The regulation of the fatty-acid composition of the triacylglycerols in microsomal preparations from avocado mesocarp and the developing cotyledons of safflower

The utilisation of [¹⁴C]glycerol 3-phosphate and [¹⁴C]linoleoyl-CoA in the synthesis of triacylglycerol has been studied in the microsomal preparations of developing cotyledons of safflower seed. The results confirm that the glycerol backbone, which flows towards triacylglycerol from phosphatidic acid through the Kennedy pathway, can enter phosphatidylcholine from diacylglycerol. The equilibration between diacylglycerol and phosphatidylcholine offers a mechanism for the return of oleate to phosphatidylcholine for desaturation to linoleate. We have established that the oleate entering position 1 of sn-phosphatidylcholine from diacylglycerol is desaturated in situ to linoleate. The results indicate that the diacylglycerol phosphatidylcholine interconvertion coupled to the acyl exchange between acyl-CoA and position 2 of sn-phosphatidylcholine brings about the continuous enrichment of the glycerol backbone with C₁₈-polyunsaturated fatty acids and hence these enzymes are of major importance in regulating the acyl quality of the accumulating triacylglycerols. Microsomal preparations from avocado mesocarp, however, did not have detectable acyl exchange between acyl-CoA and phosphatidylcholine or diacylglycerol phosphatidylcholine interconversion despite the high activity of the enzymes of the Kennedy pathway. A scheme is presented which incorporates many of the observations on triacylglycerol synthesis and provides a working model for the regulation of acyl quality in linoleate-rich vegetable oils.Abbreviation BSA bovine serum albumin     

The utilisation of fatty-acid substrates in triacylglycerol biosynthesis by tissue-slices of developing safflower (Carthamus tinctorius L.) and sunflower (Helianthus annuus L.) cotyledons

Developing cotyledons of safflower (Carthamus tinctorius L.) and sunflower (Helianthus annuus L.) readily utilised exogenously supplied ¹⁴C-labelled fatty-acid substrates for the synthesis of triacylglycerols. The other major radioactive lipids were phosphatidylcholine and diacylglycerol. In safflower cotyledons, [¹⁴C]oleate was rapidly transferred to position 2 of sn-phosphatidylcholine and concomitant with this was the appearance of radioactive linoleate. The linoleate was further utilised in the synthesis of diacyl- and triacyl-glycerol via the reactions of the so-called Kennedy pathway. Supplying [¹⁴C]linoleate, however, resulted in a more rapid labelling of the diacylglycerols than from [¹⁴C]oleate. In contrast, sunflower cotyledons readily utilised both labelled acyl substrates for rapid diacylglycerol formation as well as incorporation into position 2 of sn-phosphatidylcholine. In both species, however, [¹⁴C]palmitate largely entered sn-phosphatidylcholine at position 1 during triacylglycerol synthesis. The results support our previous in-vitro observations with isolated microsomal membrane preparations that (i) the entry of oleate into position 2 of sn-phosphatidylcholine, via acyl exchange, for desaturation to linoleate is of major importance in regulating the level of polyunsaturated fatty acids available for triacylglycerol formation and (ii) Palmitate is largely excluded from position 2 of sn-phosphatidylcholine and enters this phospholipid at position 1 probably via the equilibration with diacylglycerol. Specie differences appear to exist between safflower and sunflower in relation to the relative importance of acyl exchange and the interconversion of diacylglycerol with phosphatidylcholine as mechanisms for the entry of oleate into the phospholipid for desaturation.Abbreviations FW fresh weight - TLC thin-layer chromatography     

The biosynthesis of triacylglycerols in microsomal preparations of developing cotyledons of sunflower (Helianthus annuus L.)

The synthesis of triacylglycerols was investigated in microsomes (microsomal fractions) prepared from the developing cotyledons of sunflower (Helianthus annuus). Particular emphasis was placed on the mechanisms involved in controlling the C18- unsaturated-fatty-acid content of the oils. We have demonstrated that the microsomes were capable of: the transfer of oleate from acyl-CoA to position 2 of sn-phosphatidylcholine for its subsequent desaturation and the return of the polyunsaturated products to the acyl-CoA pool by further acyl exchange; the acylation of sn-glycerol 3-phosphate with acyl-CoA to yield phosphatidic acid, which was further utilized in diacyl- and tri-acylglycerol synthesis; and (3) the equilibrium of a diacylglycerol pool with phosphatidylcholine. The acyl exchange between acyl-CoA and position 2 of sn-phosphatidylcholine coupled to the equilibration of diacylglycerol and phosphatidylcholine brings about the continuous enrichment of the glycerol backbone with C18 polyunsaturated fatty acids for triacylglycerol production. Similar reactions were found to operate in another oilseed plant, safflower (Carthamus tinctorius L.). On the other hand, the microsomes of avocado (Persea americana) mesocarp, which synthesize triacylglycerol via the Kennedy [(1961) Fed. Proc. Fed. Am. Soc. Exp. Biol. 20, 934-940] pathway, were deficient in acyl exchange and the diacylglycerol in equilibrium phosphatidylcholine interconversion. The results provide a working model that helps to explain the relationship between C18- unsaturated-fatty-acid synthesis and triacylglycerol production in oilseeds.     

Triacylglycerols are synthesised and utilized by transacylation reactions in microsomal preparations of developing safflower (Carthamus tinctorius L.) seeds

Microsomal membrane preparations from the immature cotyledons of safflower (Carthamus tinctorius) catalysed the interconversion of the neutral lipids, mono-, di-, and triacylglycerol. Membranes were incubated with neutral lipid substrates, ¹⁴C-labelled either in the acyl or glycerol moiety, and the incorporation of radioactivity into other complex lipids determined. It was clear that diacylglycerol gave rise to triacylglycerol and monoacylglycerol as well as phosphatidylcholine. Radioactivity from added [¹⁴C] triacylglycerol was to a small extent transferred to diacylglycerol whereas added [¹⁴C] monoacylglycerol was rapidly converted to diacylglycerols and triacylglycerols. The formation of triacylglycerol from diacylglycerol occurred in the absence of acyl-CoA and hence did not involve diacylglycerol acyltransferase (DAGAT) activity. Monoacylglycerol was not esterified by direct acylation from acyl-CoA. We propose that these reactions were catalyzed by a diacylglycerol: diacylglycerol transacylase which yielded triacylglycerol and monoacylglycerol, the reaction being freely reversible. The specific activity of the transacylase was some 25% of the diacylglycerol acyltransferase activity and, hence, during the net accumulation of oil, substantial newly formed triacylglycerol equilibrated with the diacylglycerol pool. In its turn the diacylglycerol rapidly interconverted with phosphatidylcholine, the major complex lipid substrate for Δ12 desaturation. Hence, the oleate from triacylglycerols entering phosphatidylcholine via this route could be further desaturated to linoleate. A model is presented which reconciles these observations with our current understanding of fatty acid desaturation in phosphatidylcholine and oil assembly in oleaceous seeds. Received: 8 November 1996 / Accepted: 5 February 1997     

Labelling of glycerolipids in the cotyledons of developing oilseeds by [1-14C]acetate and [2-3H]glycerol

1. 3-sn-Phosphatidylcholine was identified as the major lipid in cotyledons from the developing seeds of soya bean, linseed and safflower when tissue was steamed before lipid extraction. The proportion of oleate in this lipid decreased markedly and that of the polyunsaturated C₁₈ fatty acids increased when detached developing cotyledons were incubated for up to 3h. Similar but less pronounced changes occurred in diacylglycerol, which had a fatty acid composition resembling that of the 3-sn-phosphatidylcholine from cotyledons of the same species. 2. [1-¹⁴C]Acetate supplied to detached cotyledons was incorporated into the acyl moieties of mainly 3-sn-phosphatidylcholine, 1,2-diacylglycerol and triacylglycerol. Initially label was predominantly in oleate, but subsequently entered at accelerating rates the linoleoyl moieties of the above lipids in soya-bean and safflower cotyledons and the linoleoyl and linolenyl moieties of these lipids in linseed cotyledons. In pulse–chase experiments label was rapidly lost from the oleate of 3-sn-phosphatidylcholine and accumulated in the linoleoyl and linolenoyl moieties of this phospholipid and of the di- and tri-acylglycerols. 3. [2-³H]Glycerol was incorporated into the glycerol moieties of mainly 3-sn-phosphatidylcholine and di- and tri-acylglycerols of developing linseed and soya-bean cotyledons. The label entered the phospholipid and diacylglycerol at rates essentially linear with time from the moment the substrate was supplied, and entered the triacylglycerol at an accelerating rate. With linseed cotyledons the labelled glycerol was incorporated initially mainly into species of 3-sn-phosphatidylcholine and diacylglycerol that contained oleate, but accumulated with time in more highly unsaturated species. In pulse–chase experiments with linseed cotyledons, label was lost from both 3-sn-phosphatidylcholine and diacylglycerol, preferentially from the dioleoyl species, and accumulated in triacylglycerol, mainly in species containing two molecules of linolenate. 4. The results suggest a rapid turnover of 3-sn-phosphatidylcholine during triacylglycerol accumulation in developing oilseeds, and are consistent with the operation of a biosynthetic route whereby oleate initially esterified to the phospholipid is first desaturated, then polyunsaturated fatty acids transferred to triacylglycerol, via diacylglycerol. The possible role of oleoyl phosphatidylcholine as a substrate for oleate desaturation is discussed.     

The interconversion of diacylglycerol and phosphatidylcholine during triacylglycerol production in microsomal preparations of developing cotyledons of safflower (Carthamus tinctorius L.).

Microsomal preparations from the developing cotyledons of safflower (Carthamus tinctorius) catalyse the acylation of sn-glycerol 3-phosphate in the presence of acyl-CoA. Under these conditions the radioactive glycerol in sn-glycerol 3-phosphate accumulates in phosphatidic acid, phosphatidylcholine, diacyl- and tri-acylglycerol. The incorporation of glycerol into phosphatidylcholine is via diacylglycerol and probably involves a cholinephosphotransferase. The results show that the glycerol moiety and the acyl components in phosphatidylcholine exchange with the diacylglycerol during the biosynthesis of diacylglycerol from phosphatidic acid. The continuous reversible transfer of diacylglycerol with phosphatidylcholine, which operates during active triacylglycerol synthesis, will control in part the polyunsaturated-fatty-acid quality of the final seed oil.     

Plant Acyl-CoA:Lysophosphatidylcholine Acyltransferases (LPCATs) Have Different Specificities in Their Forward and Reverse Reactions

Acyl-CoA:lysophosphatidylcholine acyltransferase (LPCAT) enzymes have central roles in acyl editing of phosphatidylcholine (PC). Plant LPCAT genes were expressed in yeast and characterized biochemically in microsomal preparations of the cells. Specificities for different acyl-CoAs were similar for seven LPCATs from five different species, including species accumulating hydroxylated acyl groups in their seed oil, with a preference for C₁₈-unsaturated acyl-CoA and low activity with palmitoyl-CoA and ricinoleoyl (12-hydroxyoctadec-9-enoyl)-CoA. We showed that Arabidopsis LPCAT1 and LPCAT2 enzymes catalyzed the acylation and de-acylation of both sn positions of PC, with a preference for the sn-2 position. When acyl specificities of the Arabidopsis LPCATs were measured in the reverse reaction, sn-2-bound oleoyl, linoleoyl, and linolenoyl groups from PC were transferred to acyl-CoA to a similar extent. However, a ricinoleoyl group at the sn-2-position of PC was removed 4–6-fold faster than an oleoyl group in the reverse reaction, despite poor utilization in the forward reaction. The data presented, taken together with earlier published reports on in vivo lipid metabolism, support the hypothesis that plant LPCAT enzymes play an important role in regulating the acyl-CoA composition in plant cells by transferring polyunsaturated and hydroxy fatty acids produced on PC directly to the acyl-CoA pool for further metabolism or catabolism.     

The role of the acyl-CoA pool in the synthesis of polyunsaturated 18-carbon fatty acids and triacylglycerol production in the microsomes of developing safflower seeds

Microsomes isolated from the developing cotyledons of the seeds of the safflower varieties, very-high-linoleate, Gila and high-oleate, were capable of exchanging the acyl groups in acyl-CoA with the fatty acids in position 2 of phosphatidylcholine. The specificity of the 'acyl-exchange' towards the acyl moiety in acyl-CoA was selective in the order: oleate greater than linoleate greater than linolenate. Stearoyl-CoA was completely selected against when presented in a mixed substrate with unsaturated 18-carbon acyl-CoAs. Microsomes, of the very-high-linoleate safflower variety, rapidly desaturated in situ-labelled [14C]oleoylphosphatidylcholine in the presence of NADH. Little oleate desaturation, however, was observed in the microsomes of the high-oleate variety. Microsomes of the Gila and high-oleate varieties of safflower rapidly synthesised phosphatidic acid by the acylation of glycerol 3-phosphate with acyl-CoA. The phosphatidic acid was metabolised to diacylglycerol, which was further acylated to triacylglycerol. A strong selectivity for linoleoyl-CoA was found for the acylation of glycerol 3-phosphate in both the Gila and high-oleate microsomes. On the basis of these results, we propose that the pattern of 18-carbon unsaturated fatty acids in the triacylglycerols of all 'oil'-producing seeds is a direct reflection of the fatty acids in the acyl-CoA pool. This, in turn, is governed by: A, the rate and specificity of the acyl exchange between acyl-CoA and phosphatidylcholine; B, the rate of oleate (and linoleate) desaturation in phosphatidylcholine; and C, the rate and specificity of the glycerophosphate acyltransferase.     

1-Acylglycerophosphocholine O-acyltransferase in maturing safflower seeds

The activity of 1-acylglycerophosphocholine (1-acyl-GPC) O-acyltransferase (EC 2.3.1.23) varied during maturation of safflower (Carthamus tinctorius L.) seeds, and activity per seed was highest in the middle period of seed development when triacylglycerol (TAG) is most rapidly synthesized. The specific activity of acyl transfer in a 20000·g particulate preparation exceeded 500nmol·min^-1·(mg protein)^-1 and was higher than those of any other enzymes involved in TAG synthesis (K. Ichihara et al., 1993, Plant Cell Physiol. 34, 557–566). This suggested the presence of a large flux of acyl-CoA to phosphatidylcholine in the cell. The reaction was specific to C₁₆ and C₁₈ acyl-CoAs with a double bond at position 9. Lauroyl- and erucoyl-CoA were completely ineffective, while ricinoleoyl- and elaidoyl-CoA were utilized efficiently. The relative order of specificity for native acyl-CoA species was linoleoyl > oleoyl stearoyl = palmitoyl. When acyl-CoA mixtures were presented, preference for the unsaturated species rather than the saturated species was even more apparent. The enzyme preferentially utilized 1-C₁₆-acyl- and 1-C₁₈-acyl-GPC molecular species, and 1-palmitoyl-, 1-stearoyl-, 1-oleoyl-and 1-linoleoyl-GPC equally served as acyl acceptor. No activity was detected with 1-octanoyl-GPC, and 1-erucoyl-GPC produced little effect. The effectiveness of 1-alkyl-GPC was comparable to that of 1-acyl-GPC. It was thus concluded that the enzyme recognizes the chain lengths of the acyl donor and acceptor, and the double bond at position 9 of the acyl donor.Abbreviations DAG diacylglycerol - DTNB 5,5-dithiobis(2-nitrobenzoic acid) - GP sn-glycerol 3-phosphate - GPC sn-glycero-3-phosphocholine - GPE sn-glycero-3-phosphoethanolamine - GPI sn-glycero-3-phosphoinositol - PC phosphatidylcholine - TAG triacylglycerol     

Combination and positional distribution of fatty acids in lipids from blue-green algae

The main glycerolipids (monogalactosyl-, digalactosyl-, sulphoquinovosyl diacylglycerol, phosphatidylglycerol) from five blue-green algae (Microcystis, Anabaena, Nostoc, Oscillatoria, Tolypothrix) were analyzed for fatty acid composition, occurrence of diglyceride species and positional distribution of fatty acids between thesn-1- andsn-2-position of glycerol. In contrast to eucaryotic plants biosynthetically closely related lipids (monogalactosyl-, digalactosyl-, trigalactosyl diacylglycerol) show nearly identical diglyceride moieties, whereas sulphoquinovosyl diacylglycerol and phosphatidylglycerol are separated from galactolipids by composition as well as occurrence of fatty acids. On the other hand the positional distribution of fatty acids in all lipids is controlled exclusively by chain length and not by degree of unsaturation with C₁₈-fatty acids at thesn-1- and C₁₆-fatty acids at thesn-2-position. These results show that in procaryotic organisms the diversity in diglyceride portions of lipids is reduced as compared to eucaryotic organisms, but nevertheless does exist.Abbreviations MGD, DGD, TGD, SQD monogalactosyl-, digalactosyl-, trigalactosyl-, sulphoquinovosyl diacylglycerol - PG phosphatidyl glycerol     

Bioassembly of acyl lipids in microspore-derived embryos of Brassica campestris L.

The native lipid composition and the capacity of cell-free extracts to biosynthesize acyl lipids in vitro were determined for the first time using the recently reported microspore-derived (MD) embryo system from the Brassica campestris low erucic acid line BC-2 (Baillie et al. 1992). The total lipid fraction isolated from midcotyledonary stage MD embryos (21 days in culture) was composed primarily of triacylglycerol (76%) with an acyl composition quite similar to that of mature BC-2 seed. When incubated in the presence of glycerol-3-phosphate, ¹⁴C 181-CoA, and reducing equivalents, homogenates prepared from 21-day cultured MD embryos were able to biosynthesize glycerolipids via the Kennedy pathway. The maximum in vitro rate of triacylglycerol biosynthesis could more than account for the known rate of lipid accumulation in vivo. The homogenate catalyzed the desaturation of 181 to 182 and to a lesser extent, 183. The newly-synthesized polyunsaturated fatty acids initially accumulated in the polar lipid fraction (primarily phosphatidic acid and phosphatidylcholine) but began to appear in the triacylglycerol fraction after longer incubation periods. As expected for a low erucic acid cultivar, homogenates of MD embryos from the BC-2 line were incapable of biosynthesizing very long chain monounsaturated fatty acyl moieties (201 and 221) from 181-CoA in vitro. Nonetheless, embryo extracts were still capable of incorporating these fatty acyl moieties into triacylglycerols when supplied with ¹⁴C 201-CoA or ¹⁴C 221-CoA. Collectively, the data suggest that developing BC-2 MD embryos constitute an excellent experimental system for studying pathways for glycerolipid bioassembly and the manipulation of this process in B. campestris.Abbreviations CPT sn-1,2-diacylglycerol cholinephosphotransferase - DAG diacylglycerol - DGAT diacylglycerol acyltransferase - DGDG digalactosyldiacylglycerol - G-3-P glycerol-3-phosphate - G-3-PAT glycerol-3-phosphate acyltransferase - LPA lyso-phosphatidic acid - LPAT lyso-phosphatidic acid acyltransferase - LPC lyso-phosphatidylcholine - LPCAT acyl-CoA: lyso-phosphatidylcholine acyltransferase - LPE lyso-phosphatidylethanolamine - MGDG monogalactosyldiacylglycerol - PA phosphatidic acid - PA Phosphatase, phosphatidic acid phosphatase - PC phosphatidylcholine - PE phosphatidylethanolamine - PG phosphatidylglycerol - TAG triacylglycerol - 181-CoA oleoyl-Coenzyme A - 181 oleic acid, cis-9-octadecenoic acid - 182 linoleic acid, cis-9,12-octadecadienoic acid - 183 -linolenic acid, cis-9,12,15-octadecatrienoic acid - 201 cis-11-eicosenoic acid - 221 erucic acid, cis-13-docosenoic acid; all other fatty acids are designated by number of carbon atoms: number of double bonds National Research Council of Canada Publication No. 35896     

The acylation of sn-glycerol 3-phosphate and the metabolism of phosphatidate in microsomal preparations from the developing cotyledons of safflower (Carthamus tinctorius L.) seed.

Microsomal preparations from the developing cotyledons of safflower (Carthamus tinctorius) catalysed the acylation of sn-glycerol 3-phosphate in the presence of acyl-CoA. The resulting phosphatidate was further utilized in the synthesis of diacyl- and tri-acylglycerol by the reactions of the so-called 'Kennedy pathway' [Kennedy (1961) Fed. Proc. Fed. Am. Soc. Exp. Biol. 20, 934-940]. Diacylglycerol equilibrated with the phosphatidylcholine pool when glycerol backbone, with the associated acyl groups, flowed from phosphatidate to triacylglycerol. The formation of diacylglycerol from phosphatidate through the action of a phosphatidate phosphohydrolase (phosphatidase) was substantially inhibited by EDTA and, under these conditions, phosphatidate accumulated in the microsomal membranes. The inhibition of the phosphatidase by EDTA was alleviated by Mg2+. The presence of Mg2+ in all incubation mixtures stimulated quite considerably the synthesis of triacylglycerol in vitro. Microsomal preparations incubated with acyl-CoA, sn-glycerol 3-phosphate and EDTA synthesized sufficient phosphatidate for the reliable analysis of its intramolecular fatty acid distribution. In the presence of mixed acyl-CoA substrates the sn-glycerol 3-phosphate was acylated exclusively in position 1 with the saturated fatty acids, palmitate and stearate. The polyunsaturated fatty acid linoleate was, however, utilized largely in the acylation of position 2 of sn-glycerol 3-phosphate. The affinity of the enzymes involved in the acylation of positions 1 and 2 of sn-glycerol 3-phosphate for specific species of acyl-CoA therefore governs the non-random distribution of the different acyl groups in the seed triacylglycerols. The acylation of sn-glycerol 3-phosphate in position 1 with saturated acyl components also accounts for the presence of these groups in position 1 of sn-phosphatidylcholine through the equilibration of diacylglycerol with the phosphatidylcholine pool, which occurs when phosphatidate is utilized in the synthesis of triacylglycerol. These results add further credence to our previous proposals for the regulation of the acyl quality of the triacylglycerols that accumulate in developing oil seeds [Stymne & Stobart (1984) Biochem. J. 220, 481-488; Stobart & Stymne (1985) Planta 163, 119-125].     

The regulation of triacylglycerol biosynthesis in cocoa (Theobroma cacao) L.

Developing cocoa cotyledons accumulate initially an unsaturated oil which is particularly rich in oleate and linoleate. However, as maturation proceeds, the characteristic high stearate levels appear in the storage triacylglycerols. In the early stages of maturation, tissue slices of developing cotyledons (105 days post anthesis, dpa) readily accumulate radioactivity from [¹⁴C]acetate into the diacylglycerols and label predominantly palmitate and oleate. In older tissues (130 dpa), by contrast, the triacylglycerols are extensively labelled and, at the same time, there is an increase in the percentage labelling of stearate. Thus, the synthesis of triacylglycerol and the production of stearate are co-ordinated during development. The relative labelling of the phospholipids (particularly phosphatidylcholine) was rather low at both stages of development which contrasts with oil seeds that accumulate a polyunsaturated oil (e.g. safflower). Microsomal membrane preparations from the developing cotyledons readily utilised an equimolar [¹⁴C]acyl-CoA substrate (consisting of palmitate, stearate and oleate) and glycerol 3-phosphate to form phosphatidate, diacylglycerol and triacylglycerol. Analysis of the [¹⁴C]acyl constituents at the sn-1 and sn-2 positions of phosphatidate and diacylglycerol revealed that the first acylase enzyme (glycerol 3-phosphate acyltransferase) selectively utilised palmitate over stearate and excluded oleate, whereas the second acylase (lysophosphatidate acyltransferase) was highly selective for the unsaturated acyl-CoA. On the other hand, the third acylase (diacylglycerol acyltransferase) exhibited an almost equal selectivity for palmitate and stearate. Thus, stearate is preferentially enriched at position sn-3 of triacylglycerol at 120–130 dpa because of the relatively higher selectivity of the diacylglycerol acyltransferase for this fatty acid compared with those of the other two acylation enzymes.Abbreviation dpa days post anthesis We are grateful to Drs. G. Pettipher (Cadbury-Schweppes, Reading, UK), M. End and P. Hadley (Department of Horticulture, University of Reading) for the supply of cocoa pods and to the Agricultural and Food Research Council for financial support. We also wish to thank Dr. S. Stymne (Swedish University of Agricultural Sciences, Uppsala, Sweden) for a generous gift of acyl-CoA substrates.     

Coupling of ethanol metabolism to lipid biosynthesis: labelling of the glycerol moieties of sn-glycerol-3-phosphate,a phosphatidic acid and a phosphatidylcholine in liver of rats given [1,1-2H2]ethanol

The mechanism behind ethanol-induced fatty liver was investigated by administration of [1,1-²H₂]ethanol to rats and analysis of intermediates in lipid biosynthesis. Phosphatidic acid and phosphatidylcholine were isolated by chromatography on a lipophilic anion exchanger and molecular species were isolated by high-performance liquid chromatography in a non-aqueous system. The glycerol moieties of palmitoyl-linoleoylphosphatidic acid, the corresponding phosphatidylcholine and free sn-glycerol-3-phosphate were analysed by GC/MS of methyl ester t-butyldimethylsilyl derivatives. The deuterium labelling in the glycerol moiety of the phosphatidic acid was 2–3-times higher than in free sn-glycerol-3-phosphate, indicating that a specific pool of sn-glycerol-3-phosphate was used for the synthesis of phosphatidic acid in liver. The results indicate that NADH formed during ethanol oxidation is used in the formation of a pool of sn-glycerol-3-phosphate that gives rise to triacylglycerol and possibly fatty liver.     

Does triacylglycerol biosynthesis require diacylglycerol acyltransferase (DAGAT)?

Microsomal membrane preparations from the developing seeds of sunflower (Helianthus annuus L.) catalyse the conversion of sn-glycerol-3-phosphate and acyl-CoA to triacylglycerol via phosphatidic acid and diacylglycerol. The formation of diacylglycerol from phosphatidic acid was Mg2+ dependent and in the presence of EDTA phosphatidic acid accumulated. This property was used to generate large quantities of endogenous radioactive phosphatidic acid in the membranes. On addition of Mg2+ the phosphatidic acid was used in triacylglycerol formation. Acyl-CoA had little effect on the label which accumulated in triacylglycerol from phosphatidic acid. Diacylglycerol acyltransferase, therefore, may not play a major role in oil formation as originally envisaged and other enzymes, including diacylglycerol:diacylglycerol transacylase [Stobart, Mancha, Lenman, Dahlqvist and Stymne (1997) Planta 203, 58-66] may have important biosynthetic functions.     

Substrate specificities of the membrane-bound and partially purified microsomal acyl-CoA:1-acylglycerol-3-phosphate acyltransferase from etiolated shoots of Pisum sativum (L.)

Membrane fractions enriched in rough endoplasmic reticulum and not contaminated with plastidial membranes were isolated from etiolated shoots of Pisum sativum (L.). From these fractions the acyl-CoA:1-acyl-sn-glycerol-3-phosphate acyltransferase (EC 2.3.1.51) was solubilized by extracting the membranes with the zwitterionic detergent 3-[(3-cholamidopropyl)-dimethyl-ammonio]-1-propanesulfonate at high ionic strength. The subsequent separation of the solubilized fractions on a Mono Q column resulted in a tenfold enriched enzymic activity, which could be stabilized by polyethyleneglycol precipitation. A comparison of the substrate specificities and selectivities of the solubilized, enriched 1-acylglycerol-3-phosphate acyltransferase and the corresponding membrane-bound activity revealed no appreciable difference. Both enzymic forms specifically utilized acyl-CoA thioesters as acyl donors whereas the corresponding acyl-acyl carrier protein thioesters were not used. Furthermore, the membrane-bound as well as the solubilized enriched form showed not only higher activities with 1-oleoylthan with 1-palmitoylglycerol-3-phosphate but also pronounced specificities and selectivities for unsaturated C₁₈-CoA thioesters. Hence, the extraplastidial 1-acylglycerol-3-phosphate acyltransferase which catalyses the formation of phosphatidic acid with an eukaryotic fatty-acid pattern was partially purified.Abbreviations ACP acyl carrier protein - CHAPS 3-[(3-cholamidopropyl)-dimethylammonio]-1-propanesulfonate - LPA-AT acyl-CoA:1-acylglycerol-3-phosphate acyltransferase - PEG polyethyleneglycol The authors are grateful to the Deutsche Forschungsgemeinschaft for financial support. We wish to thank Miss Ute Hammer for the analysis of the lipid composition of the microsomal fractions.     

Activities of acyl-CoA:diacylglycerol acyltransferase (DGAT) and phospholipid:diacylglycerol acyltransferase (PDAT) in microsomal preparations of developing sunflower and safflower seeds

The last step in triacylglycerols (TAG) biosynthesis in oil seeds, the acylation of diacylglycerols (DAG), is catalysed by two types of enzymes: the acyl-CoA:diacylglycerol acyltransferase (DGAT) and phospholipid:diacylglycerol acyltransferase (PDAT). The relative contribution of these enzymes in the synthesis of TAG has not yet been defined in any plant tissue. In the presented work, microsomal preparations were obtained from sunflower and safflower seeds at different stages of development and used in DGAT and PDAT enzyme assays. The ratio between PDAT and DGAT activity differed dramatically between the two different species. DGAT activities were measured with two different acyl acceptors and assay methods using two different acyl-CoAs, and in all cases the ratio of PDAT to DGAT activity was significantly higher in safflower than sunflower. The sunflower DGAT, measured by both methods, showed significant higher activity with 18:2-CoA than with 18:1-CoA, whereas the opposite specificity was seen with the safflower enzyme. The specificities of PDAT on the other hand, were similar in both species with 18:2-phosphatidylcholine being a better acyl donor than 18:1-PC and with acyl groups at the sn-2 position utilised about fourfold the rate of the sn-1 position. No DAG:DAG transacylase activity could be detected in the microsomal preparations.     

Glycerolipid synthesis in Chlorella kessleri 11h: I. Existence of a eukaryotic pathway

The fatty acid distributions at the sn-1 and sn-2 positions in major chloroplast lipids of Chlorella kessleri 11h, monogalactosyl diacylglycerol (MGDG) and digalactosyl diacylglycerol (DGDG), were determined to show the coexistence of both C₁₆ and C₁₈ acids at the sn-2 position, i.e. of prokaryotic and eukaryotic types in these galactolipids. For investigation of the biosynthetic pathway for glycerolipids in C. kessleri 11h, cells were fed with [¹⁴C]acetate for 30 min, and then the distribution of the radioactivity among glycerolipids and their constituent fatty acids during the subsequent chase period was determined. MGDG and DGDG were labeled predominantly as the sn-1-C₁₈-sn-2-C₁₆ (C₁₈/C₁₆) species as early as by the start of the chase, which suggested the synthesis of these lipids within chloroplasts via a prokaryotic pathway. On the other hand, the sn-1-C₁₈-sn-2-C₁₈ (C₁₈/C₁₈) species of these galactolipids gradually gained radioactivity at later times, concomitant with a decrease in the radioactivity of the C₁₈/C₁₈ species of phosphatidylcholine (PC). The change at later times can be explained by the conversion of the C₁₈/C₁₈ species of PC into galactolipids through a eukaryotic pathway. The results showed that C. kessleri 11h, distinct from most of other green algal species that were postulated mainly to use a prokaryotic pathway for the synthesis of chloroplast lipids, is similar to a group of higher plants designated as 16:3 plants in terms of the cooperation of prokaryotic and eukaryotic pathways to synthesize chloroplast lipids. We propose that the physiological function of the eukaryotic pathway in C. kessleri 11h is to supply chloroplast membranes with 18:3/18:3-MGDG for their functioning, and that the acquisition of a eukaryotic pathway by green algae was favorable for evolution into land plants.     

Regulation of triacylglycerol biosynthesis in embryos and microsomal preparations from the developing seeds of Cuphea lanceolata.

Embryos of Cuphea lanceolata have more than 80 mol% of decanoic acid ('capric acid') in their triacylglycerols, while this fatty acid is virtually absent in phosphatidylcholine (PtdCho). Seed development was complete 25-27 days after pollination, with rapid triacylglycerol deposition occurring between 9 and 24 days. PtdCho amounts increased until day 15 after pollination. Analysis of embryo lipids showed that the diacylglycerol (DAG) pool consisted of mainly long-chain molecular species, with a very small amount of mixed medium-chain/long-chain glycerols. Almost 100% of the fatty acid at position sn-2 in triacylglycerols (TAG) was decanoic acid. When equimolar mixtures of [14C]decanoic and [14C]oleic acid were fed to whole detached embryos, over half of the radioactivity in the DAG resided in [14C]oleate, whereas [14C]decanoic acid accounted for 93% of the label in the TAG. Microsomal preparations from developing embryos at the mid-stage of TAG accumulation catalysed the acylation of [14C]glycerol 3-phosphate with either decanoyl-CoA or oleoyl-CoA, resulting in the formation of phosphatidic acid (PtdOH), DAG and TAG. Very little [14C]glycerol entered PtdCho. In combined incubations, with an equimolar supply of [14C]oleoyl-CoA and [14C]decanoyl-CoA in the presence of glycerol 3-phosphate, the synthesized PtdCho species consisted to 95% of didecanoic and dioleic species. The didecanoyl-glycerols were very selectively utilized over the dioleoylglycerols in the production of TAG. Substantial amounts of [14C]oleate, but not [14C]decanoate, entered PtdCho. The microsomal preparations of developing embryos were used to assess the acyl specificities of the acyl-CoA:sn-glycerol-3-phosphate acyltransferase (GPAT, EC 2.3.1.15) and the acyl-CoA:sn-1-acyl-glycerol-3-phosphate acyltransferase (LPAAT, EC 2.3.1.51) in Cuphea lanceolata embryos. The efficiency of acyl-CoA utilization by the GPAT was in the order decanoyl = dodecanoyl greater than linoleoyl greater than myristoyl = oleoyl greater than palmitoyl. Decanoyl-CoA was the only acyl donor to be utilized to any extent by the LPAAT when sn-decanoylglycerol 3-phosphate was the acyl acceptor. sn-1-Acylglycerol 3-phosphates with acyl groups shorter than 16 carbon atoms did not serve as acyl acceptors for long-chain (greater than or equal to 16 carbon atoms) acyl-CoA species. On the basis of the results obtained, we propose a schematic model for triacylglycerol assembly and PtdCho synthesis in a tissue specialized in the synthesis of high amounts of medium-chain fatty acids.     

The molecular species of phosphatidic acid, diacylglycerol and phosphatidylcholine synthesized from sn-glycerol 3-phosphate in rat lung microsomes

The species pattern of phosphatidic acid, diacylglycerol and phosphatidylcholine synthesized from [14C]glycerol 3-phosphate was measured using a newly developed HPLC technique yielding 13 molecular species. A direct comparison of these species patterns presupposes determination of the lipolytic activity of lung microsomes. The lipolytic activity was quantitatively determined by measuring the changes of the endogenous concentration of diacylglycerol, triacylglycerol and free fatty acids. The species pattern of endogenous diacylglycerol measured in the time-course of lipolysis did not show any changes up to an incubation period of 20 min, suggesting that the lipolytic activity showed only a very low selectivity for individual substrate species. Diisopropylfluorophosphate (5 mumol/mg microsomal protein) strongly decreased the lipolytic activities as well as the microsomal phosphatidate phosphohydrolase activity, as measured by means of exogenous phosphatidic acid, and also the generation of phosphatidic acid from [14C]glycerol 3-phosphate. In lung microsomes, labeled phosphatidic acid and diacylglycerols were synthesized from the endogenous free fatty acids and sn-[14C]glycerol 3-phosphate, which had previously been added. By addition of CDPcholine to the prelabeled microsomes the synthesis of phosphatidylcholine was measured. After hydrolysis of phosphatidic acid and phosphatidylcholine with cytoplasmatic phosphatidate phosphohydrolase or phospholipase C, respectively, the de novo synthesized species patterns of these two lipids and of the diacylglycerol were determined. Comparison of the species pattern of de novo synthesized phosphatidic acid with that of diacylglycerol largely showed the same distribution of radioactivity among the individual species, except that the relative proportion of label was higher in the 16:0/16:0 and 16:0/18:0 species of phosphatidic acid and lower in the 16:0/20:4 and 18:0/20:4 species than in the corresponding species of diacylglycerol. The species pattern of de novo-synthesized diacylglycerol showed no differences from that of the phosphatidylcholine synthesized from it. From this result we concluded that the cholinephosphotransferase of lung microsomes is nonselective for individual species of the diacylglycerol substrate. The 16:0/18:1 and 16:0/18:2 species of phosphatidic acid, diacylglycerol and phosphatidylcholine showed a higher synthesis rate than their 18:0 counterparts, whereas the 16:0 or 18:0 analogues of species containing 20:4 and 22:6 fatty acids showed nearly the same synthesis rates.(ABSTRACT TRUNCATED AT 400 WORDS)