Fatty Acid synthesis in endosperm of young castor bean seedlings

Mapping the castor bean endosperm proteome revealed a metabolic interaction between plastid, mitochondria, and peroxisomes to optimize seedling growth

In this work, we studied castor-oil plant Ricinus communis as a classical system for endosperm reserve breakdown. The seeds of castor beans consist of a centrally located embryo with the two thin cotyledons surrounded by the endosperm. The endosperm functions as major storage tissue and is packed with nutritional reserves, such as oil, proteins, and starch. Upon germination, mobilization of the storage reserves requires inter-organellar interplay of plastids, mitochondria, and peroxisomes to optimize growth for the developing seedling. To understand their metabolic interactions, we performed a large-scale organellar proteomic study on castor bean endosperm. Organelles from endosperm of etiolated seedlings were isolated and subjected to liquid chromatography-tandem mass spectrometry (LC-MS/MS). Computer-assisted deconvolution algorithms were applied to reliably assign the identified proteins to their correct subcellular localization and to determine the abundance of the different organelles in the heterogeneous protein samples. The data obtained were used to build a comprehensive metabolic model for plastids, mitochondria, and peroxisomes during storage reserve mobilization in castor bean endosperm.

The influence of endogenous acyl-acyl carrier protein concentrations on fatty acid compositions of chloroplast glycerolipids

The concentrations of long-chain acyl-acyl carrier proteins (acyl-ACP) occurring during fatty acid synthesis from [1-14C]acetate within chloroplasts isolated from spinach, pea, and amaranthus leaves were manipulated by making minor changes to a basal incubation medium containing sn-glycerol 3-phosphate (G3P). Pools of oleoyl-, stearoyl-, and palmitoyl-ACP were compared with those of the corresponding 1-acyl glycerol 3-phosphates to determine how endogenous acyl-ACP concentrations affected the fatty acid compositions of chloroplast glycerolipids. The 1-acyl G3P synthesized by isolated chloroplasts contained more palmitate than would be expected for the precursor of thylakoid phosphatidylglycerol in the different plant species. However, treatments which increased ratios of oleoyl- to palmitoyl-ACP by about 50% increased synthesis of sn-1-oleoyl G3P to the extent anticipated from known fatty acid compositions of the different phosphatidylglycerols. Since stearate constituted 70-73% of the acyl-ACP and 48-51% of the 1-acyl-G3P pool of spinach and pea chloroplasts incubated in the presence of cyanide, it is transferred to G3P much more efficiently in situ than would be predicted from competition studies using mixtures of acyl donors and purified acyltransferases. Increasing concentrations of G3P in incubation media from 0.1 to 2 mM had relatively little effect on the amounts and proportions of acyl-ACPs but forced the synthesis of palmitoyl-G3P and, ultimately, disaturated glycerolipid. It is concluded that the chloroplast G3P acyltransferases are primarily responsible for determining the fatty acid compositions of procaryotic glycerolipids in plants, but that acyl-ACP concentrations may play a more important role than would be anticipated from the kinetics of the purified enzyme. However, those kinetics may be quite complex; allosteric effectors may influence the affinities of the enzyme for oleoyl-ACP and for G3P.

3-Oxoacyl-[ACP] reductase from oilseed rape (Brassica napus)

3-Oxoacyl-[ACP] reductase (E.C. 1.1.1.100, alternatively known as beta-ketoacyl-[ACP] reductase), a component of fatty acid synthetase has been purified from seeds of rape by ammonium sulphate fractionation, Procion Red H-E3B chromatography, FPLC gel filtration and high performance hydroxyapatite chromatography. The purified enzyme appears on SDS-PAGE as a number of 20-30 kDa components and has a strong tendency to exist in a dimeric form, particularly when dithiothreitol is not present to reduce disulphide bonds. Cleveland mapping and cross-reactivity with antiserum raised against avocado 3-oxoacyl-[ACP] reductase both indicate that the multiple components have similar primary structures. On gel filtration the enzyme appears to have a molecular mass of 120 kDa suggesting that the native structure is tetrameric. The enzyme has a strong preference for the acetoacetyl ester of acyl carrier protein (Km = 3 microM) over the corresponding esters of the model substrates N-acetyl cysteamine (Km = 35 mM) and CoA (Km = 261 microM). It is inactivated by dilution but this can be partly prevented by the inclusion of NADPH. Using an antiserum prepared against avocado 3-oxoacyl-[ACP] reductase, the enzyme has been visualised inside the plastids of rape embryo and leaf tissues by immunoelectron microscopy. Amino acid sequencing of two peptides prepared by digestion of the purified enzyme with trypsin showed strong similarities with 3-oxoacyl-[ACP] reductase from avocado pear and the Nod G gene product from Rhizobium meliloti.

Characterization of Fatty Acid biosynthesis in isolated pea root plastids

Fatty acid biosynthesis from Na[1-(14)C]acetate was characterized in plastids isolated from primary roots of 7-day-old germinating pea (Pisum sativum L.) seeds. Fatty acid synthesis was maximum at 82 nanomoles per hour per milligram protein in the presence of 200 micromolar acetate, 0.5 millimolar each of NADH, NADPH, and coenzyme A, 6 millimolar each of ATP and MgCl(2), 1 millimolar each of MnCl(2) and glycerol-3-phosphate, 15 millimolar KHCO(3), 0.31 molar sucrose, and 0.1 molar Bis-Tris-propane, pH 8.0, incubated at 35 degrees C. At the standard incubation temperature of 25 degrees C, fatty acid synthesis was essentially linear for up to 6 hours with 80 to 120 micrograms per milliliter plastid protein. ATP and coenzyme A were absolute requirements, whereas divalent cations, potassium bicarbonate, and reduced nucleotides all variously improved activity two- to 10-fold. Mg(2+) and NADH were the preferred cation and nucleotide, respectively. Glycerol-3-phosphate had little effect, whereas dithiothreitol and detergents generally inhibited the incorporation of [(14)C]acetate into fatty acids. On the average, the principal radioactive products of fatty acid biosynthesis were approximately 39% palmitic, 9% stearic, and 52% oleic acid. The proportions of these fatty acids synthesized depended on the experimental conditions.

3-Oxoacyl-(acyl-carrier protein) reductase from avocado (Persea americana) fruit mesocarp

The NADPH-linked 3-oxoacyl-(acyl-carrier protein) (ACP) reductase (EC 1.1.1.100), also known as 'beta-ketoacyl-ACP reductase', has been purified from the mesocarp of mature avocado pears (Persea americana). The enzyme is inactivated by low ionic strength and low temperature. On SDS/PAGE under reducing conditions, purified 3-oxoacyl-ACP reductase migrated as a single polypeptide giving a molecular mass of 28 kDa. Gel-filtration chromatography gave an apparent native molecular mass of 130 kDa, suggesting that the enzyme is tetrameric. The enzyme is inactivated by dilution, but some protection is afforded by the presence of NADPH. Kinetic constants have been determined using synthetic analogues as well as the natural ACP substrate. It exhibits a broad pH optimum around neutrality. Phenylglyoxal inactivates the enzyme, and partial protection is given by 1 mM-NADPH. Antibodies have been raised against the protein, which were used to localize it using immunogold electron microscopy. It is localized in plastids. N-Terminal amino-acid-sequence analysis was performed on the enzyme, and it shows close structural similarity with cytochrome f. Internal amino-acid-sequence data, derived from tryptic peptides, shows similarity with the putative gene products encoded by the nodG gene from the nitrogen-fixing bacterium Rhizobium meliloti and the gra III act III genes from Streptomyces spp.

Plants contain multiple biotin enzymes: discovery of 3-methylcrotonyl-CoA carboxylase, propionyl-CoA carboxylase and pyruvate carboxylase in the plant kingdom

Acetyl-CoA carboxylase is the sole biotin enzyme previously reported in plants. Western analysis with 125I-streptavidin of proteins extracted from carrot somatic embryos visualized six biotin-containing polypeptides, the relative molecular masses of which are 210,000, 140,000, 73,000, 50,000, 39,000, and 34,000. This multiplicity of the biotin-containing polypeptides can be partly explained by the discovery of 3-methylcrotonyl-CoA carboxylase, propionyl-CoA carboxylase, and pyruvate carboxylase in extracts of somatic carrot embryos, biotin enzymes previously unknown in the plant kingdom. These biotin enzymes seem to be widely distributed in the plant kingdom.

A preliminary analysis of Fatty Acid synthesis in pea roots

Subcellular fractions from pea (Pisum sativum L.) roots have been prepared by differential centrifugation techniques. Greater than 50% of the recovered plastids can be isolated by centrifugation at 500g for 5 minutes. Plastids of this fraction are largely free from mitochondrial and microsomal contamination as judged by marker enzyme analysis. De novo fatty acid biosynthesis in pea roots occurs in the plastids. Isolated pea root plastids are capable of fatty acid synthesis from acetate at rates up to 4.3 nanomoles per hour per milligram protein. ATP, bicarbonate, and either Mg(2+) or Mn(2+) are all absolutely required for activity. Coenzyme A at 0.5 millimolar improved activity by 60%. Reduced nucleotides were not essential but activity was greatest in the presence of 0.5 millimolar of both NADH and NADPH. The addition of 0.5 millimolar glycerol-3-phosphate increased activity by 25%. The in vitro and in vivo products of fatty acid synthesis from acetate were primarily palmitate, stearate, and oleate, the proportions of which were dependent on experimental treatments. Fatty acids synthesized by pea root plastids were recovered in primarily phosphatidic acid and diacylglycerol or as water soluble derivatives and the free acids. Lesser amounts were found in phosphatidylcholine, phosphatidylethanolamine, phosphatidylglycerol, and monogalactosyldiacylglycerol.

Fatty-acid synthesis in plastids from maturing safflower and linseed cotyledons

Plastids isolated from maturing, nongreen safflower (Carthamus tinctorius L.) cotyledons yielded unesterified fatty acids as the predominant product of fatty-acid synthesis from [1-(14)C]acetate. Exogenous reduced pyridine nucleotides were not required for this synthesis, but [1-(14)C]acetate incorporation was absolutely dependent on addition of ATP. Linseed (Linum usitatissimum L.) cotyledons are green during development and plastids isolated from them resembled leaf chloroplasts with developed grana. In contrast to the safflower plastids, those from linseed were able to carry out fatty-acid synthesis at low irradiances without the addition of either pyridine nucleotides or ATP. Intact linseed cotyledons were capable of net photosynthesis at rates up to 95 μmol·mg(-1) chlorophyll·h(-1). However, the low-light environment inside the linseed capsule (approx. 15% of external) means that photosynthesis will not contribute appreciably to the carbon economy of the developing seed and its main role may be to supply cofactors for fatty-acid synthesis.

Subcellular distribution of acetyl-coenzyme A carboxylase in mesophyll cells of barley and sorghum leaves

The subcellular distribution of acetyl-CoA carboxylase [acetyl-CoA-carbon dioxide ligase (ADP-forming), EC 6.4.1.2] was determined in mesophyll protoplasts isolation from barley, a C3 plant, and sorghum, a C4 plant. In both species, all of the mesophyll acetyl-CoA carboxylase was demonstrated to be chloroplastic. In barley leaves and mesophyll protoplasts, a single biotinyl protein of 60,000 Da was identified by a modified Western-blotting procedure. The subcellular distribution of this biotinyl protein was identical to that found for acetyl-CoA carboxylase. These results are discussed in relation to the compartmentation of reactions requiring malonyl-CoA as a substrate.

Acyl-CoA Synthetase Is Located in the Outer Membrane and Acyl-CoA Thioesterase in the Inner Membrane of Pea Chloroplast Envelopes

Both acyl-CoA synthetase and acyl-CoA thioesterase activities are present in chloroplast envelope membranes. The functions of these enzymes in lipid metabolism remains unresolved, although the synthetase has been proposed to be involved in either plastid galactolipid synthesis or the export of plastid-synthesized fatty acids to the cytoplasm. We have examined the locations of both enzymes within the two envelope membranes of pea (Pisum sativum var Laxton's Progress No. 9) chloroplasts. Inner and outer envelope membranes were purified from unfractionated envelope preparations by linear density sucrose gradient centrifugation. Acyl-CoA synthetase was located in the outer envelope membrane while acyl-CoA thioesterase was located in the inner envelope membrane. Thus, it seems unlikely that the synthetase is directly involved in galactolipid assembly. Instead, its localization supports the hypothesis that it functions in the transport of plastid-synthesized fatty acids to the endoplasmic reticulum.

Polar lipid composition of a plastid ribosome-deficient barley mutant

Green and white leaves of the barley mutant line ;albostrians' were compared for their polar lipid content and fatty acid composition. The mutant plastids of the white leaves have a double-layered envelope, but in contrast with the normal chloroplasts, lack 70 S ribosomes and thylakoids. In the green leaves, the amount of monogalactosyldiacylglycerol (MGDG) consistently exceeds the amount of digalactosyldiacylglycerol (DGDG) and the amount of galactolipids exceeds the amount of phospholipids. In contrast, in white leaves the amount of DGDG exceeds the amount of MGDG and the amount of phospholipids exceeds the amount of galactolipids. In white leaves, the galactolipid composition reflects the plastid envelope composition which is rich in DGDG, whereas in green leaves the galactolipid composition reflects the thylakoid composition which is rich in MGDG. These results demonstrate the likelihood that all the enzymes involved in galactolipid, sulfolipid and fatty acid synthesis are coded by the nuclear genome.

The characteristics of some components of the fatty acid synthetase system in the plastids from the mesocarp of avocado (Persea americana) fruit

Preparations of NADH-specific and NADPH-specific 3-oxoacyl-[acyl-carrier-protein] reductase enzymes (EC 1.1.1.100), enoyl-[acyl-carrier-protein] reductase (EC 1.3.1.9) and [acyl-carrier-protein] malonyltransferase (EC 2.3.1.39) have been purified from preparations of avocado mesocarp plastids and characterised. The enzymes are quite similar in molecular and kinetic characteristics to analogous enzymes known in Escherichia coli and Euglena and are clearly components of a type-II fatty acid synthetase system.

Intracellular distribution of enzymes associated with nitrogen assimilation in roots

The cellular distribution of enzymes involved in nitrogen assimilation: nitrate reductase (EC 1.6.6.2), nitrite reductase (EC 1.6.6.4), glutamine synthetase (EC 6.3.1.2), glutamate synthase (EC 2.6.1.53), and glutamate dehydrogenase (EC 1.4.1.3) has been studied in the roots of five plants: maize (Zea mays L. hybrid W 64A x W 182E), rice (Oryza sativa L. cv. Delta), bean (Phaseolus vulgaris L. cv. Contender), pea (Pisum sativum L. cv. Demi-nain), and barley (Hordeum vulgare L.). Initially, cell organelles were separated from soluble proteins by differential centrifugation. Cell organelles were also subjected to sucrose density gradients. The results obtained by these two methods indicate that nitrite reductase and glutamate synthase are localized in plastids, nitrate reductase and glutamine synthetase are present in the cytosol, and glutamate dehydrogenase is a mitochondrial enzyme.

Light control of fatty acid synthesis and diurnal fluctuations of fatty acid composition in leaves

1. Although isolated spinach chloroplasts were almost entirely (greater than 99%) dependent on light for fatty acid synthesis, leaf discs were capable of fatty acid synthesis in the dark (up to 500nmol of 3H/h per mg of chlorophyll equivalent to approx. 400nmol of carbon/h per mg of chlorophyll), which represented 12-20% of the corresponding 'light rates'. 2. Net fatty acid accumulation by greening maize leaves occurred largely or entirely during the light period. 3. There was a diurnal fluctuation in the proportions of C18 unsaturated fatty acids in the lipids of developing spinach leaves, where an increase in the concentration of oleate during the day and a subsequent decline at night was observed; a complementary change occurred in the concentration of alpha-linolenate. The rhythm is interpreted as reflecting the continuation of oleate and linoleate desaturation at high rates when oleate synthesis is markedly decreased at night. 4. Changes in the fatty acid composition of 3-sn-phosphatidylcholine accounted for at least 60% of the total decrease in oleate over the dark period. This result is consistent with suggestions that this lipid is the substrate for the leaf microsomal oleate desaturase and an intermediate in leaf glycerolipid biosynthesis.

The role of chloroplasts and microsomal fractions in polar-lipid synthesis from [1-14C]acetate by cell-free preparations from spinach (Spinacia oleracea) leaves

1. Isolated spinach (Spinacia oleracea) chloroplasts were incapable of accumulating polar lipids when incubated with [1-14C]acetate in a cofactor-free medium. When CoA, ATP and glycerol 3-phosphate were added to incubation media, the accumulated products were non-esterified fatty acids, acyl-CoA and 1,2-diacylglycerol, all intermediates of lipid metabolism. 2. Chloroplast acyl-CoA was used to synthesize phosphatidylcholine only when a microsomal fraction was added back to the incubation medium. 3. The 1,2-diacylglycerol synthesized by isolated chloroplasts was converted almost quantitatively into diacylgalactosylglycerol when exogenous UDP-galactose was available. 4. Stereospecific analyses of the isolated lipids suggested that the diacylglycerol synthesized by isolated chloroplasts may be an important precursor for the synthesis in vivo of diacylgalactosylglycerol and phosphatidylglycerol but was unlikely to be a precursor of phosphatidylcholine. 5. A scheme for plant-lipid biosynthesis is presented that integrates the functions of chloroplasts, the cytoplasm and the endoplasmic reticulum.

Pyruvate dehydrogenase complex from germinating castor bean endosperm

Subcellular organelles from castor bean (Ricinus communis) endosperm were isolated on discontinuous sucrose gradients from germinating seeds which were 1 to 7 days postimbibition. Marker enzyme activities of the organelles were measured (fumarase, catalase, and triose phosphate isomerase) and the homogeneity of the organelle fractions was examined by electron microscopy. Pyruvate dehydrogenase complex activity was measured only in the mitochondrial fraction and attempts to activate or release the enzyme from the proplastid were not successful. A pathway is proposed for the most efficient use of endosperm carbon for de novo fatty acid biosynthesis that does not require the presence of the pyruvate dehydrogenase complex in the proplastid to provide acetyl-coenzymeA.

On the control of long-chain-fatty acid synthesis in isolated intact spinach (Spinacia oleracea) chloroplasts

1. Chloroplasts isolated from spinach leaves by using the low-ionic-strength buffers of Nakatani & Barber [(1977) Biochim. Biophys. Acta.461, 510-512] had higher rates of HCO(3) (-)-dependent oxygen evolution (up to 369mumol/h per mg of chlorophyll) and higher rates of [1-(14)C]acetate incorporation into long-chain fatty acids (up to 1500nmol/h per mg of chlorophyll) than chloroplasts isolated by using alternative procedures. 2. Acetate appeared to be the preferred substrate for fatty acid synthesis by isolated chloroplasts, although high rates of synthesis were also measured from H(14)CO(3) (-) in assays permitting high rats of photosynthesis. Incorporation of H(14)CO(3) (-) into fatty acids was decreased by relatively low concentrations of unlabelled acetate. Acetyl-CoA synthetase activity was present 3-4 times in excess of that required to account for rates of [1-(14)C]acetate incorporation into fatty acids, but pyruvate dehydrogenase was either absent or present in very low activity in spinach chloroplasts. 3. Rates of long-chain-fatty acid synthesis from [1-(14)C]acetate in the highly active chloroplast preparations, compared with those used previously, were less dependent on added cofactors, but showed a greater response to light. The effects of added CoA plus ATP, Triton X-100 and sn-glycerol 3-phosphate on the products of [1-(14)C]acetate incorporation were similar to those reported for less active chloroplast preparations. 4. Endogenous [(14)C]acetyl-CoA plus [(14)C]malonyl-CoA was maintained at a constant low level even when fatty acid synthesis was limited by low HCO(3) (-) concentrations. Endogenous [(14)C]acyl-(acyl-carrier protein) concentrations increased with increasing HCO(3) (-) concentration and higher rates of fatty acid synthesis, but were slightly lower in the presence of Triton X-100. It is proposed that rates of long-chain-fatty acid synthesis in isolated chloroplasts at saturating [1-(14)C]acetate concentrations and optimal HCO(3) (-) concentrations may be primarily controlled by rates of removal of the products of the fatty acid synthetase.

Subcellular distribution of gluconeogenetic enzymes in germinating castor bean endosperm

The intracellular distribution of enzymes capable of catalyzing the reactions from oxaloacetate to sucrose in germinating castor bean endosperm has been studied by sucrose density gradient centrifugation. One set of glycolytic enzyme activities was detected in the plastids and another in the cytosol. The percentages of their activities in the plastids were less than 10% of total activities except for aldolase and fructose diphosphatase. The activities of several of the enzymes present in the plastids seem to be too low to account for the in vivo rate of gluconeogenesis whereas those in the cytosol are quite adequate. Furthermore, phosphoenolypyruvate carboxykinase, sucrose phosphate synthetase, and sucrose synthetase, which catalyze the first and final steps in the conversion of oxaloacetate to sucrose, were found only in the cytosol. It is deduced that in germinating castor bean endosperm the complete conversion of oxaloacetate to sucrose and CO(2) occurs in the cytosol. The plastids contain some enzymes of the pentose phosphate pathway, pyruvate dehydrogenase and fatty acid synthetase in addition to the set of glycolytic enzymes. This suggests that the role of the plastid in the endosperm of germinating castor bean is the production of fatty acids from sugar phosphates, as it is known to be in the endosperm during seed development.

ATP Citrate Lyase from Germinating Castor Bean Endosperm: Localization and some Properties

ATP citrate lyase (EC 4.1.3.8) has been found in crude extracts from endosperm tissue of germinating castor bean and shows its maximum activity in 4- to 5-day-old seedlings. A strict requirement for coenzyme A and adenosine 5'-triphosphate was demonstrated. The pH optimum for the reaction is around 7.5. The unstable enzyme can be stabilized by freezing and addition of citrate and glycerol. (-)-Hydroxycitrate is a potent inhibitor. The molecular weight is about 400,000. The adenosine 5'-triphosphate citrate lyase is localized in the plastids, where it possibly plays a role in providing acetyl coenzyme A for lipid biosynthesis.

Subcellular localization of acyl carrier protein in leaf protoplasts of Spinacia oleracea

This communication demonstrates that all de novo fatty acid biosynthesis in spinach leaf cells requires acyl carrier protein (ACP) and occurs specifically in the chloroplasts. Antibodies raised to purified spinach ACP inhibited at least 98% of malonyl CoA-dependent fatty acid synthesis by spinach leaf homogenates. Therefore, the presence of ACP in a compartment of the spinach leaf cell would serve as a marker for de novo fatty acid biosynthesis. A radioimmunoassay capable of detecting 10(15) mol (10(-11) g) of spinach ACP was developed to measure the levels of ACP in leaf cell components isolated by sucrose gradient centrifugation of a gentle lysate of spinach leaf protoplasts. All of the ACP of the leaf cell could be attributed to the chloroplast. Less than 1% of the ACP associated with chloroplasts resulted from binding of free ACP to chloroplasts. Of interest, ACP from Escherichia coli, soybean, and sunflower showed only partial crossreactivity with spinach ACP by the radioimmunoassay. These results strongly suggest that, in the leaf cell, chloroplasts are the sole site for the de novo synthesis of C16 and C18 fatty acids. These fatty acids are then transported into the cytoplasm for further modification and are either inserted into extrachloroplastic membrane lipids or returned to the chloroplast for insertion into lamellar membrane lipids.

Inhibition of carotenoid biosynthesis by the herbicide SAN 9789 and its consequences for the action of phytochrome on plastogenesis

Treatment of the mustard (Sinapis alba L.) seedling with the herbicide SAN 9789 inhibits synthesis of colored carotenoids and interferes with the formation of plastid membrane lipids without affecting growth and morphogenesis significantly. In farred light, which is hardly absorbed by chlorophyll, development of plastid ultrastructure, synthesis of ribulosebisphosphate carboxylase and synthesis of chlorophyll are not affected by SAN 9789. It is concluded that normal phytochrome actions on plastid structural development, protein and chlorophyll syntheses are not affected by the absence of carotenoids provided that there is no significant light absorption in chlorophyll. The findings show that the inhibition of synthesis of one set of plastid membrane components (the carotenoids) does not stop synthesis of other components such as chlorophyll and does not halt membrane assembly. Supplementary experiments with the closely related compound SAN 9785, which affects the amount and composition of plastid lipids but not carotenoid and chlorophyll syntheses, suggest that the effect of the herbicide SAN 9789 is due exclusively to its inhibition of synthesis of colored carotenoids. In the presence of SAN 9789 white or red light at high fluence rate causes photodestruction of chlorophyll and ribulosebisphosphate carboxylase and photodecomposition of thylakoids. These effects are interpreted as resulting exclusively from the self-photooxidation and photosensitizing action of chlorophyll once the protection by carotenoids of chlorophyll against self- and sensitized photooxidation is lost.