Role and location of NAD malic enzyme in thermogenic tissues of Araceae

Measurement of ferrochelatase activity using a novel assay suggests that plastids are the major site of haem biosynthesis in both photosynthetic and non-photosynthetic cells of pea (Pisum sativum L.)

Ferrochelatase is the terminal enzyme of haem biosynthesis, catalysing the insertion of ferrous iron into the macrocycle of protoporphyrin IX, the last common intermediate of haem and chlorophyll synthesis. Its activity has been reported in both plastids and mitochondria of higher plants, but the relative amounts of the enzyme in the two organelles are unknown. Ferrochelatase is difficult to assay since ferrous iron requires strict anaerobic conditions to prevent oxidation, and in photosynthetic tissues chlorophyll interferes with the quantification of the product. Accordingly, we developed a sensitive fluorimetric assay for ferrochelatase that employs Co(2+) and deuteroporphyrin in place of the natural substrates, and measures the decrease in deuteroporphyrin fluorescence. A hexane-extraction step to remove chlorophyll is included for green tissue. The assay is linear over a range of chloroplast protein concentrations, with an average specific activity of 0.68 nmol x min(-1) x mg of protein(-1), the highest yet reported. The corresponding value for mitochondria is 0.19 nmol x min(-1) x mg of protein(-1). The enzyme is inhibited by N-methylprotoporphyrin, with an estimated IC(50) value of approximately 1 nM. Using this assay we have quantified ferrochelatase activity in plastids and mitochondria from green pea leaves, etiolated pea leaves and pea roots to determine the relative amounts in the two organelles. We found that, in all three tissues, greater than 90% of the activity was associated with plastids, but ferrochelatase was reproducibly detected in mitochondria, at levels greater than the contaminating plastid marker enzyme, and was latent. Our results indicate that plastids are the major site of haem biosynthesis in higher plant cells, but that mitochondria also have the capacity for haem production.

NAD malic enzyme and the control of carbohydrate metabolism in potato tubers

Potato (Solanum tuberosum) plants were transformed with a cDNA encoding the 59-kD subunit of the potato tuber NAD-dependent malic enzyme (NADME) in the antisense orientation. Measurements of the maximum catalytic activity of NADME in tubers revealed a range of reductions in the activity of this enzyme down to 40% of wild-type activity. There were no detrimental effects on plant growth or tuber yield. Biochemical analyses of developing tubers indicated that a reduction in NADME activity had no detectable effects on flux through the tricarboxylic acid cycle. However, there was an effect on glycolytic metabolism with significant increases in the concentration of 3-phosphoglycerate and phosphoenolpyruvate. These results suggest that alterations in the levels of intermediates toward the end of the glycolytic pathway may allow respiratory flux to continue at wild-type rates despite the reduction in NADME. There was also a statistically significant negative correlation between NADME activity and tuber starch content, with tubers containing reduced NADME having an increased starch content. The effect on plastid metabolism may result from the observed glycolytic perturbations.

A single precursor protein for ferrochelatase-I from Arabidopsis is imported in vitro into both chloroplasts and mitochondria

Ferrochelatase is the last enzyme of heme biosynthesis and in higher plants is found in both chloroplasts and mitochondria. We have isolated cDNAs for two isoforms of ferrochelatase from Arabidopsis thaliana, both of which are imported into isolated chloroplasts. In this paper we show that ferrochelatase-I is also imported into isolated pea mitochondria with approximately the same efficiency as into chloroplasts. Processing of the precursor was observed with both chloroplast stroma and mitochondrial matrix extracts. This was inhibited by EDTA, indicating it was due to the specific processing proteases. The specificity of import was verified by the fact that the mitochondrial preparation did not import the precursor of the light-harvesting chlorophyll a/b protein precursor or the precursor of porphobilinogen deaminase, an earlier enzyme of tetrapyrrole biosynthesis, both of which are exclusively chloroplast-located. Furthermore, import of ferrochelatase-I precursor into mitochondria was inhibited by valinomycin, but this had no effect on its import into chloroplasts. Thus a single precursor molecule is recognized by the import machinery of the two organelles. The implications for the targeting of ferrochelatase in a possible protective role against photooxidative stress are discussed.

Investigation of the subcellular location of the tetrapyrrole-biosynthesis enzyme coproporphyrinogen oxidase in higher plants

The subcellular location of two enzymes in the biosynthetic pathway for protoporphyrin IX, coproporphyrinogen (coprogen) oxidase (EC 1.3.3.3) and protoporphyrinogen (protogen) oxidase (EC 1.3.3.4) has been investigated in etiolated pea (Pisum sativum) leaves and spadices of cuckoo-pint (Arum maculatum). Plant tissue homogenized in isotonic buffer was subjected to subcellular fractionation to prepare mitochondria and plastids essentially free of contamination by other cellular organelles, as determined by marker enzymes. Protogen oxidase activity measured fluorimetrically was reproducibly found in both mitochondria and etioplasts. In contrast, coprogen oxidase could be detected only in etioplasts, using either a coupled fluorimetric assay or a sensitive radiochemical method. The implications of these results for the synthesis of mitochondrial haem in plants is discussed.

Tansley Review No. 37 Circadian rhythms: their origin and control

This article reviews the circadian rhythm of carbon dioxide metabolism in leaves of the Crassulacean plant Bryophyllum (Kalanchoë) fedtsckenkoi which persists both in continuous darkness and a CO₂ -free atmosphere, and in continuous light and normal air. Under both conditions the rhythm is due to the periodic activity of the enzyme phosphoenolpyruvate carboxylase (PEPc). The physiological characteristics of the rhythm are described in detail and, from these characteristics, hypotheses are advanced to account for both the generation of the rhythm and the regulation of its phase and period by environmental factors. The periodic activity of PEPc is ascribed to the periodic accumulation of an allosteric inhibitor, malate, in the cytoplasm and its subsequent removal either to the vacuole in continuous darkness, or by metabolism in continuous light. Also involved in the generation of the rhythm is a periodic change in the sensitivity of PEPc to malate inhibition due to the periodic phosphorylation and dephosphorylation of PEPc which changes its K₁ by a factor of 10 from 30 to 0.3 mM and vice versa. This periodic phosphorylation of PEPc is apparently achieved by the periodic synthesis and breakdown of a PEPc kinase which phosphorylates the enzyme on a serine residue; dephosphorylation is achieved by a type 2A phosphatase which shows no rhythmic variation. The induction of phase shifts in the rhythm in continuous darkness and CO₂ -free air has been explained in terms of light and high-temperature activated gates or channels in the tonoplast which, when open, allow malate to diffuse between the vacuole and cytoplasm. For the rhythm in continuous light and normal air phase, control by environmental signals can be attributed to changes in the malate levels in critical cell compartments, or in particular cell populations such as the stomatal guard cells, due to regulation of the malate synthesizing enzyme system involving PEPc, and malic enzyme which is responsible for malate metabolism. The role of the stomata in the generation of the rhythm is also discussed. The biochemical events which appear to give rise to the well-studied circadian rhythms in leaf movement in Samanea and Albizza, in luminescence in Gonyaulax polyedra and in the synthesis of the chlorophyll a/b binding protein are also reviewed in an attempt to identify similarities between these events and those involved in the Bryophyllum rhythm. Finally, the somewhat similar nature of the genes apparently responsible for circadian rhythmicity in Neurospora and Drosophila are discussed, and suggestions made for utilizing anti-sense nucleic acid technology in the further elucidation of the critical biochemical events involved in the basic, temperature-compensated circadian oscillator in living organisms. CONTENTS Summary 347 I. Introduction 348 II. Occurrence of circadian rhythms 348 III. Physiological characteristics of circadian rhythms 349 IV. Biochemical and molecular events involved in the circadian rhythm in Bryophyllum leaves 362 V. Biochemical and molecular events involved in the origin and control of circadian rhythmicity in other organisms 366 VI. Genetic studies 370 VII. Conclusion 371 References 372.

The capacity of plastids from developing pea cotyledons to synthesise acetyl CoA

In order to determine whether the enzymes required to convert triose phosphate to acetyl CoA were present in pea (Pisum sativum L.) seed plastids, a rapid, mechanical technique was used to isolate plastids from developing cotyledons. The plastids were intact and the extraplastidial contamination was low. The following glycolytic enzymes, though predominantly cytosolic, were found to be present in plastids: glyceraldehyde 3-phosphate dehydrogenase (EC 1.2.1.12), phosphoglycerate kinase (EC 2.7.2.3), and pyruvate kinase(EC 2.7.1.40). Evidence is presented which indicates that plastids also contained low activities of enolase (EC 4.2.1.11) and phosphoglycerate mutase (EC 2.7.5.3). Pyruvate dehydrogenase, although predominantly mitochondrial, was also present in plastids. The plastidial activities of the above enzymes were high enough to account for the rate of lipid synthesis observed in vivo.

Subcellular localization of two porphyrin-synthesis enzymes in Pisum sativum (pea) and Arum (cuckoo-pint) species

The subcellular location of the two porphyrin-synthesis enzymes 5-aminolaevulinate dehydratase (ALAD) and porphobilinogen deaminase (PBGD) was investigated in Pisum sativum (pea) leaves and spadices of Arum (cuckoo-pint). Throughout the tissue-fractionation procedures the distribution of the two enzymes paralleled that of the plastid marker enzyme (ADP-glucose pyrophosphorylase), even in Arum, a tissue where the synthesis of non-plastid haem is predominant. The distribution of cytosolic marker enzyme (lactate dehydrogenase) was significantly different from that of ALAD and PBGD and, although purified mitochondria from both species had some residual activity, this was always less than contaminating plastid marker enzyme. The results suggest that ALAD and PBGD are exclusively plastid enzymes. The significance of this for the role of plastids in cellular porphyrin synthesis is discussed.

Regulation of aerobic fermentation in Leishmania donovani promastigotes by NADP+-dependent malic enzyme

NADP+-dependent malic enzyme (decarboxylating) was extracted from Leishmania donovani promastigotes with Triton X-100. The enzyme was specific for NADP+ and did not decarboxylate oxaloacetate (OA). The substrate activity relationship was hyperbolic for both L-malate and NADP+, and Km values were calculated as 0.18 and 0.12 mM, respectively. The enzyme exhibited a broad pH optimum of 7.5-8.0. Pyruvate, NADPH and OA inhibited the reaction in a competitive manner with apparent Ki values of 0.2, 0.04 and 0.04 mM, respectively, while oxalate inhibition was of the mixed type. The kinetic results obtained indicate that malic enzyme is involved in the regulation of carbon flow towards aerobic fermentation, complete oxidation of dicarboxylic acids or biosynthetic purposes.

Fatty-acid synthesis in chloroplasts from mustard (Sinapis alba L.) cotyledons: formation of acetyl coenzyme A by intraplastid glycolytic enzymes and a pyruvate dehydrogenase complex

Chloroplasts from the cotyledons of mustard (Sinapis alba L.) seedlings were isolated on Percoll gradients, and showed a high degree of intactness (92%) and purity as judged by electron microscopy and marker-enzyme analysis (cytoplasmic contamination lower than 0.4% on a protein basis). The chloroplasts synthesized longchain fatty acids from both precursors [1-(14)C] acetate and [2-(14)C]pyruvate; maximum incorporation rates were 96 nmol·(mg Chl)(-1)·h(-1) for acetate and 213 nmol·(mg Chl)(-1)·h(-1) for pyruvate. Acetyl-CoA-producing enzymatic activities, namely acetyl-CoA synthetase (EC 6.2.1.1.) and a pyruvate dehydrogenase complex, showed specific activities of 14.8 nmol·(mg protein)(-1)·min(-1) and 18.2 nmol·(mg protein)(-1)·min(-1), respectively. The glycolytic enzymes phosphoglyceromutase (EC 2.7.5.3) phosphopyruvate hydratase (EC 4.2.1.11) and pyruvate kinase (EC 2.7.1.40) were all found to be components of these chloroplasts, thus indicating a possible pathway for intraplastid acetyl-CoA formation.

Alkaline inorganic pyrophosphatase and starch synthesis in amyloplasts

The aim of this work was to see if amyloplasts contained inorganic pyrophosphatase. Alkaline pyrophosphatase activity, largely dependant upon MgCl2 but not affected by 100 μM ammonium molybdate or 60-100 mM KCl, was demonstrated in exracts of developing and mature clubs of the spadix of Arum maculatum L. and of suspension cultures of Glycine max L., but not in extracts of the developing bulb of Allium cepa L. The maximum catalytic activity of alkaline pyrophosphatase in the above tissues showed a positive correlation with starch synthesis, and in the first two tissues was shown to exceed the activity of ADPglucose pyrophosphorylase. Of the alkaline pyrophosphatase activity in lysates of protoplasts of suspension cultures of Glycine max, 57% was latent. Density-gradient centrifugation of these lysates showed a close correlation between the distribution of alkaline pyrophosphatase and the plastid marker, nitrite reductase. It is suggested that much, if not all, of the alkaline pyrophosphatase in suspension cultures of Glycine max is located in the plastids.

Pyrophosphate:fructose 6-phosphate 1-phosphotransferase and glycolysis in non-photosynthetic tissues of higher plants

The activity of pyrophosphate:fructose-6-phosphate 1-phosphotransferase [PFK (PPi); EC 2.7.1.90] in extracts of the storage tissues of leek (Allium porrum), beetroot (Beta vulgaris) and roots of darnel (Lolium temulentum) exceeded 0.15 mumol/min per g fresh wt. As net flux from fructose 1,6-bisphosphate to fructose 6-phosphate in these tissues is unlikely, it is suggested that PFK (PPi) does not contribute to gluconeogenesis or starch synthesis. The maximum catalytic activities of PFK (PPi) in apex, stele and cortex of the root of pea (Pisum sativum) and in the developing and the thermogenic club of the spadix of cuckoo-pint (Arum maculatum) were measured and compared with those of phosphofructokinase, and to estimates of the rates of carbohydrate oxidation. PPi and fructose 2,6-bisphosphate in Arum clubs were measured. The above measurements are consistent with a glycolytic role for PFK (PPi) in tissues where there is marked biosynthesis, but not in the thermogenic club of Arum. The possibility that PFK (PPi) is a means of synthesizing pyrophosphate is discussed.

Intracellular location of ATP citrate lyase in leaves of Pisum sativum L

The aim of this work was to discover if there is enough ATP citrate lyase (EC 4.1.3.8) in the cytosol of the leaves of Pisum sativum L. to catalyse the synthesis of the acetyl CoA needed for terpenoid synthesis. Estimates of the maximum catalytic activity of the enzyme in leaves of 7-d-old peas gave values of 113 nmol min(-1) g(-1) fresh weight. The rate of carotenoid accumulation in these leaves corresponded to a requirement for acetyl CoA of 0.7 nmol min(-1) g(-1) fresh weight. The distribution of marker enzymes during fractionation of homogenates of leaves from 7 to 10-d-old peas showed that differential centrifugation led to the isolation in reasonable yields of chloroplasts, mitochondria, peroxisomes and the endomembrane system. None of the above components of the leaf contained appreciable detectable activity of ATP citrate lyase, the distribution of which closely paralleled that of the cytosolic marker. It was concluded that in young leaves of pea most of the ATP citrate lyase is in the cytosol.