The origin and turnover of organelle membranes in castor bean endosperm

Flip-flop of phospholipids in proteoliposomes reconstituted from detergent extract of chloroplast membranes: kinetics and phospholipid specificity

Eukaryotic cells are compartmentalized into distinct sub-cellular organelles by lipid bilayers, which are known to be involved in numerous cellular processes. The wide repertoire of lipids, synthesized in the biogenic membranes like the endoplasmic reticulum and bacterial cytoplasmic membranes are initially localized in the cytosolic leaflet and some of these lipids have to be translocated to the exoplasmic leaflet for membrane biogenesis and uniform growth. It is known that phospholipid (PL) translocation in biogenic membranes is mediated by specific membrane proteins which occur in a rapid, bi-directional fashion without metabolic energy requirement and with no specificity to PL head group. A recent study reported the existence of biogenic membrane flippases in plants and that the mechanism of plant membrane biogenesis was similar to that found in animals. In this study, we demonstrate for the first time ATP independent and ATP dependent flippase activity in chloroplast membranes of plants. For this, we generated proteoliposomes from Triton X-100 extract of intact chloroplast, envelope membrane and thylakoid isolated from spinach leaves and assayed for flippase activity using fluorescent labeled phospholipids. Half-life time of flipping was found to be 6±1 min. We also show that: (a) intact chloroplast and envelope membrane reconstituted proteoliposomes can flip fluorescent labeled analogs of phosphatidylcholine in ATP independent manner, (b) envelope membrane and thylakoid reconstituted proteoliposomes can flip phosphatidylglycerol in ATP dependent manner, (c) Biogenic membrane ATP independent PC flipping activity is protein mediated and (d) the kinetics of PC translocation gets affected differently upon treatment with protease and protein modifying reagents.

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.

Acquisition of membrane lipids by differentiating glyoxysomes: role of lipid bodies

Glyoxysomes in cotyledons of cotton (Gossypium hirsutum, L.) seedlings enlarge dramatically within 48 h after seed imbibition (Kunce, C.M., R.N. Trelease, and D.C. Doman. 1984. Planta (Berl.). 161:156-164) to effect mobilization of stored cotton-seed oil. We discovered that the membranes of enlarging glyoxysomes at all stages examined contained a large percentage (36-62% by weight) of nonpolar lipid, nearly all of which were triacylglycerols (TAGs) and TAG metabolites. Free fatty acids comprised the largest percentage of these nonpolar lipids. Six uncommon (and as yet unidentified) fatty acids constituted the majority (51%) of both the free fatty acids and the fatty acids in TAGs of glyoxysome membranes; the same six uncommon fatty acids were less than 7% of the acyl constituents in TAGs extracted from cotton-seed storage lipid bodies. TAGs of lipid bodies primarily were composed of palmitic, oleic, and linoleic acids (together 70%). Together, these three major storage fatty acids were less than 10% of both the free fatty acids and fatty acids in TAGs of glyoxysome membranes. Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) constituted a major portion of glyoxysome membrane phospholipids (together 61% by weight). Pulse-chase radiolabeling experiments in vivo clearly demonstrated that 14C-PC and 14C-PE were synthesized from 14C-choline and 14C-ethanolamine, respectively, in ER of cotyledons, and then transported to mitochondria; however, these lipids were not transported to enlarging glyoxysomes. The lack of ER involvement in glyoxysome membrane phospholipid synthesis, and the similarities in lipid compositions between lipid bodies and membranes of glyoxysomes, led us to formulate and test a new hypothesis whereby lipid bodies serve as the dynamic source of nonpolar lipids and phospholipids for membrane expansion of enlarging glyoxysomes. In a cell-free system, 3H-triolein (TO) and 3H-PC were indeed transferred from lipid bodies to glyoxysomes. 3H-PC, but not 3H-TO, also was transferred to mitochondria in vitro. The amount of lipid transferred increased linearly with respect to time and amount of acceptor organelle protein, and transfer occurred only when lipid body membrane proteins were associated with the donor lipid bodies. 3H-TO was transferred to and incorporated into glyoxysome membranes, and then hydrolyzed to free fatty acids. 3H-PC was transferred to and incorporated into glyoxysome and mitochondria membranes without subsequent hydrolysis. Our data are inconsistent with the hypothesis that ER contributes membrane lipids to glyoxysomes during postgerminative seedling growth.(ABSTRACT TRUNCATED AT 400 WORDS)

Microbody transition in greening watermelon cotyledons Double immunocytochemical labeling of isocitrate lyase and hydroxypyruvate reductase

Microbody transition during the greening of watermelon cotyledons (Citrullus vulgaris Schrad.) was studied by double immunocytochemical labeling of the glyoxysomal marker enzyme isocitrate lyase and the peroxisomal marker enzyme hydroxypyruvate reductase. In order to analyze the immunocytochemistry, developmental stages representing the glyoxysomal, microbodytransition and peroxisomal stages were chosen, taking into account the time course of enzyme activity and the amounts of the respective antigens. It was shown that during microbody transition, between 83 and 91% of all the tested microbodies contained isocitrate lyase as well as hydroxypyruvate reductase, which was significantly higher than in the glyoxysomal and peroxisomal stages of development. Comprehensive controls precluded labeling artifacts. Our results support the one-population hypothesis first proposed by Trelease et al. (1971, Plant Physiol. 48, 461-465).

Investigation of the glyoxysome-peroxisome transition in germinating cucumber cotyledons using double-label immunoelectron microscopy

Microbodies in the cotyledons of cucumber seedlings perform two successive metabolic functions during early postgerminative development. During the first 4 or 5 d, glyoxylate cycle enzymes accumulate in microbodies called glyoxysomes. Beginning at about day 3, light-induced activities of enzymes involved in photorespiratory glycolate metabolism accumulate rapidly in microbodies. As the cotyledonary microbodies undergo a functional transition from glyoxysomal to peroxisomal metabolism, both sets of enzymes are present at the same time, either within two distinct populations of microbodies with different functions or within a single population of microbodies with a dual function. We have used protein A-gold immunoelectron microscopy to detect two glyoxylate cycle enzymes, isocitrate lyase (ICL) and malate synthase, and two glycolate pathway enzymes, serine:glyoxylate aminotransferase (SGAT) and hydroxypyruvate reductase, in microbodies of transition-stage (day 4) cotyledons. Double-label immunoelectron microscopy was used to demonstrate directly the co-existence of ICL and SGAT within individual microbodies, thereby discrediting the two-population hypothesis. Quantitation of protein A-gold labeling density confirmed that labeling was specific for microbodies. Quantitation of immunolabeling for ICL or SGAT in microbodies adjacent to lipid bodies, to chloroplasts, or to both organelles revealed very similar labeling densities in these three categories, suggesting that concentrations of glyoxysomal and peroxisomal enzymes in transition-stage microbodies probably cannot be predicted based on the apparent associations of microbodies with other organelles.

The fine structure of Cymbidium ringspot virus infections in host tissues. III. Role of peroxisomes in the genesis of multivesicular bodies

Ultrastructural studies of Nicotiana clevelandii plants systemically infected with Cymbidium ringspot virus, a member of the tombusvirus group, have shown that a clear-cut relationship exists between perioxisomes and multivesicular bodies (MVB). In infected cells, peroxisomes undergo a progressive vesiculation of the bounding membrane through the possible addition of membranous material by the endoplasmic reticulum and become very plastic. Portions of the ground cytoplasm are engulfed either through the invagination of the limiting membrane or the production of membranous appendages that fold back on the main body. Cytochemical tests have shown MVB to possess catalase and glycolate oxidase activity in the matrix. The vesicles contain RNA, a substantial amount of which is double stranded, as indicated by differential RNase digestion tests in high- and low-salt media. The double-stranded RNA may consist of replicative forms or replicative intermediates of the viral nucleic acid. If so, MVB (i.e., modified peroxisomes) may be directly involved in the replication of Cymbidium ringspot virus.

Synthesis and incorporation of phospholipid by peroxisomes of mouse liver

The uptake of radioactively labelled glycerol into the phospholipid fractions of mouse liver has been studied. The incorporation of phospholipid into peroxisomal and microsomal membranes was found to be rapid, and of a similar timescale, whereas mitochondrial membranes were appreciably slower in their uptake of label. Discernible differences were shown to exist between these membrane types in relation to phospholipid composition and lipid turnover. These data are interpreted as supportive of a model for peroxisomal biogenesis which involves formation of these organelles by a budding process from the smooth endoplasmic reticulum.

Senescence and the Fluidity of Rose Petal Membranes : RELATIONSHIP TO PHOSPHOLIPID METABOLISM

In previous work, senescence of rose petal cells has been shown to be accompanied by a gradual decrease of membrane fluidity, as measured by a fluorescence polarization technique. Concomitantly, an increase in the free sterol-to-phospholipid ratio was found. Both observations were verified in this study. Further, experiments carried out on whole tissue and isolated protoplasts during senescence revealed that there was no quantitative change in the level of free sterols. The content of phospholipids decreased without any significant change in their composition. Results from experiments measuring the incorporation of [(32)P]orthophosphate indicated a reduced capacity for phospholipid synthesis in senescent cells. Both young and old tissue showed phospholipase A and D activity, the former increasing with age.It was concluded that the fluidity of rose petal membranes decreases with age as a result of a decrease in phospholipid content, brought about by both reduced synthesis and enhanced degradation. Evidence supporting the view that the phenomena observed are related specifically to changes in the plasmalemma is discussed.

Organelle-specific isozymes of citrate synthase in the endosperm of developing ricinus seedlings

Chromatographic analysis of organelle-associated citrate synthase activity revealed distinct mitochondrial and glyoxysomal forms of the enzyme. The chromatographic elution patterns on hydroxylapatite, carboxymethylcellulose and DEAE-cellulose of citrate synthase from the endosperm of 4.5-day-old castor bean seedlings revealed significant differences for mitochondrial and glyoxysomal activities of the enzyme. The endoplasmic reticulum-associated citrate synthase activity eluted from DEAE-cellulose in a pattern that was identical to that of the glyoxysomal activity. The same kinds of organelle specific isozyme elution patterns were observed with young, developing seedlings. Gibberellic acid-treatment of young seedlings increased total recoverable citrate synthase activity from endosperm tissue but did not modify the organelle specific isozyme relationships.

Induction of glyconeogenic enzymes by gibberellin a(3) in endosperm of castor bean seedlings

Seedlings of castor bean (Ricinus communis cv. Hale) were exposed to gibberellin A(3) (GA(3)) (100 micromolar) for periods up to 20 hours. Endosperm homogenates were fractionated on linear sucrose gradients and enzymes in mitochondria, glyoxysome, and cytosol fractions were assayed. Gibberellin treatment resulted in increases in the activities of enzymes in all three compartments. There were also enzymes in all three compartments which were not affected by exogenous applications of GA(3). The isozymes of l-asparate-alpha-ketoglutarate aminotransferase in both mitochondria and glyoxysomes were induced coordinately, whereas the isozymes of citrate synthase and malate dehydrogenase were not. All gluconeogenic enzymes in glyoxysomes are induced by GA(3). With the exception of the mitochondrial malate dehydrogenase isozyme, all enzymes of the tricarboxylic acid cycle believed to participate in glyconeogenesis were increased. The cytosolic enzymes malate dehydrogenase, phosphoenolpyruvate carboxykinase, and fructose bisphosphatase were induced, but the levels of pyruvate kinase and enolase were not affected by GA(3) treatment.

Organelle Membranes from Germinating Castor Bean Endosperm: II. ENZYMES, CYTOCHROMES, AND PERMEABILITY OF THE GLYOXYSOME MEMBRANE

Glyoxysome ghosts were isolated from germinating castor bean endosperms using established methods. Electron microscopic examination showed that some matrix material was retained within the glyoxysomal membrane. Two cytochrome reductases and phosphorylcholine glyceride transferase co-sedimented with the alkaline lipase, a known component of the glyoxysome membrane, in sucrose gradient centrifugation of osmotically shocked glyoxysomes. The activities of these enzymes in the glyoxysome membranes were compared to those in the endoplasmic reticulum relative to phospholipid content. On this basis, the phosphorylcholine glyceride transferase was 10-fold more active in the endoplasmic reticulum, whereas the lipase was 50-fold more active in the glyoxysome membrane. The cytochrome reductases were only 2-fold more active in the endoplasmic reticulum, indicating that they are components of the two membranes. Difference spectroscopy of the glyoxysome membrane suspension revealed the presence of a b5-type cytochrome similar to that found in the endoplasmic reticulum. Since the glyoxysome membrane is apparently derived from the endoplasmic reticulum, components of the endoplasmic reticulum such as these are likely to be incorporated into the glyoxysome membrane during biogenesis.Enzyme activities involving the cofactors NADH or CoA were measurable in broken, but not in intact, glyoxysomes. Thus, it appears that cofactors for enzymes within the organelle cannot pass through the membrane.

Quantitative and qualitative changes in the endoplasmic reticulum of barley aleurone layers

Changes in the level of the endoplasmicreticulum (ER) marker enzyme cytochrome-c reductase (EC 1.6.2.1) were followed with time of imbibition of de-embryonated half-seeds of barley (Hordeum vulgare L.) and the subsequent incubation of their aleurone layers in gibberellic acid (GA3) and H2O. During imbibition there is an increase in the level of cytochrome-c-reductase activity and in the amount of 280-nm absorbance associated with this enzyme. When aleurone layers are incubated for a further 42 h in water, there is a doubling of the cytochrome-c-reductase activity. In GA3, the activity of cytochrome-c reductase reaches a maximum at 24 h of incubation and thereafter falls to below 70% of its level at the beginning of the incubation period. Changes in the cytochrome-c-reductase activity correlate with changes in the fine structure of the aleurone cell. The ER isolated in low Mg(2+) from aleurone layers incubated in buffer for up to 18 h has buoyant density of 1.13-1.14 g cc(-1) while that from layers incubated in GA3 for 7.5-18 h has a density of 1.11-1.12 g cc(-1). The α-amylase (EC3.2.1.1) isolated with the organelle fraction by Sepharose gel filtration is associated with the ER on isopycnic and rate-zonal density gradients, and its activity can be enhanced by Triton X-100. The soluble α-amylase fraction from Separose-4B columns, on the other hand, is not Triton-activated but is acid-labile. Acid phosphatase (EC3.1.3.2) is distributed in at least three peaks on isopycnic gradients. In low Mg(2+) the second peak of activity has a density of 1.12 g cc(-1) in GA3-treated tissue and 1.13-1.14 g cc(-1) in H2O-treated tissue. With high-Mg(2+) buffers, this peak of phosphatase activity disappears. Acid-phosphatase activity is not enhanced by Triton X-100 nor is it acid-labile.

Protoplast Fusion: EFFECT OF LOW TEMPERATURE ON THE MEMBRANE FLUIDITY OF CULTURED CELLS

The relation between the composition of the phospholipid molecular species in a cell membrane and the velocity of protoplast fusion was studied using cells cultured at a low temperature (10 C). Cells cultured at a low temperature contained larger proportions of phospholipids of low phase transition point, the 1,2-dilinoleoyl-type, than those cultured at a normal temperature (25 C). When treated with polyethylene glycol 6000, protoplasts from cells cultured at 10 C fused and progressed to the fused sphere stage more rapidly than did those from cells cultured at 25 C.

Lipid metabolism in germinating seeds. Purification of ethanolamine kinase from soya bean

Ethanolamine kinase has been purified to homogeneity from germinating soya bean (Glycine max L.) seeds. The purified enzyme had a molecular weight of 17--19 000 as estimated by gel filtration and sodium dodecyl suphate-polyacrylamide gel electrophoresis. It would not phosphorylate choline, had a Km for ethanolamine of 8 microM and utilised Mg-ATP. The kinase could be purified in a 37 000 molecular weight form (dimer) which would easily dissociate on storage. In contrast to ethanolamine kinase whose activity was unaffected by the presence of choline in the assay system, soya bean choline kinase, although not phosphorylating ethanolamine, was competitively inhibited by the latter. The purification of specific choline and ethanolamine kinases from germinating soya bean confirmed in vivo observations which had indicated separate enzymes.

Apparent Catalase Synthesis in Sunflower Cotyledons during the Change in Microbody Function: A Mathematical Approach for the Quantitative Evaluation of Density-labeling Data

Density-labeling with 10 mm K(15)NO(3)/70% (2)H(2)O has been used to investigate catalase synthesis in different developmental stages of sunflower (Helianthus annuus L.) cotyledons. A mathematical approach is introduced for the quantitative evaluation of the density-labeling data. The method allows, in the presence of preexisting enzyme activity, calculation of this synthesized activity (apparent enzyme synthesis) which results from the balance between actual enzyme synthesis and the degradation of newly synthesized enzyme at a given time. During greening of the cotyledons, when the catalase activity declines and the population of leaf peroxisomes is formed, the apparent catalase synthesis is lower than, or at best equal to, that occurring during a developmental stage when the leaf peroxisome population is established and catalase synthesis and degradation of total catalase are in equilibrium. This result suggests a formation, in fatty cotyledons, of the leaf peroxisomes by transformation of the glyoxysomes rather than by de novo synthesis.

Effect of gibberellin a(3) on the endoplasmic reticulum and on the formation of glyoxysomes in the endosperm of germinating castor bean

Seedlings of castor bean (Ricinus communis cv. Hale) were exposed to a range of concentrations of gibberellin A(3) (GA(3)). Treatments for 20 hours with GA(3) concentrations of 0.5 muM or higher resulted in increased levels of NADH-cytochrome c reductase, phosphorylcholine glyceride transferase, and malate synthase in endoplasmic reticulum (ER) isolated from endosperm on linear sucrose gradients. GA(3) treatment also resulted in increased RNA associated with ER. Malate synthase and catalase in crude homogenates were enhanced by 1 to 100 muM GA(3) concentrations. Isocitrate lyase, citrate synthase, malate synthase, catalase, and glycolate oxidase in isolated glyoxysomes were enhanced by 60, 20, 18, 40, and 28%, respectively, over controls. Treatment with abscisic acid led to decreased levels of glyoxysomal enzymes and reduced glyoxysomal protein. The effect of GA(3) and abscisic acid on the specific activities of glyoxysomes of different densities suggests that GA(3) influences enzyme levels and glyoxysome assembly.

Control of Enzyme Activities in Cotton Cotyledons during Maturation and Germination: II. Glyoxysomal Enzyme Development in Embryos

The sequence of glyoxysomal enzyme development was investigated in cotyledons of cotton (Gossypium hirsutum L. cv. Deltapine 16) embryos from 16 to 70 days after anthesis (DAA). Catalase, malate dehydrogenase, and citrate condensing enzyme activities were barely detectable prior to 22 DAA, but showed dramatic increases from 22 to 50 DAA. Development of malate synthase activity, however, was delayed during this period, rising to peak activity from 45 to 50 DAA (just prior to desiccation) in the absence of any detectable isocitrate lyase activity. Substantial activities of all of these enzymes (except isocitrate lyase) persisted in the dry seeds. Isopycnic centrifugations on sucrose gradients demonstrated that the enzymes were compartmentalized within particles increasing in buoyant density with time of development (1.226 to 1.245 grams per cubic centimeter from 22 to 50 DAA). Of particular significance were the observations in 22-day embryos of smooth surfaced membrane dilations of rough endoplasmic reticulum having cytochemical catalase reactivity, and the demonstrations of catalase activities in microsomal fractions isolated throughout the 16- to 50-DAA period. Our data do not allow determination of the mechanism(s) for enzyme activation and/or addition to previously existing or newly formed microbodies, but do show that development and acquisition of enzyme activities within glyoxysomes occur sequentially and thus are not regulated in concert as previously thought.

Evidence that glyoxysomal malate synthase is segregated by the endoplasmic reticulum

At the onset of castor bean (Ricinus communis) germination, 76% of the cellular malate synthase activity of the endosperm tissue was located in the microsomal fraction, with the remainder in the glyoxysomal fraction. During later developmental stages, when rapid malate synthase synthesis was occurring, an increasing proportion of the enzyme was recovered in glyoxysomes. The kinetics of [(35)S]methionine incorporation into microsomal and glyoxysomal malate synthase in 2-day-old endosperm tissue was followed by employing antiserum raised against glyoxysomal malate synthase to precipitate specifically the enzyme from KCl extracts of these organelle fractions. This experiment showed that microsomal malate synthase was labeled before the glyoxysomal enzyme. When such kinetic experiments were interrupted by the addition of an excess of unlabeled methionine, (35)S-labeled malate synthase was rapidly lost from the microsomal fraction and was quantitatively recovered in the glyoxysomal fraction.Free cytoplasmic ribosomes were separated from bound ribosomes (rough microsomes) using endosperm tissue labeled with [(35)S]methionine or (14)C-amino-acids. Nascent polypeptide chains were released from polysome fractions using a puromycin-high salt treatment, and radioactive malate synthase was shown to be exclusively associated with bound polysomes.Together these data establish that malate synthase is synthesized on bound ribosomes and vectorially discharged into the endoplasmic reticulum cisternae prior to its ultimate sequestration in glyoxysomes.

Purification and comparative properties of microsomal and glyoxysomal malate synthase from castor bean endosperm

Sucrose density gradient centrifugation was employed to separate microsomes, mitochondria, and glyoxysomes from homogenates prepared from castor bean (Ricinus communis) endosperm. In the case of tissue removed from young seedlings, a significant proportion of the characteristic glyoxysomal enzyme malate synthase was recovered in the microsomal fraction. Malate synthase was purified from both isolated microsomes and glyoxysomes by a procedure involving osmotic shock, KCI solubilization, and sucrose density gradient centrifugation. All physical and catalytic properties examined were identical for the enzyme isolated from both organelle fractions. These properties include a molecular weight of 575,000, with a single subunit type of molecular weight 64,000, a pH optimum of 8, apparent K(m) for acetyl-CoA of 10 mum and glyoxylate of 2 mm. Microsomal and glyoxysomal malate synthases showed identical responses to various inhibitors. Adenine nucleotides were competitive inhibitors with respect to acetyl-CoA, and oxalate (K(i) 110 mum) and glycolate (K(i) 150 mum) were competitive inhibitors with respect to glyoxylate. Antiserum raised in rabbits against purified glyoxysomal malate synthase was used to confirm serological identity between the microsomal and glyoxysomal enzymes, and was capable of specifically precipitating (35)S-labeled malate synthase from KCI extracts of both microsomes and glyoxysomes isolated from [(35)S]methionine-labeled endosperm tissue.

Phospholipid turnover in soybean tissue cultures

The degradation rates of phospholipids in soybean (Glycine max L. Merrill) suspension cultures were studied by pulse-chase experiments. The only chloroform-soluble product of incorporation of radioactive choline was phosphatidylcholine, the bulk of which had a half-life of 36 hours. Ethanolamine was incorporated primarily into phosphatidylethanolamine, phosphatidylcholine at an intermediate level, and phosphatidylmonomethylethanolamine to a small extent. The phosphatidylethanolamine decayed in a triphasic fashion with half-lives of 12, 34, and 136 hours. Phosphatidylcholine in this case increased in radioactivity up to day 4 and thereafter declined with a 92-hour half-life. The radioactivity rose slightly to day 4 in phosphatidylmonomethylethanolamine after an initial rapid decline. When serine was used as a substrate, half-lives similar to those obtained with ethanolamine were obtained. Phosphatidylcholine contained the greatest amount of label, however, with phosphatidylethanolamine containing slightly less, and phosphatidylserine contained the least. Data also are presented for glycerol and acetate phospholipid product degradation.

Lipid composition of organelles from germinating castor bean endosperm

Glyoxysome, endoplasmic reticulum, mitochondria, and proplastid fractions were isolated from endosperm of castor beans (Ricinus communis) germinated for 5 days at 30 C. Samples from sucrose density gradients were diluted with 0.15 m KCI and the membranes pelleted. Lipid extracts of these membranes were analyzed for phosphoglyceride, acyl lipid, and sterol content. The endoplasmic reticulum contains 1.24 mumol of phosphoglyceride per mg of protein; the mitochondria, 0.65 mumol/mg; and the glyoxysome membranes, 0.55 mumol/mg. Phosphatidyl choline and phosphatidyl ethanolamine are the most abundant lipids in all membranes studied, accounting for 70% or more of the lipid phosphorus and 50% or more of the fatty acid. Glyoxysome membranes and endoplasmic reticulum also contain phosphatidyl inositol (respectively, 9 and 17% of the lipid phosphorus) and free fatty acids (13% of the total fatty acid in each). Compared with other organelles, mitochondrial membranes have more phosphatidyl ethanolamine relative to phosphatidyl choline and are characterized by the presence of cardiolipin, in which 80% of the fatty acid is linoleate. The relative amounts of linoleate, palmitate, oleate, stearate, and linolenate in each of the phosphotoglycerides are constant regardless of the membrane source. Stimasgasterol and beta-sitosterol are present in the membranes (1-9 nmol each/mg protein).The data provide further evidence that glyoxysome membranes are derived from the endoplasmic reticulum but at the same time indicate some differentiation.

Glyoxysomal malate dehydrogenase of watermelon cotyledons: De novo synthesis on cytoplasmic ribosomes

The development of glyoxysomal malate dehydrogenase (gMDH, EC 1.1.1.37) during early germination of watermelon seedlings (Citrullus vulgaris Schrad.) was determined in the cotyledons by means of radial immunodiffusion. The active isoenzyme was found to be absent in dry seeds. By density labelling with deuterium oxide and incorporation of [(14)C] amino acids it was shown that the marked increase of gMDH activity in the cotyledons during the first 4 days of germination was due to de novo synthesis of the isoenzyme. The effects of protein synthesis inhibitors (cycloheximide and chloramphenicol) on the synthesis of gMDH indicated that the glyoxysomal isoenzyme was synthesized on cytoplasmic ribosomes. Possible mechanisms by which the glyoxysomal malate dehydrogenase isoenzyme reaches its final location in the cell are discussed.

Serological and developmental relationships between endoplasmic reticulum and glyoxysomal proteins of castor bean endosperm

Glyoxysomes isolated from the endosperm of castor bean (Ricinus communis L.) by sucrose density gradient centrifugation were fractionated into their matrix protein and membrane components. Antisera were raised in rabbits against both the matrix proteins and sodium dodecyl sulphate (SDS)-solubilized membrane proteins. SDS-polyacrylamide gel electrophoresis (PAGE) analysis established that such antisera precipitate all major polypeptide components present in their respective glyoxysomal mixedantigen preparations. Furthermore, when soluble constituents recovered from the microsomal vesicles or solubilized microsomal membranes were challenged with the appropriate glyoxysomal antiserum, serological determinants were again found to be present. Intact endosperm tissue was incubated with [(35)S]methionine and the kinetics of (35)S-incorporation into protein recovered in immunoprecipitates when the glyoxysomal matrix fraction or the soluble fraction released from the microsomes were incubated with anti-glyoxysomal matrix serum were followed. [(35)S]antigens rapidly appeared in the microsomal fraction whereas a lag period preceded their appearance in glyoxysomes. Interupting such kinetic experiments by the addition of an excess of unlabelled methionine resulted in a rapid decrease in the microsomal content of [(35)S]antigens and a concomitant increase in glyoxysomal content.

Membrane lipid metabolism in germinating castor bean endosperm

Castor bean (Ricinus communis L. var. Hale) endosperms, excised after 2 days germination at 30 C, were incubated 5 min to 8 hr with (14)C-acetate and (3)H-glycerol. Homogenates were fractionated by sucrose gradient centrifugation. Organelles found to be active in lipid synthesis were the lipid bodies and the endoplasmic reticulum. The products of incorporation in the lipid bodies were (3)H-diglycerides containing (14)C-fatty acids of more than 20 carbons. In contrast, the endoplasmic reticulum produced (3)H-phospholipids as well as (3)H-diglycerides rich in (14)C-linoleate. The phospholipids synthesized and their acyl contents were of the types known to be the major components of organelle membranes in this tissue. Phospholipids and diglycerides containing (14)C and (3)H were found in the glyoxysomes and mitochondria subsequent to their appearance in the endoplasmic reticulum. The results show that germinating castor bean endosperm synthesizes membrane lipids de novo from acetate rather than reutilizing stored lipid components directly. It is also apparent that the endoplasmic reticulum is responsible for several steps in membrane lipid production.

Some properties of a microsomal oleate desaturase from leaves

1. When [1-14C]oleoyl-CoA was incubated with a pea-leaf homogenate oleate was both incorporated into microsomal 3-sn-phosphatidylcholine and released as the unesterified fatty acid. The proportion of oleate incorporated into this phospholipid was dependent on the relative amounts of thiol ester and microsomal preparation present in reactions. 2. At the concentrations of microsomal preparation and [14C]oleoyl-CoA used to study oleate desaturation the metabolism of the thiol ester was essentially complete after 5 min incubation, but the loss of label from 3-sn-phosphatidylcholine oleate and the concomitant increase in radioactivity in the linoleate of this phospholipid proceeded at approximately linear rates over a 60 min period. The kinetics of labelling of unesterified linoleate was consistent with the view that this labelled fatty acid was derived from 3-sn-phosphatidylcholine. 3. Oleate desaturation required oxygen and with unwashed microsomal fractions was stimulated either by NADPH or by the 105 000g supernatant. Washed microsomal preparations did not catalyse desaturation, but actively was restored by the addition of NADPH, 105 000G supernatant or Sephadex-treated supernatant. NADPH could be replaced by NADH or NADP+, but not by NAD+. 4. Microsomal fractions from mature and immature maize lamina and expanding spinach leaves also rapidly incorporated oleate from ([14C]oleoyl-CoA into 3-sn-phosphatidylcholine, but desaturation of 3-sn-phosphatidylcholine oleate was detected only with microsomal preparations from immature maize lamina. 5. It is proposed that leaf microsomal preparations posses an oleate desaturase for which 3-sn-phosphatidylcholine oleate is either the substrate or an immediate precursor of the substrate.

Role of the endoplasmic reticulum in glyoxysome formation in castor bean endosperm

Homogenates of the endosperm of castor bean (Ricinus communis var. Hale) were prepared at intervals during germination and fractionated on sucrose gradients. Early in germination when glyoxysomes were being produced, a substantial proportion (50%) of the activities of malate synthetase and citrate synthetase was recovered in the membranes of the endoplasmic reticulum (mean density 1.12 grams per cubic centimeter). This proportion declined to less than 10% at 4 days when the glyoxysomes were fully developed.Gradient fractions challenged by antiglyoxysome-protein antiserum in double immunodiffusion assay revealed strong antigenic response in the endoplasmic reticulum membranes. The results support the view advanced earlier that glyoxysomes are derived directly from the endoplasmic reticulum.

Studies on lipid synthesis and degradation in developing soybean cotyledons

The metabolic activity of individual lipid classes found in developing soybean cotyledons (Glycine max.) is estimated by determining the degradation rate of the compound under given conditions. Pulse-labeling and dual substrate labeling are used to evaluate this parameter. These studies indicate first order decay kinetics for phosphatidic acid, phosphatidylinositol, phosphatidylcholine, phosphatidylethanolamine, N-acyl-phosphatidylethanolamine, diglyceride, and zero order kinetics for triglyceride in cotyledons var. "Harosoy 63" at 30 days after flowering. Decay coefficients for acyl groups and lipid-glycerol moieties within specific lipid classes from either method are comparable. Half-life (t((1/2))) calculations from the decay coefficients indicate extremely rapid turn-over rates (0.08 to 3.4 hours at 25 C) and suggest similar turnover rates of acyl groups and lipid-glycerol in diglyceride and all phospholipids except N-acylphosphatidylethanolamine where acyl groups are replaced independent of the glycerol moiety. These experiments reveal not only different metabolic activity between lipid components of soybean cotyledons, but also describe a new method for measuring lipid turnover in plants.

The cellular origin of glyoxysomal proteins in germinating castor-bean endosperm

The capacity of castor-bean endosperm tissue to incorporate [35S]methionine into proteins of the total particulate fraction increased during the first 3 days of germination and subsequently declined. At the onset of germination 66% of the incorporated 35S was found in the separated endoplasmic-reticulum fraction, with the remainder in mitochondria, whereas at later developmental stages an increasing proportion of 35S was recovered in glyoxysomes. The kinetics of [35S]methionine incorporation into the major organelle fractions of 3-day-old endosperm tissue showed that the endoplasmic reticulum was immediately labelled, whereas a lag period preceded the labelling of mitochondria and glyoxysomes. When kinetic experiments were interrupted by the addition of an excess of unlabelled methionine, incorporation of [35S]methionine into the endoplasmic reticulum rapidly ceased, but incorporation into mitochondia and glyoxysomes continued for a further 1h. Examination of isolated organelle membranes during this period showed that the addition of unlabelled methionine resulted in a stimulated incorporation of [35S]no methionine into the endoplasmic-reticulum membrane for 30 min, after which time the 35S content of this fraction declined, whereas that of the glyoxysomal membranes continued to increase slowly. The 35S-labelling kinetics of organelles and fractions derived therefrom are discussed in relation to the role of the endoplasmic reticulum in protein synthesis during glyoxysome biogenesis.

Phospholipid synthesis and exchange in castor bean endosperm homogenates

Crude organelle preparations from castor bean (Ricinus communis L.) endosperm rapidly incorporate CDP-((14)C)choline and CDP-((14)C)-ethanolamine into phosphatidylcholine and phosphatidylethanolamine, respectively. Separation of organelles by sucrose density gradient centrifugation following incubation with these substrates demonstrated that most of the (14)C phospholipids thus formed were present in the endoplasmic reticulum membranes, although label was also found in mitochondria, proplastids, and glyoxysomes. The phospholipid-synthesizing enzymes, cholinephosphotransferase and ethanolaminephosphotransferase, are exclusively confined to the endoplasmic reticulum membrane fraction, suggesting that the appearance of (14)C-phospholipid in other organelles was due to phospholipid exchange. Phospholipid synthesis was inhibited by the cytoplasmic supernatant fraction. The active inhibitor in this fraction was not identified, but the inhibition was not significantly relieved by either dialyzing or boiling the supernatant. Phosphatidylcholine synthesis showed an absolute requirement for Mg(2+); the Michaelis constant was 1 mm. Ca(2+) was a potent inhibitor of Mg(2+)-stimulated phospholipid synthesis and enhanced the decay of (14)C-phospholipids from pre-labeled membranes, particularly when the membranes were resuspended in the cytoplasmic supernatant.The data are consistent with the concept that the endoplasmic reticulum is a major site of membrane proliferation where structural lipids, and possibly proteins, are inserted into, and thus expand, a pre-existing membrane fraction. Other organelle and cellular membranes could therefore originate from the proliferating endoplasmic reticulum by a process of membrane flow and differentiation.

Phytochrome-mediated transformation of glyoxysomes into peroxisomes in the cotyledons of mustard (Sinapis alba L.) seedlings

The specific changes in the temporal pattern of glyoxysomal and peroxisomal enzymes in dark-grown and continuously far-red irradiated mustard seedlings are accompanied by specific changes in the spatial associations of microbodies with other cell organelles which can be quantitatively estimated from electron micrographs. The association (surface contact) with oleosomes (lipid bodies) and with plastids have been used as operational criteria for the glyoxysomal and peroxisomal engagement, respectively, of individual microbodies. The time course of these specific associations during the phytochrome-mediated changeover from glyoxysomal to peroxisomal character reveals the transient formation of functionally intermediary microbodies ("glyoxyperoxisomes") which are associated to oleosomes as well as to plastids. In continuous far-red light, up to 50% of the microbody profiles detectable on electron micrographs fall into this category, compared to about 10% in darkness. It is concluded that peroxisomes of cotyledons neither originate de novo as an independent population nor are formed from pre-existing glyoxysomes by repackaging of enzymes. We suggest rather that a transition from glyoxysomal to peroxisomal enzyme formation in the presence of continuous turnover of microbody particles leads to a gradual replacement of microbodies of glyoxysomal character by microbodies of intermediary character and ultimately by microbodies of peroxisomal character.

The kinetics of incorporation in vivo of (14C)acetate and (14C)carbon dioxide into the fatty acids of glycerolipids in developing leaves

1. The patterns of incorporation of (14)C into glycerolipid fatty acids of developing maize leaf lamina from supplied [1-(14)C]acetate and from (14)CO(2) during steady-state photosynthesis were similar. Oleate of phosphatidylcholine and palmitate of phosphatidylglycerol attained linear rates of labelling more rapidly than did other fatty acids, particularly the linoleate and linolenate of monogalactosyl diacylglycerol. 2. After the transfer of lamina from labelled to unlabelled acetate, there was a decrease in labelled oleate and linoleate of phosphatidylcholine and a concomitant increase in the amount of radioactivity in the linoleate and linolenate of monogalactosyl diacylglycerol. 3. The rapidly labelled phospholipids, phosphatidylcholine and phosphatidylglycerol, were shown by differential and sucrose-density-gradient centrifugation to be associated with different organelles, the former being mainly in a low-density membrane fraction, probably microsomal, and the latter mainly in chloroplasts. 4. During a 48h period after supplying spinach leaves with [(14)C]acetate, radioactivity was lost from the oleate of phosphatidylcholine present in fractions sedimented at 12000g and 105000g, and accumulated in the linolenate of monogalactosyl diacylglycerol of the chloroplast. 5. It is proposed that the phosphatidylcholine of some non-plastid membranes is intimately involved in the process of oleate desaturation and that this lipid serves as a donor of unsaturated C(18) fatty acids to other lipids, principally monogalactosyl diacylglycerol, of the chloroplasts.

Studies on the Development and Localization of Catalase and H(2)O(2)-generating Oxidases in the Endosperm of Germinating Castor Beans

During germination of castor bean seeds (Ricinus communis var. Hale), the changes of activity of catalase, uricase, and alpha-hydroxyacid oxidase of the endosperm follow a rise and fall pattern with a peak between day 4 and 5 similar to that observed for the glyoxylate cycle enzymes. After 3 days of germination, most of the activities of these enzymes are recovered from the glyoxysomal fraction separated by isopycnic sucrose density gradient centrifugation.The activities of the oxidases of the glyoxysomal fractions increase somewhat more slowly than those of catalase or isocitrate lyase during the first 3 days of germination. But separation of the oxidative enzyme activities from glyoxysomal isocitrate lyase activity was not achieved by density gradient centrifugation or by density gradient electrophoresis. The sedimentation velocities of these two kinds of enzyme activities were found to be identical. The role and regulation of the glyoxysomal uricase, glycolate oxidase, lactate oxidase, and catalase activity in endosperm cells are discussed.

The development of microbodies (glyoxysomes and leaf peroxisomes) in cotyledons of germinating watermelon seedlings

The ontogeny of glyoxysomes and leaf peroxisomes has been examined in the cotyledons of germinating watermelon (Citrullus vulgaris) seedlings. Organelles from the cotyledons were extracted by razor blade homogenization and microbodies were separated by sucrose density gradient fractionation. Both kinds of microbodies have the same mean equilibrium density on sucrose gradients.The development of leaf peroxisomes was examined in seedlings transferred to light at 4 days and 10 to 12 days. In seedlings maintained in darkness to the age of 10 to 12 days, glyoxysomal enzymes virtually disappeared, and the losses were paralleled by a corresponding loss in microbody protein. During this period peroxisomal activity was low and changed only slightly. On transfer to light at this stage, the activity of peroxisomal enzymes rose strikingly. The residual glyoxysomal activity disappeared completely, and the developmental pattern of microbody catalase and microbody protein paralleled the light-induced glyoxysomal disappearance.Similar patterns of microbody development were observed when 4-day-old dark-grown seedlings with maximum glyoxysomal activities were exposed to light. The activity of the peroxisomal enzymes increased and the glyoxysomal enzymes disappeared at a faster rate than in darkness. These changes were again paralleled by the accelerated demise of microbody catalase and microbody protein. Thus under both conditions glyoxysomes were selectively destroyed during peroxisomal development, and the amount of peroxisomes produced was insufficient to offset the loss of glyoxysomal protein. The results do not support the contention that glyoxysomes are transformed to leaf peroxisomes in developing cucurbit cotyledons and favor the view that the two kinds of microbody arise independently of each other.

Studies on Phospholipid-synthesizing Enzyme Activities during the Growth of Etiolated Cucumber Cotyledons

The enzymatic incorporation of sn-glycerol 3-phosphate into lipid by extracts of cucumber (Cucumis sativus) cotyledons showed an absolute requirement for ATP (saturation 2 mM). The incorporation was stimulated 4-fold by 0.2 mM oleate. Ethyldiaminetetraacetate stimulated the incorporation at concentrations below 1 mM and inhibited at higher concentrations. Mg(2+) did not affect the reaction. Triton X-100 and Cutscum inhibited the reaction, while a third detergent, Span 80, was stimulatory. p-Mercuribenzoate was inhibitory. The enzymatic reaction has a pH optimum in the range of 8.8 to 9.6. The Michaelis constant was 112 muM for sn-glycerol 3-phosphate. The major amount of product was phosphatidic acid, the remainder was diacylglycerol, monoacylglycerol, and an unknown phospholipid.The activity profiles for two glyoxysmal enzymes, malate synthetase and catalase, were compared to the activities of four enzymes involved in phospholipid synthesis. Phosphatidylcholine and phosphatidylethanolamine synthesis paralleled the activity profiles of catalase and malate synthetase, as well as the levels of endogenous diglycerides. sn-Glycerol 3-phosphate incorporation peaked at a later stage of cucumber cotyledon growth than the glyoxysomal enzymes and seemed to be the major pathway of phosphatidic acid synthesis. Diglyceride phosphokinase activity did not reach appreciable levels during the first 11 days of cucumber cotyledon growth.