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.     

Ontogeny of microbodies (glyoxysomes) in cotyledons of dark-grown watermelon (Citrullus vulgaris Schrad.) seedlings

The development of glyoxysomal marker enzyme activities and concomitant ultrastructural evidence for the ontogeny of glyoxysomes has been studied in cotyledons of dark-grown watermelon seedlings (Citrullus vulgaris Schrad., var. Florida Giant). Catalase (CAT, EC 1.11.1.6) was stained in glyoxysomal structures with the 3,3-diaminobenzidine procedure. Serial sections and high-voltage electron microscopy were used to analyze the three-dimensional structure of the glyoxysomal population. With early germination CAT was localized in three distinct cell structures: spherical microbodies already present in freshly imbibed cotyledons; in appendices on lipid bodies; and in small membrane vesicles between the lipid bodies. Due to their ribosome-binding capacity, both appendices and small vesicles were identified as derivatives of the endoplasmic reticulum (ER). In the following period, glyoxysome formation and lipid body degradation were found to be inseparable processes. The small CAT-containing vesicles attach to a lipid body on a restricted area. Both lipid body appendices and attached cisternae enlarge around and between tightly packed lipid bodies and eventually become pleomorphic glyoxysomes with lipid bodies entrapped into cavities. The close contact between lipid body and glyoxysomes is maintained until the lipid body is digested and the glyoxysomal cavity becomes filled with cytoplasm. During the entire period of increase in glyoxysomal enzyme activities, no evidence was obtained for destruction of glyoxysomes, but small CAT-containing vesicles were observed from day 2 through day 6 after imbibition, indicating a continuous de novo formation of glyoxysomes. This study does not substantiate the hypothesis that glyoxysomes bud directly from the ER. Rather, ER-derivatives, e.g., lipid body appendices or cisternae attached to lipid bodies are interpreted as being glyoxysomal precursors that grow in close contact with lipid bodies both in volume and surface membrane area.Abbreviations CAT catalase - DAB 3,3 diaminobenzidine tetrahydrochloride - ER endoplasmic reticulum - GOX glycolate oxidase - HPR hydroxypyruvate reductase - HVEM high-voltage electron microscopy - ICL isocitrate lyase - MS malate synthase - RER rough endoplasmic reticulum In the figures bars represent 0.1 m (if not stated otherwise)     

Direct interaction between glyoxysomes and lipid bodies in cotyledons of theArabidopsis thaliana ped1 mutant

Summary During germination and subsequent growth of fatty seeds, higher plants obtain energy from the glyconeogenic pathway in which fatty acids are converted to succinate in glyoxysomes, which contain enzymes for fatty acid -oxidation and the glyoxylate cycle. TheArabidopsis thaliana ped1 gene encodes a 3-ketoacyl-CoA thiolase (EC 2.3.1.16) involved in fatty acid -oxidation. Theped1 mutant shows normal germination and seedling growth under white light. However, etiolated cotyledons of theped1 mutant grow poorly in the dark and have small cotyledons. To elucidate the mechanisms of lipid degradation during germination in theped1 mutant, we examined the morphology of theped1 mutant. The glyoxysomes in etiolated cotyledons of theped1 mutant appeared abnormal, having tubular structures that contained many vesicles. Electron microscopic analysis revealed that the tubular structures in glyoxysomes are derived from invagination of the glyoxysomal membrane. By immunoelectron microscopic analysis, acyl-CoA synthetase (EC 6.2.1.3), which was located on the membrane of glyoxysomes in wild-type plants, was located on the membranes of the tubular structures in the glyoxysomes in theped1 mutant. These invagination sites were always in contact with lipid bodies. The tubular structure had many vesicles containing substances with the same electron density as those in the lipid bodies. From these results, we propose a model in which there is a direct mechanism of transporting lipids from the lipid bodies to glyoxysomes during fatty acid -oxidation.     

Development of Endoplasmic Reticulum and Glyoxysomal Membrane Redox Activities during Castor Bean Germination

Redox activities, NADH:ferricyanide reductase, NAD(P)H:cytochrome reductases, and NADH:ascorbate free-radical reductase, are present in endoplasmic reticulum (ER) and glyoxysomal membranes from the endosperm of germinating castor bean (Ricinus comminus L. var Hale). The development of these functions was followed in glyoxysomes and ER isolated on sucrose gradients from castor bean endosperm daily from 0 through 6 days of germination. On a per seed basis, glyoxysomal and ER protein, glyoxysomal and ER membrane redox enzyme activities, and glyoxylate cycle activities peaked at day 4 as did the ER membrane content of cytochrome P-450. NADH:ferricyanide reductase was present in glyoxysomes and ER isolated from dry seed. This activity increased only about twofold in glyoxysomes and threefold in ER during germination relative to the amount of protein in the respective fractions. The other reductases, NADH:cytochrome reductase and NADH:ascorbate free-radical reductase, increased about 10-fold in the ER relative to protein up to 4 to 5 days, then declined. NADPH:cytochrome reductase reached maximum activity relative to protein at day 2 in both organelles. The increases in redox activities during germination indicate that the membranes of the ER and glyoxysome are being enriched with redox proteins during their development. The development of redox functions in glyoxysomes was found to be coordinated with development of the glyoxylate cycle.     

Beta-Oxidation in Glyoxysomes from Euglena*

SYNOPSIS. We demonstrated previously the presence of glyoxysomes containing the glyoxylate cycle enzymes in Euglena gracilis grown in the dark on ethanol. We have now established that the glyoxysomes of Euglena grown on hexanoate also contain the following enzymes of the pathway for β-oxidation of fatty acids: hexanoyl-CoA synthetase, 3-β-hydroxyacyl-CoA dehydrogenase and thiolase. Estimations of specific activities indicate that these enzymes are over 20 times as active in glyoxysomes as they are in mitochondria, suggesting that the β-oxidation of fatty acids occurs almost entirely in Euglena glyoxysomes under these conditions. Thus, the entire portion of the gluconeogenic pathway from fatty acid to succinate is localized in the glyoxysome of Euglena.     

Similarities in the polypeptide composition of glyoxysomal and endoplasmic-reticulum membranes from castor-bean endosperm.

Microsomal fractions, glyoxysomes and mitochondria were isolated from homogenates of germinating castor-bean (Ricinus communis) endosperm by sucrose-density-gradient centrifugation. Washed membrane preparations from these cellular fractions were examined by sodium dodecyl sulphate/polyacrylamide-gel electrophoresis. At corresponding developmental stages the endoplasmic-reticulum and glyoxysomal membranes were strikingly similar in polypeptide composition, at least 16 polypeptides being present in membranes isolated from 3-day-old tissue. Supplying [35S]methionine to intact endosperm tissue resulted in the labelling of all membrane polypeptides, the specific radioactivity in the endoplasmic reticulum being greater than for equivalent polypeptides of the glyoxysomal membrane. Washing these membranes with sodium deoxycholate solution extensively solubilized protein components, with the exception of a predominant polypeptide of mol.wt. 55000. Mitochondrial membrane preparations differed from those of the endoplasmic reticulum and glyoxysomes in polypeptide molecular-weight distribution and the [35S]methionine-labelling pattern. The similarity in polypeptide composition between endoplasmic-reticulum and glyoxysomal membranes is discussed in relation to glyoxysome biogenesis.     

Oleate desaturation and acyl turnover in sunflower (Helianthus annuus L.) seed lipids during rapid temperature adaptation

In-vivo experiments with developing sunflower (Helianthus annuus L.) seeds demonstrated that oleate desaturase activity was stimulated by low temperature (10 °C), repressed by high temperature (30 °C) and rapidly restored by returning the seeds to low temperature. Within time periods of 2–4 h, in which the de-novo fatty acid synthesis was negligible, the percentages of oleate (18:1) and linoleate (18:2) were modified in the seed lipids as a consequence of temperature adaptation. When the seeds were transferred to low temperature, the 18:2 content increased in all lipids from both microsomal membranes and oil bodies. After shifting to high temperature, the overall 18:2 content remained constant, but the 18:2 content decreased in diacylglycerols, phosphatidylcholine (PC) and other polar lipids of the two fractions and also in triacylglycerols (TAGs) of the microsomes but increased in TAGs of the oil bodies. The results indicate that the mechanism for the rapid adaptation of sunflower seeds to temperature changes involves (i) the synthesis or activation of oleate desaturase at low temperature and the reversible inhibition of this enzyme at high temperature and (ii) the exchange of 18:1 and 18:2 between TAGs and PC. Under both low and high temperature, 18:1 is transferred from reserve TAGs to PC and 18:2 is transferred from PC to reserve TAGs. At low temperature, 18:1 is desaturated to 18:2 thus allowing the enrichment of membrane lipids with 18:2, the excess being stored in reserve TAGs. At high temperature, however, and provided that oleate desaturase is repressed, the membrane lipids become enriched in 18:1 and the oil-body TAGs become enriched in 18:2. Received: 11 August 1997 / Accepted: 10 November 1997     

Chloroplast lipid synthesis and lipid trafficking through ER-plastid membrane contact sites

Plant chloroplasts contain an intricate photosynthetic membrane system, the thylakoids, and are surrounded by two envelope membranes at which thylakoid lipids are assembled. The glycoglycerolipids mono- and digalactosyldiacylglycerol, and sulfoquinovosyldiacylglycerol as well as phosphatidylglycerol, are present in thylakoid membranes, giving them a unique composition. Fatty acids are synthesized in the chloroplast and are either directly assembled into thylakoid lipids at the envelope membranes or exported to the ER (endoplasmic reticulum) for extraplastidic lipid assembly. A fraction of lipid precursors is reimported into the chloroplast for the synthesis of thylakoid lipids. Thus polar lipid assembly in plants requires tight co-ordination between the chloroplast and the ER and necessitates inter-organelle lipid trafficking. In the present paper, we discuss the current knowledge of the export of fatty acids from the chloroplast and the import of chloroplast lipid precursors assembled at the ER. Direct membrane contact sites between the ER and the chloroplast outer envelopes are discussed as possible conduits for lipid transfer.     

Accumulation of Free Fatty Acids during Hardening of Chlorella ellipsoidea

Chlorella ellipsoidea Gerneck (IAM C-27) was synchronously grown, and cells at an intermediate stage in the ripening phase of the cell cycle were hardened at 3°C for 48 hours. A nonpolar lipid which increased greatly during hardening was analyzed by gas-liquid chromatography. Palmitic, oleic, linoleic, and linolenic acids were the main components of the lipid. Electron micrographs revealed the appearance of lipid bodies in hardened cells. When formation of free fatty acids and lipid bodies was inhibited with cycloheximide, oligomycin, and 3-(3,4-dichlorophenyl)-1,1-dimethylurea, the development of a high level of hardiness was always inhibited. However, the converse results were not always realized. Cells hardened in the dark in the absence of glucose developed a measurable hardiness in spite of their failure to form free fatty acids. The appearance of lipid bodies was invariably accompanied by the formation of the fatty acids. In pulse-labeling with [¹⁴C]NaHCO₃ for 4 minutes at zero time and at the 12th hour of hardening, initial incorporation rates of ¹⁴C into total lipids of whole cells and the cellular membrane fraction were significantly higher than that into free fatty acids. These results suggest that, although fatty acids are inserted into membrane lipids during hardening, the accumulation of free fatty acids and the appearance of lipid bodies per se are not involved in the development of frost hardiness.     

Evidence that a Proliferation of the Endoplasmic Reticulum Precedes the Formation of Glyoxysomes and Mitochondria in Germinating Castor Bean Endosperm

Excised castor bean endosperm halves incubated with CDP-[Me-¹⁴C]cholineactively incorporated this compound into membrane phosphatidylcholine.The capacity of the tissue to synthesize phosphatidyl-[¹⁴C]cholineincreased during the first 3 d of germination and subsequentlydeclined. At the onset of germination phosphatidyl-[^l4C]cholinewas exclusively recovered in the ER membrane fraction. The rateof incorporation into the ER membranes increased strikinglyduring the first 24 h of germination while that into mitochondriaand glyoxysomes remained low. At later developmental stagesan increasing proportion of the newly synthesized phosphatidyl-[¹⁴C]cholinewas present in mitochondria and glyoxysomes; the rate of incorporationinto the membranes of these organelles increased while thatinto the ER membrane began to level off. The kinetics of CDP-[¹⁴C]cholineincorporation into membrane phosphatidylcholine of the majororganelle fractions of 3-d-old endosperm tissue showed thatthe ER was immediately labelled, whereas a lag period precededthe labelling of mitochondria and glyoxysomes. Assuming that the incorporation of CDP-[¹⁴C]choline into phosphatidylcholineserves as a reliable indicator of membrane synthesis, the resultsobtained suggest that a proliferation of ER membranes precedesthe formation of glyoxysomes and mitochondria in germinatingcastor bean endosperm. A comparison of developmental changesin (a) total ER and glyoxysomal phospholipid content and (b)ER and mitochondrial NADH cytochrome c reductase activity providedadditional evidence supporting this conclusion.     

Neutral and Phospholipids of the Myxococcus xanthus Lipodome during Fruiting Body Formation and Germination

Myxobacteria are well-known for their complex life cycle, including the formation of spore-filled fruiting bodies. The model organism Myxococcus xanthus exhibits a highly complex composition of neutral and phospholipids, including triacylglycerols (TAGs), diacylglycerols (DAGs), phosphatidylethanolamines (PEs), phosphatidylglycerols (PGs), cardiolipins (CLs), and sphingolipids, including ceramides (Cers) and ceramide phosphoinositols (Cer-PIs). In addition, ether lipids have been shown to be involved in development and signaling. In this work, we describe the lipid profile of M. xanthus during its entire life cycle, including spore germination. PEs, representing one of the major components of the bacterial membrane, decreased by about 85% during development from vegetative rods to round myxospores, while TAGs first accumulated up to 2-fold before they declined 48 h after the induction of sporulation. Presumably, membrane lipids are incorporated into TAG-containing lipid bodies, serving as an intermediary energy source for myxospore formation. The ceramides Cer(d-19:0/iso-17:0) and Cer(d-19:0/16:0) accumulated 6-fold and 3-fold, respectively, after 24 h of development, identifying them to be novel putative biomarkers for M. xanthus sporulation. The most abundant ether lipid, 1-iso-15:0-alkyl-2,3-di-iso-15:0-acyl glycerol (TG1), exhibited a lipid profile different from that of all TAGs during sporulation, reinforcing its signaling character. The absence of all these lipid profile changes in mutants during development supports the importance of lipids in myxobacterial development. During germination of myxospores, only the de novo biosynthesis of new cell membrane fatty acids was observed. The unexpected accumulation of TAGs also during germination might indicate a function of TAGs as intermediary storage lipids during this part of the life cycle as well.     

The lipoxygenase-dependent oxygenation of lipid body membranes is promoted by a patatin-type phospholipase in cucumber cotyledons

Oilseed germination is characterized by the mobilization of storage lipids as a carbon and energy source for embryonic growth. In addition to storage lipid degradation in germinating oilseeds via the direct action of a triacylglycerol lipase (TGL) on the storage lipids, a second degradation pathway that is dependent on a specific lipid body trilinoleate 13-lipoxygenase (13-LOX) has been proposed in several plant species. The activity of this specific 13-LOX leads first to the formation of ester lipid hydroperoxides. These hydroperoxy fatty acids are then preferentially cleaved off by a TGL and serve as a substrate for glyoxysomal β-oxidation. As a prerequisite for triacylglycerol (TAG) mobilization, a partial degradation of the phospholipid monolayer and/or membrane proteins of the oil body has been discussed. Evidence has now been found for both processes: partial degradation of the proteins caleosin and oleosin was observed and simultaneously a patatin-like protein together with transient phospholipase (PLase) activity could be detected at the oil body membranes during germination. Moreover, in vitro experiments with isolated oil bodies from mature seeds revealed that the formation of 13-LOX-derived lipid peroxides in lipid body membranes is increased after incubation with the purified recombinant patatin-like protein. These experiments suggest that in vivo the degradation of storage lipids in cucumber cotyledons is promoted by the activity of a specific oil body PLase, which leads to an increased decomposition of the oil body membrane by the 13-LOX and thereby TAGs may be better accessible to LOX and TGL.     

Electron transport in glyoxysomal membranes

Glyoxysomes were isolated from germinating castor bean endosperm by equilibrium density gradient centrifugation in a vertical rotor. To recover the membranes, glyoxysome ghosts were prepared by osmotic shock and then subjected to differential centrifugation. The glyoxysomal membranes and the endoplasmic reticulum (ER), isolated by the same methods, were assayed for electron transport components. Both organelles contained NADH ferricyanide reductase, NADH cytochrome c reductase, and cytochromes b₅ and P-420. The ER also contained cytochrome P-450. Pyridine hemochrome derivatives of the organelle membranes and hemin produced coincident difference spectra, indicating that only b-type cytochromes are present in glyoxysomal and ER membranes. The maximal activities of ferricyanide reductase and cytochrome c reductase in glyoxysomes, 2.19 and 0.33 μmol min^?1 mg membrane protein^?1, respectively, represent 30 and 18% of the activities in the ER. The cytochrome b₅ content of the glyoxysomal membrane is 0.108 nmol mg^?1, 31% of the level found in ER. The reductases from both organelles were resistant to solubilization by salt (0.2 m KCl) and were easily solubilized by detergent (1% Triton X-100). Flavin analysis of the organelles from germinating castor beam endosperm confirmed spectral evidence that the flavin content of glyoxysomes is quite high, 100 pmol mg protein^?1, more than twice that of mitochondria. Three-quarters of the glyoxysomal flavin was solubilized by KCl, but even after salt treatment the glyoxysomal membrane flavin content, 98 pmol mg membrane protein^?1, is three times greater than that of the ER.     

Different routes for integral protein insertion into Ricinus communis protein-body and glyoxysome membranes

Total polyadenylated RNA from ripening or germinating Ricinus communis L. endosperm was translated in rabbit reticulocyte lysate in the absence or presence of canine pancreatic microsomes. The products were immunoprecipitated using antibodies raised againts Triton X-114-extracted integral membrane proteins of protein bodies or glyoxysomes. While the proteins of proteinbody membranes were found to insert co-translationally into added microsomes, this was not observed in the case of glyoxysomal proteins. This observation was confirmed using antibodies raised against a purified glyoxysome membrane protein, alkaline lipase. These results indicate that different routes exist for the insertion of membrane proteins into the two organelles. In both cases membrane-protein insertion does not appear to be accompanied by proteolytic processing.Abbreviations anti-PB antiserum to integral protein-body membrane proteins - anti-G antiserum to integral glyoxysomal membrane proteins - anti-L antiserum to alkaline lipase - ER endoplasmic reticulum - M_r relative molecular mass - mRNA poly(A)-rich messenger RNA - PAGE polyacrylamide gel electrophoresis - poly(A) polyadenylic acid - SDS sodium dodecyl sulphate     

Intramitochondrial phospholipid trafficking

Mitochondrial functions and architecture rely on a defined lipid composition of their outer and inner membranes, which are characterized by a high content of non-bilayer phospholipids such as cardiolipin (CL) and phosphatidylethanolamine (PE). Mitochondrial membrane lipids are synthesized in the endoplasmic reticulum (ER) or within mitochondria from ER-derived precursor lipids, are asymmetrically distributed within mitochondria and can relocate in response to cellular stress. Maintenance of lipid homeostasis thus requires multiple lipid transport processes to be orchestrated within mitochondria. Recent findings identified members of the Ups/PRELI family as specific lipid transfer proteins in mitochondria that shuttle phospholipids between mitochondrial membranes. They cooperate with membrane organizing proteins that preserve the spatial organization of mitochondrial membranes and the formation of membrane contact sites, unravelling an intimate crosstalk of membrane lipid transport and homeostasis with the structural organization of mitochondria.This article is part of a Special Issue entitled: Lipids of Mitochondria edited by Guenther Daum.     

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.     

Lipid analysis of the plasma membrane and mitochondria of brewer’s yeast

The plasma membrane and mitochondria of bottom fermenting brewer's yeast obtained as a by-product of industrial beer production were isolated and the lipid fraction was analyzed. The phospholipid content accounted for 78 mg/g protein in the plasma membrane and 59 mg/g protein in the mitochondria. Major phospholipids in both preparations were phosphatidylinositol, phosphatidylcholine and phosphatidylethanolamine but their proportions differed significantly. In the plasma membrane phosphatidylinositol, and in the mitochondria phosphatidylcholine were present in the highest concentration (37 and 30%, respectively). The main classes of neutral lipids (triacylglycerols, ergosterol, squalene and steryl esters) were twice more abundant in the plasma membrane than in the mitochondria (61 and 33 mg/g protein, respectively). A characteristic of the neutral lipid composition of both organelles was the low content of ergosterol (12 and 7 mg/g protein, respectively) and a high content of squalene (25 and 22 mg/g protein). The main feature of the fatty acid composition of both organelles was the preponderance of saturated fatty acids (78 and 79%, respectively), among which palmitic acid was the principal one. The most expressed characteristics of lipid fractions of the analyzed plasma membranes and mitochondria, high concentration of squalene and preponderance of saturated fatty acids are the consequences of anaerobic growth conditions. The lack of oxygen had possibly the strongest effect on the lipid composition of the plasma membranes and mitochondria of bottom fermenting brewer's yeast.     

Characterization of nuclear membranes and endoplasmic reticulum isolated from plant tissue

Nuclei, nuclear membranes and rough endoplasmic reticulum (rER) were isolated from onion root tips and stems. Structural preservation and purity of the fractions was determined by electron microscopic and biochemical methods. Gross compositional data (protein, phospholipid, nonpolar lipids, sterols, RNA, DNA), phospholipid and fatty acid patterns, enzyme activities (ATPases, ADPase, IDPase, glucose-6-phosphatase, 5'-nucleotidase, acid phosphatase, and NADH- and NADPH-cytochrome C reductases), and cytochrome contents were determined. A stable, high salt-resistant attachment of some DNA with the nuclear membrane was observed as well as the association of some RNA with high salt-treated nuclear and rER membranes. The phospholipid pattern was identical for both nuclear and rER membranes and showed a predominance of lecithin (about 60%) and phosphatidyl ethanolamine (20-24%). Special care was necessary to minimize lipid degradation by phospholipases during isolations. Nonpolar lipids, mostly sterols and triglycerides, accounted for 35-45% of the membrane lipids. Sterol contents were relatively high in both membrane fractions (molar ratios of sterols to phospholipids ranged from 0.12 to 0.43). Sitosterol accounted for about 80% of the total sterols. Palmitic, oleic, and linoleic acids were the most prevalent acids in membrane-bound lipids as well as in storage lipids and occurred in similar proportions in phospholipids, triglycerides and free fatty acids of the membrane. About 80% of the fatty acids in membrane phospholipids and triglycerides were unsaturated. A cytochrome of the b5 type was characterized in these membranes, but P-450-like cytochromes could not be detected. Both NADH and NADPH-cytochrome c reductases were found in nuclear and rER membranes and appeared to be enriched in rER membranes. Among the phosphatases, Mg2+-ATPase and, to lesser extents, ADPase, IDPase and acid phosphatase activities occurred in the fractions, but significant amounts of monovalent ion-stimulated ATPase, 5'-nucleotidase and glucose-6-phosphatase activities did not. The results obtained emphasize that the close biochemical similarities noted between rER and nuclear membranes of animal cells extend to these fractions from plant cells.     

特定电磁波对小麦幼苗膜脂肪酸组成的影响

本文研究了特定电磁波处理小麦种子对其幼苗叶片的总膜脂肪酸组成及线粒体膜脂肪酸组成的影响。在正常情况下,ＴＤＰ处理的总膜脂肪酸组成及线粒体膜脂肪酸组成与对照的区不大。     

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 ¹⁴C-acetate and ³H-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 ³H-diglycerides containing ¹⁴C-fatty acids of more than 20 carbons. In contrast, the endoplasmic reticulum produced ³H-phospholipids as well as ³H-diglycerides rich in ¹⁴C-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 ¹⁴C and ³H 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.