The synthesis of linoleate and phosphatidic acid and its relationship to oil production in the microsomes of developing seeds of safflower (Carthamus tinctorius, L. var. Gila)

Effect of temperature on polyunsaturated fatty acid accumulation in soybean seeds

Soybean oil contains around 60 % of polyunsaturated fatty acids, which are responsible for the low oxidative stability of soy-derived products. Soybean lines with low linolenic acid content can be obtained by genetic manipulation; however, a high proportion of the variation in fatty acids content is due to environmental factors. This work aimed to determine the effect of temperature on oil composition and on the activity of the enzymes CDP-choline:1,2-diacylglycerolcholine phosphotransferase (CPT) and acyl-CoA:lysophosphatidylcholine acyltransferase (LPCAT), responsible for maintenance of polyunsaturated fatty acids in the cytoplasmic acyl-CoA pool, that is used for oil synthesis in the seeds. CAC-1, a soybean variety with linolenic acid content of about 8 % and CC4, a BC3F4 CAC-1 derived line, with about 4 % linolenic acid, were used. The lines were cultivated under two temperature: 34/28ºC or 22/13ºC - day/night. The seeds were collected along seven development stages, according to their fresh weight. Fatty acid analysis was carried out by gas chromatography and CPT and LPCAT activities were determined by measuring the radioactivity incorporated in their products, phosphatidyl-[14C]choline and phosphatidylcholine-[14C]oleoyl, respectively. Linolenic acid contents were 3.89 and 6.92 % for line CC4 and 7.39 and 12.49 % for variety CAC-1, when submitted to high and low temperature conditions, respectively. Both enzymes were more active, in the development stages analyzed, in seeds produced under low temperature. Kinetics characterization of CPT and LPCAT were conducted previously.

Solubilisation, partial purification and properties of acyl-CoA: glycerol-3-phosphate acyltransferase from avocado (Persea americana) fruit mesocarp

Glycerol-3-phosphate acyltransferase (G3PAT) activity was studied using a microsomal membrane fraction from avocado (Persea americana) mesocarp. G3PAT was shown to be an integral membrane protein, having an active site that appeared to be accessible to the cytoplasmic face of the endoplasmic reticulum, in experiments using limited proteolytic digestion. CHAPS solubilisation (0.25%, w/v) of microsomal G3PAT activity was used as an initial step in purification of this enzyme. Both CHAPS-solubilised and microsomal G3PAT activities were characterised and compared. Affinity chromatography was used to purify microsomal G3PAT for the first time from a plant source. Glycerophosphorylethanolamine coupled to cyanogen bromide-activated Sepharose was used for this purpose. Specific elution of G3PAT by a solution of glycerol-3-phosphate resulted in about 150-fold purification. The significance of the results and the potential usefulness of the purification method for further studies of G3PAT in plants are discussed.

The delta 12-desaturase from the house cricket, Acheta domesticus (Orthoptera: Gryllidae): characterization and form of the substrate

A novel delta 12-desaturase from animals, which converts oleic acid (18:1n-9) to linoleic acid (18:2n-6), was characterized in the house cricket, Acheta domesticus. The delta 12-desaturase product, linoleic acid, was determined by silver nitrate thin-layer chromatography, radio-gas-liquid chromatography and radio-high-performance liquid chromatography with the latter being used for routine analyses. Enzyme activity was located in the microsomal fraction of whole insect homogenates. NADPH or NADH was required for activity, with NADPH being the more efficient electron donor. In short incubation times with oleoyl-CoA as substrate, the highest amount of product, linoleic acid, was found as linoleoyl-CoA. With longer incubation periods, most of the linoleic acid was recovered in the polar lipid fraction containing phospholipid. Preincubation of the microsomal preparation in the absence of NADPH, which allowed 90% of the oleoyl moiety to be transacylated into complex lipid, resulted in no detectable desaturation upon addition of NADPH. These data indicate that the oleic acid moiety used as substrate was in the form of a CoA derivative and not in the form of a phospholipid, as it is for the plant delta 12-desaturase. This is the first characterization of a delta 12-desaturase from an animal system and the first report of a delta 12-desaturase that uses oleoyl-CoA as substrate.

Correlations between gametophytic (pollen) and sporophytic (seed) generations for polyunsaturated fatty acids in oilseed rape Brassica napus L

Lipids were extracted from the diploid seed and haploid pollen of Brassica napus L. Two fractions of pollen lipids, namely the diploid-specified pollen-coat and the haploid-specified internal cytoplasmic lipids were obtained. Significant correlations exist between pollen and seed generations for linoleic (18∶2) and linolenic (18∶3) acids. In pollen internal storage lipids, the level of 18∶3 is positively correlated and the level of 18∶2 is negatively correlated with the level of 18∶3 in seed lipids. Evidence is presented that strongly supports the hypothesis that lipid biosynthesis occurs within the pollen and that synthesis is specified by haploid genes. These data support the concept of pollen selection, so that selecting among living pollen grains for superior individuals has potential as a new plant breeding tool for improving seed oil quality.

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 (14)C-labelled fatty-acid substrates for the synthesis of triacylglycerols. The other major radioactive lipids were phosphatidylcholine and diacylglycerol. In safflower cotyledons, [(14)C]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 [(14)C]linoleate, however, resulted in a more rapid labelling of the diacylglycerols than from [(14)C]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, [(14)C]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.

Characterization of the seed and leaf lipids of high and low linolenic acid flax genotypes

The total seed lipids of four flax (Linum usitatissimum) genotypes, differing markedly in their acyl composition, were extracted and fractionated using column, preparative, and thin-layer chromatography. In the total lipid extract of seeds, the lower linolenate content of the cultivar Glenelg (39.1% compared to that of cv. Croxton (50.5%) was associated with a higher oleate content. Further reductions in linolenate content in the induced mutants of cv. Glenelg, M1722 (17.2%) and "Zero" (1.9%) were accompanied by equivalent increases in linoleate but only minor increases in oleate. Similar changes were observed in the major triacylglycerol fraction of the simple lipids (fatty acid esters of glycerol and sterols), but there was considerable heterogeneity for acyl composition in the minor simple lipid components, including both diacylglycerols and sterol esters, and the complex lipids (glycolipids and phospholipids). The induced mutations substantially reduced linolenate content of all lipid fractions but in no case was it eliminated. Maturation of "Zero" seed at 15/10 degrees C (compared to 24/19 degrees C) increased linoleate and decreased stearate and oleate contents in all lipid fractions. In contrast to seed lipids, the acyl composition of the leaf lipids of the mutant genotypes was the same as those of their parent.

Glycerolipid synthesis by homogenate and oil bodies from developing mustard (Sinapis alba L.) seed

[1-(14)C]Oleic acid and [14-(14)C]erucic acid were converted to their acyl-CoA derivatives and incorporated into acyl lipids by a homogenate from developing mustard (Sinapis alba L.) seed and oil bodies, as well as supernatant isolated by centrifugation at 20000 g. In both homogenate and oil bodies, the oleoyl moieties from exogenous [1-(14)C]oleoyl-CoA were most extensively incorporated into phosphatidic acids, but very little into phosphatidylcholines. The pattern of labelling of acyl lipids by oleoyl versus erucoyl moieties from either of the corresponding fatty acids, added individually or as a mixed substrate, indicates that oleoyl-CoA directly acylates sn-glycerol-3-phosphate to yield lysophosphatidic acids and phosphatidic acids that are subsequently converted to mono- and diacylglycerols. In contrast, erucoyl-CoA predominantly acylates preformed mono-and diacylglycerols containing oleoyl moieties to yield triacylglycerols containing erucoyl moieties.

Involvement of acyl exchange between acyl-CoA and phosphatidylcholine in the remodelling of phosphatidylcholine in microsomal preparations of rat lung

Microsomal membrane preparations from rat lung catalyse the incorporation of radioactive linolenic acid from [14C]linolenoyl-CoA into position 2 of sn-phosphatidylcholine. The incorporation was stimulated by bovine serum albumin and free CoA. Free fatty acids in the incubation mixtures were not utilised in the incorporation into complex lipids. Fatty acids were transferred to the acyl-CoA pool during the incorporation of linolenic acid into phosphatidylcholine. An increase in lysophosphatidylcholine occurred in incubations containing both bovine serum albumin and free CoA and in the absence of acyl-CoA. The results were consistent with an acyl-CoA: lysophosphatidylcholine acyltransferase operating in both a forwards and backwards direction and thus catalysing the acyl exchange between acyl-CoA and position 2 of sn-phosphatidylcholine. In incubations with mixed species of acyl-CoAs, palmitic acid was the major fatty acid substrate transferred to phosphatidylcholine in acyl exchange, whereas this acid was completely selected against in the acylation of added lysophosphatidylcholine. The selectivity for palmitoyl-CoA was particularly enhanced when the mixed acyl-CoA substrate was presented to the microsomes in molar concentrations equivalent to the molar ratios of the fatty acids in position 2 of sn-phosphatidylcholine. During acyl exchange, the predominant fatty acid transferred to phosphatidylcholine from acyl-CoA was palmitic acid, whereas arachidonic acid was particularly selected for in the reverse reaction from phosphatidylcholine to acyl-CoA. A hypothesis is presented to explain the differential selectivity for acyl species between the forward and backward reactions of the acyltransferase that is based upon different affinities of the enzyme for substrates at high and low concentrations of acyl donor. Acyl exchange between acyl-CoA and phosphatidylcholine offers, therefore, a possible mechanism for the acyl-remodelling of phosphatidylcholine for the production of lung surfactant.

Oil synthesis in vitro in microsomal membranes from developing cotyledons of Linum usitatissimum L

Microsomal preparations from developing linseed (Linum usitatissimum L.) cotyledons catalyzed i) acyl exchange between acyl-CoA and position 2 of sn-phosphatidylcholine, ii) acylation of sn-glycerol 3-phosphate to yield phosphatidic acid, and iii) the utilisation of phosphatidic acid in the production of diacylglycerol and triacylglycerol. Selectivity studies for C18 acyl species of acyl-CoA indicated a bias for the channelling of oleate to phosphatidylcholine for, presumably, its desaturation, and the utilisation of the polyunsaturated fatty-acid products in the acyl-CoA pool for phosphatidic acid and subsequent triacylglycerol synthesis. The microsomal preparations were capable of returning glycerol backbone with associated acyl components to phosphatidylcholine from diacylglycerol where it may be further enriched with polyunsaturated C18 acids by desaturation. The acyl quality in linolenate-rich oilseeds appears to be under similar control to that found in linoleate-rich species.

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 [(14)C]glycerol 3-phosphate and [(14)C]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.

sn-Glycerol-3-phosphate acyltransferase in a particulate fraction from maturing safflower seeds

The properties of the acyl-CoA:sn-glycerol-3-phosphate O-acyltransferase in a 20,000g particulate fraction from maturing safflower seeds were investigated. The optimum pH of the reaction was 7.2. The apparent Km for glycerophosphate was 0.54 mM. Only monoacylglycerophosphate was accumulated in the particulate fraction under normal conditions. Position 1 of glycerophosphate was exclusively esterified with either palmitoyl-CoA or linoleoyl-CoA as acyl donor, while 2-acylglycerophosphate was the minor product. The specificity and selectivity of the acyltransferase for acyl-CoA were broad and somewhat affected by temperature. The concentration of glycerophosphate did not affect the selectivity. These observations suggested that the fatty acid composition of position 1 of safflower triacylglycerol must primarily depend on the composition of the acyl-CoA pool in the site of synthesis, and that growth temperature and the acyl-CoA selectivity of the glycerophosphate acyltransferase may be rather minor factors regarding regulation of the fatty acid composition of position 1 in triacylglycerol.