were low and could only be determined in the green stage of the fruit. However, peaks of various enzyme activities coincided in identical mitochondrial fractions on the sucrose density gradient. Moreover, the levels of malate dehydrogenase and cytochrome c oxidase were constant during the ripening process while the other two enzymes, succinate dehydrogenase and NADH oxidase, declined. This might indicate that mitochondria retain some of their essential functions through the ripening process.     

转BADH基因烟草的光系统Ⅱ和呼吸酶活性变化

测定了导入甜菜碱醛脱氢酶（ＢＡＤＨ） 基因烟草（ Ｎｉｃｏｔｉａｎａｔａｂａｃｕｍ Ｌ．） 植株的叶绿素荧光诱导瞬变特性、呼吸酶和光呼吸酶的活性,并与亲本植株比较。结果表明,转基因植株的Ｆｖ／Ｆｏ 、Ｆｖ／Ｆｍ 和Ｆｄ／Ｆｓ 没有明显的变化;三羧酸循环中的苹果酸脱氢酶、异柠檬酸脱氢酶和琥珀酸脱氢酶活性略有增加;末端氧化的细胞色素氧化酶活性明显提高;光呼吸途径中的羟基丙酮酸还原酶、乙醇酸氧化酶和过氧化氢酶活性明显提高。对这些变化的可能意义进行了讨论。     

The relationship between mitochondrial heterogeneity and gluconeogenesis in liver mitochondria of the rat, pigeon and guinea pig.

Isolated mitochondria of pigeon and guinea pig liver were subjected to zonal centrifugation. With pigeon liver mitochondria there was uniform distribution of pyruvate carboxylase, phosphoenolpyruvate carboxykinase, malate dehydrogenase, aspartate aminotransferase and glutamate dehydrogenase activities. Guinea pig liver mitochondria demonstrated two pyruvate carboxylase and phosphoenolpyruvate carboxykinase maxima but only one maximum with aspartate aminotransferase, malate dehydrogenase and glutamate dehydrogenase. Mitochondrial enzyme levels in rat, pigeon and guinea pig indicate different roles of certain gluconeogenic enzymes in the transport of carbon and hydrogen in and out of mitochondria.     

Ethanol-induced redox imbalance in rat kidneys

This study reports the effects of long-term ethanol consumption on kidney redox status, in terms of enzymatic mechanisms involved in regulating the cytosolic [NADH]/[NAD(+) ] balance. Wistar rats were treated with ethanol (2 g/kg body weight/24 h) via intragastric intubation for 10 and 30 weeks, respectively. Ethanol administration induced an enhancement of alcohol dehydrogenase activities and affected the capacity of the kidney to prevent NADH accumulation in the cytosol. After 10 weeks, the excess of NADH was balanced by increased activities of malate dehydrogenase and aspartate transaminase. In the event of a longer period of ethanol intake, the kidney was not able to balance the NADH excess, even though an increase in malate dehydrogenase, lactate dehydrogenase, aspartate transaminase, and alanine transaminase activities was noted. The electrophoretic analysis of alcohol dehydrogenase, lactate dehydrogenase, and malate dehydrogenase isoforms revealed differences between control and ethanol-treated animals. The results suggest that rat kidneys have a multicomponent metabolic response to the same daily dose of ethanol that functions to maintain the redox status and which varies with the length of the administration period.     

Activities and subcellular localization of enzymes responsible for lipolysis and gluconeogenesis during the germination of Brassica campestris cv. esculenta seeds

During the growth of turnip seedlings, two new lipases have been demonstrated, one with a maximum activity at pH 4.5 (acid lipase) and the other with a maxima at pH 8.6 (alkaline lipase). Many different enzymes are involved in gluconeogenesis: catalase, isocitrate lyase, malate synthetase, malate dehydrogenase, aconitase, citrate synthetase, fumarase, glycolate oxidase, phosphoenol-pyruvate carboxykinase. All of these show maximum activity coinciding with the stage in which lipid hydrolysis is maximal and when the accumulation of soluble carbohydrates has also reached its peak. The alkaline lipase as found to be located mainly in the spherosomes, whereas the glyoxysomes contained the following main activities: catalase, isocitrate lyase, malate synthetase, malate dehydrogenase and citrate synthetase. Aconitase, together with cytochrome oxidase and fumarase showed their highest activity in the mitochondria, and the presence of malate dehydrogenase, citrate synthetase and glycolate oxidase was also observed in these organelles. In the membrane-bound fraction, the activities of cytochrome reductase, glycolate oxidase and phosphoenol-pyruvate kinase were marked, although the latter enzyme was even more active in the soluble fraction.     

Studies on polyphenol content, activities and isozymes of polyphenol oxidase and peroxidase during air-curing in three tobacco types

The change in polyphenol content in the primed leaves of burley, flue-cured, and Turkish tobaccos during air-curing was related to the activities and isozymes of polyphenol oxidase and peroxidase. The quantity of chlorogenic acid was rapidly reduced during the first week of curing. The decrease in rutin content during curing was less significant, especially when the concentration of chlorogenic acid was high in leaf tissues. This result was further confirmed by in vitro assays with partially purified tobacco polyphenol oxidase.     

Water-deficit effects on carbon and nitrogen metabolism of pea nodules

Two experiments were carried out to investigate the effects of water-deficit stress on carbon and nitrogen metabolism of Pisum sativum nodules. In the first experiment, leaf w was allowed to reach -1.0 MPa over a period of 14 d whilst in the second experiment -1.5 MPa was reached during the same time period. Nodule activities of phosphoenol pyruvate carboxylase, glutamine synthetase, alkaline invertase, pyruvate decarboxylase, alcohol dehdyrogenase, uridine pyro-phosphorylase, and malate dehydrogenase activities were not affected by water-deficit stress. In the first experiment (-1.0 MPa), sucrose synthase (SS), an enzyme which hydrolyses sucrose to support nodule metabolism, declined by 50% in activity and about 25% in content, according to Western immunoblot data. In the second experiment (-1.5 MPa), SS activity decreased by 75% together with glutamate synthase and aspartate aminotransferase which declined by 60% and 40%, respectively. Coincident with the decline of these activities, a dramatic increase in the nodule content of sucrose and a slight increase in the levels of total free amino acids were found. It has been recently suggested that the decline in SS activity and, therefore, a reduced potential to metabolize sucrose may be an important factor contributing to the overall response of soybean nodules to water stress. These results suggest that this observation may be also correct for temperate legumes with indeterminate nodules. However, in this latter case, the activity of some enzymes involved in nitrogen assimilation (glutamate synthase and aspartate aminotransferase) were also affected by water-deficit stress).Key words: Pisum sativum, water stress, nitrogen metabolism, nodule metabolism, pea, sucrose synthase.      

Activities of enzymes concerned with pyruvate and oxaloacetate metabolism in the heart and liver of developing sheep.

1. In order to assess whether the potential ability of heart ventricular muscle and liver to metabolise substrates such as alanine, aspartate and lactate varies as the sheep matures and its nutrition changes, the activities of the following enzymes were determined in tissues of lambs obtained at varying intervals between 50 days after conception to 16 weeks after birth and in livers from adult pregnant ewes: lactate dehydrogenase (EC 1.1.1.27), alanine aminotransferase (EC 2.6.1.2), pyruvate kinase (EC 2.7.1.40), pyruvate carboxylase (EC 6.4.1.1), phosphoenolpyruvate carboxykinase (GTP)(EC 4.1.1.32), malate dehydrogenase (EC 1.1.1.37), aspartate aminotransferase (EC 2.6.1.1) and citrate (si)-synthase (EC 4.1.3.7). 2. In the heart a most marked increase in alanine aminotransferase activity was found throughout development. During this period the activities of citrate (si)-synthase, lactate dehydrogenase and pyruvate carboxylase also increased. There were no substantial changes in the activities of aspartate aminotransferase, malate dehydrogenase or pyruvate kinase. Pyruvate kinase activities were five times greater in the heart compared with those found in the liver. No significant activity of phosphoenolpyruvate carboxykinase (GTP) was detected in heart muscle. 3. In the liver the activities of both alanine aminotransferase and aspartate aminotransferase increased immediately following birth although the activity of alanine aminotransferase was lower in livers of pregnant ewes than in any of the lambs. As with alanine aminotransferase the highest activities of lactate dehydrogenase were found during the period of postnatal growth. No marked changes were observed in malate dehydrogenase or citrate (si)-synthase activities during development. A small decline in pyruvate kinase activity occurred whilst the activities of pyruvate carboxylase and phosphoenolpyruvate carboxykinase (GTP) tended to rise during development.     

Activities of pyruvate dehydrogenase,enzymes of citric acid cycle,and aminotransferases in the subcellular fractions of cerebral cortex in normal and hyperammonemic rats

Activity levels of pyruvate dehydrogenase, enzymes of citric acid cycle, aspartate and alanine aminotransferases were estimated in mitochondria, synaptosomes and cytosol isolated from brains of normal rats and those injected with acute and subacute doses of ammonium acetate. In mitochondria isolated from animals treated with acute dose of ammonium acetate, there was an elevation in the activities of pyruvate, isocitrate and succinate dehydrogenases while the activities of malate dehydrogenase (malateoxaloacetate), aspartate and alanine aminotransferases were suppressed. In subacute conditions a similar profile of change was noticed excepting that there was an elevation in the activity of -ketoglutarate dehydrogenase in mitochondria. In the synaptosomes isolated from animals administered with acute dose of ammonium acetate, there was an increase in the activities of pyruvate, isocitrate, -ketoglutarate and succinate dehydrogenases while the changes in the activities of malate dehydrogenase, asparatate and alanine amino transferases were suppressed. In the subacute toxicity similar changes were observed in this fraction except that the activity of malate dehydrogenase (oxaloacetatemalate) was enhanced. In the cytosol, pyruvate dehydrogenase and other enzymes of citric acid cycle except malate dehydrogenase were enhanced in both acute and subacute ammonia toxicity though their activities are lesser than that of mitochondria. In this fraction malate dehydrogenase (oxaloacetatemalate), was enhanced while activities of malate dehydrogenase (malateoxaloacetate), aspartate, and alanine aminotransferases were suppressed in both the conditions. Based on these results it is concluded that the decreased activities of malate dehydrogenase (malateoxaloacetate) in mitochondria and of aspartate, aminotransferase in mitochondria and cytosol may be responsible for the disruption of malate-aspartate, shuttle in hyperammonemic state. Possible existence of a small vulnerable population of mitochondria in brain which might degenerate and liberate their contents into cytosol in hyperammonemic states is also suggested.     

Physiological role of microbodies in the yeast Trichosporon cutaneum during growth on ethylamine as the source of energy,carbon and nitrogen

Compartmentation of the metabolism of ethylamine in Trichosporon cutaneum X4 was studied in cells, grown on this compound as the sole source of energy, carbon, and nitrogen. Transfer experiments indicated that an amine oxidase is involved in the early metabolism of ethylamine. The synthesis of this enzyme was induced by primary amines and was subject to partial carbon catabolite repression. Repression by ammonium ions was not observed. Adaptation of glucose-grown cells to growth on ethylamine was associated with the development of many microbodies, which developed from already existing organelles present in the inoculum cells and multiplied by division. Cytochemical experiments indicated that the organelles contained amine oxidase and catalase. Therefore, they were considered to play a key role in the metabolism of ethylamine. The physiological significance of the microbodies was investigated by fractionation studies of homogenized protoplasts from ethylamine-grown cells by differential- and sucrose-gradient centrifugation of subcellular organelles. Intact microbodies were only obtained when the isolation procedure was performed at pH 5.8 in the absence of Mg²⁺-ions. Analysis of the different fractions indicated that the key enzymes of the glyoxylate cycle, namely isocitrate lyase and malate synthase, cosedimented together with catalase and amine oxidase. In addition, activities of malate dehydrogenase, glutamate:oxaloacetate aminotransferase (GOT) and (NAD-dependent) glutamate dehydrogenase were detected in these fractions. Electron microscopy revealed that they mainly contained microbodies. Cytochemical experiments indicated that the above enzymes were all present in the same organelle. These findings suggest that microbodies of ethylamine-grown T. cutaneum X4 produce aspartate, so allowing NADH generated in the oxidation of malate by malate dehydrogenase to be quantitatively reoxidized inside the organelles in a series of reactions involving GOT and glutamate dehydrogenase. Aspartase and fumarase were not detected in the microbodies; activities of these two enzymes were present in the cytoplasm.Abbreviations ABTS 2,2-Azino-di(3-ethylbenzthiazoline sulfonate [6]) - DTT dithiothreitol - GOT glutamate:oxaloacetate aminotransferase - DTNB 5,5-dithiobis-2-nitrobenzoate - DAB diaminobenzidine - BSPT 2-(2-benzothiazolyl)-3-(4-phthalhydrazidyl)-t-styryl-sH-tetrazolium chloride - PF convex fracture face - EF concave fracture face     

A Survey of Plants for Leaf Peroxisomes

Leaves of 10 plant species, 7 with photorespiration (spinach, sunflower, tobacco, pea, wheat, bean, and Swiss chard) and 3 without photorespiration (corn, sugarcane, and pigweed), were surveyed for peroxisomes. The distribution pattern for glycolate oxidase, glyoxylate reductase, catalase, and part of the malate dehydrogenase indicated that these enzymes exist together in this organelle. The peroxisomes were isolated at the interface between layers of 1.8 to 2.3 m sucrose by isopycnic nonlinear sucrose density gradient centrifugation or in 1.95 m sucrose on a linear gradient. Chloroplasts, located by chlorophyll, and mitochondria by cytochrome c oxidase, were in 1.3 to 1.8 m sucrose.In leaf homogenates from the first 7 species with photorespiration, glycolate oxidase activity ranged from 0.5 to 1.5 mumoles x min(-1) x g(-1) wet weight or a specific activity of 0.02 to 0.05 mumole x min(-1) x mg(-1) protein. Glyoxylate reductase activity was comparable with glycolate oxidase. Catalase activity in the homogenates ranged from 4000 to 12,000 mumoles x min(-1) x g(-1) wet weight or 90 to 300 mumoles x min(-1) x mg(-1) protein. Specific activities of malate dehydrogenase and cytochrome oxidase are also reported. In contrast, homogenates of corn and sugarcane leaves, without photorespiration, had 2 to 5% as much glycolate oxidase, glyoxylate reductase, and catalase activity. These amounts of activity, though lower than in plants with photorespiration, are, nevertheless, substantial.Peroxisomes were detected in leaf homogenates of all plants tested; however, significant yields were obtained only from the first 5 species mentioned above. From spinach and sunflower leaves, a maximum of about 50% of the marker enzyme activities was found to be in these microbodies after homogenization. The specific activity for peroxisomal glycolate oxidase and glyoxylate reductase was about 1 mumole x min(-1) x mg(-1) protein; for catalase. 8000 mumoles x min(-1) x mg(-1) protein, and for malate dehydrogenase, 40 mumoles x min(-1) x mg(-1) protein. Only small to trace amounts of marker enzymes for leaf peroxisomes were recovered on the sucrose gradients from the last 5 species of plants. Bean leaves, with photorespiration, had large amounts of these enzymes (0.57 mumole of glycolate oxidase x min(-1) x g(-1) tissue) in the soluble fraction, but only traces of activity in the peroxisomal fraction. Low peroxisome recovery from certain plants was attributed to particle fragility or loss of protein as well as to small numbers of particles in such plants as corn and sugarcane.Homogenates of pigweed leaves (no photorespiration) contained from one-third to one-half the activity of the glycolate pathway enzymes as found in comparable preparations from spinach leaves which exhibit photorespiration. However, only traces of peroxisomal enzymes were separated by sucrose gradient centrifugation of particles from pigweed. Data from pigweed on the absence of photorespiration yet abundance of enzymes associated with glycolate metabolism is inconsistent with current hypotheses about the mechanism of photorespiration.Most of the catalase and part of the malate dehydrogenase activity was located in the peroxisomes. Contrary to previous reports, the chloroplast fractions from plants with photo-respiration did not contain a concentration of these 2 enzymes, after removal of peroxisomes by isopycnic sucrose gradient centrifugation.     

Aspartate stimulation of malate decarboxylation in Zea mays bundle sheath cells: possible role in regulation of C4 photosynthesis.

Aspartate stimulated by as much as three fold the rate of malate decarboxylation by Zea mays bundle sheath cells. Both the basal and aspartate stimulated rates of malate decarboxylation were light-dependent. Stimulation appeared to be due to aspartate as such, rather than depending on aspartate metabolism, and was due partly to a reduction in the malate concentration required for maximum decarboxylation and partly to an increased maximum velocity of decarboxylation. The extractable activities of NADP malic enzyme, glyceraldehyde phosphate dehydrogenase, and 3-phosphoglycerate kinase recoverable from cells were not increased by preincubating cells with aspartate, and aspartate did not affect the activity of these enzymes in cell-free extracts. It is suggested that aspartate may influence the transport of either malate into or pyruvate out of bundle sheath chloroplasts.