Factors enhancing diacylglycerol acyltransferase activity in microsomes from cell-suspension cultures of oilseed rape

Several factors, including an unidentified endogenous component, were found to stimulate microsomal diacylglycerol acyltransferase (DGAT, EC 2.3.1.20) from a microspore-derived cell-suspension culture of oilseed rape (Brassica napus L. cv. Jet Neuf). At a concentration of 25mM, MgSO₄ and MgCl₂ stimulated microsomal DGAT 25- and 10-fold, respectively. AIP and CoA at concentrations of 2 and 1 mM stimulated the enzyme 2.4- and 12-fold, respectively, although the effects were lessened in the presence of higher Mg²⁺ concentrations. Although microsomal DGAT activity was increased only slightly by the addition of exogenous sn-1,2-diacylglycerol to the reaction mixture, it was increased substantially by the addition of exogenous phosphatidate. sn-Glycerol-3-phosphate and other phospholipids tested did not have this stimulatory effect. DGAT activity did not decrease when microsomes were incubated with ATP in the presence of the cytosolic fraction. This fraction, however, contained a small organic compound(s) that stimulated microsomal DGAT activity.     

A protein tyrosine kinase associated with the ATP-dependent inactivation of adipose diacylglycerol acyltransferase

The activity that has been previously reported to reversibly inactivate adipose glycerolphosphate acyltransferase (GPAT) and diacylglycerol acyltransferase (DGAT)in vitro in the presence of ATP is shown here to be partially purified from adipose tissue with an apparent molecular weight of 68 kDa. The activity responsible for inactivating DGAT is associated with a kinase activity as determined by phosphate incorporation both into microsomal proteins and into a synthetic tyrosine-containing peptide as substrate for protein tyrosine kinase. Two microsomal polypeptides of 53 and 69 kDa are major substrates of this kinase. Both DGAT inactivating and kinase activities assayed from the purified sample have been found to be insensitive to the Ser/Thr kinase inhibitor H-7 while being sensitive to genistein and tyrphostin-25. A crude protein phosphatase preparation from liver was capable of reversing the effects of both activities. The purified sample was also shown to inactivate GPAT in the presence of ATP. These results suggest that a protein tyrosine kinase, in concert with a protein tyrosine phosphatase, may regulate the activities of DGAT and GPAT by a phosphorylation-dephosphorylation mechanism.     

Characterization of the diacylglycerol acyltransferase activity in the membrane fraction from a fungus

In an attempt to clarify the mechanism of lipid accumulation inMortierella ramanniana var.angulispora, diacylglycerol acyltransferase (DGAT) in the membrane fraction from this fungus was characterized. The enzyme had an optimum pH of 7.0–7.5, and enzyme activity was blocked by SH-reagents. Metal ions were not essential for maintaining DGAT activity.n-Octyl-β-d-glucoside, 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate and Tween 80 were found to preserve activity, while Triton X-100 and sucrose monolaurate inhibited it. As the inhibition of DGAT activity by Triton X-100 was overcome by the addition of diacylglycerol (DG), the dependency of DGAT activity on exogenous DG was determined in the presence of 0.1% Triton X-100. DGAT activity in the membrane fraction was traced in fungi cultured for different time periods or in media at different carbon to nitrogen (C/N) ratios. Although the increase in total lipid content with culture time was accompanied by an increase in DGAT activity, total lipid changes related to changes in C/N ratio did not correlate with DGAT activity. Factors other than DGAT activity in the membrane fraction would appear to be involved in the regulation of total lipid content in this fungus.     

The Utilization of the Acyl-CoA and the Involvement PDAT and DGAT in the Biosynthesis of Erucic Acid-Rich Triacylglycerols in Crambe Seed Oil

The triacylglycerol of Crambe abyssinica seeds consist of 95 % very long chain (>18 carbon) fatty acids (86 % erucic acid; 22:1^?13) in the sn‐1 and sn‐3 positions. This would suggest that C. abyssinica triacylglycerols are not formed by the action of the phospholipid:diacylglycerol acyltransferase (PDAT), but are rather the results of acyl‐CoA:diacylglycerol acyltransferase (DGAT) activity. However, measurements of PDAT and DGAT activities in microsomal membranes showed that C. abyssinica has significant PDAT activity, corresponding to about 10 % of the DGAT activity during periods of rapid seed oil accumulation. The specific activity of DGAT for erucoyl‐CoA had doubled at 19 days after flowering compared to earlier developmental stages, and was, at that stage, the preferred acyl donor, whereas the activities for 16:0‐CoA and 18:1‐CoA remained constant. This indicates that an expression of an isoform of DGAT with high specificity for erucoyl‐CoA is induced at the onset of rapid erucic acid and oil accumulation in the C. abyssinica seeds. Analysis of the composition of the acyl‐CoA pool during different stages of seed development showed that the percentage of erucoyl groups in acyl‐CoA was much higher than in complex lipids at all stages of seed development except in the desiccation phase. These results are in accordance with published results showing that the rate limiting step in erucic acid accumulation in C. abyssinica oil is the utilization of erucoyl‐CoA by the acyltransferases in the glycerol‐3‐phosphate pathway.     

Relation between diacylglycerol acyltransferase activity and oil concentration in soybean

Diacylglycerol acyltransferase (EC 2.3.1.20; DGAT) catalyzes synthesis of triacylglycerol from acyl-CoA and diacylglycerol. Activity of this enzyme and developmental changes in oil accumulation were estimated at various stages of seed growth in soybean germplasm with phenotypic differences in oil content. Oil deposition in seed of these genotypes followed a sigmoid pattern that was modeled to predict incremental rates of oil accumulation during seed development. A strong positive correlation was found between the estimated peak rate of oil deposition (near the mid-term of seed development) and oil concentration in mature seed. At saturating substrate levels, DGAT activity measured near the peak rate of oil deposition also was correlated positively with oil phenotype. In the latter stages of seed development, a positive correlation between estimates of enzyme activity at or below the apparent K _m for diolein and comparable oil accumulation rates was attributed to reduced synthesis of substrates and/or potential change in affinity for substrate as suggested by an increase in apparent K _m for diolein in older seed. These data indicated that DGAT activity may be a rate-limiting step in triacylglycerol synthesis. However, it is difficult to accept the idea of a single rate-limiting step at the end of a complex metabolic pathway. Because oil is a quantitatively inherited trait, several genes determine genotypic differences in oil content among soybeans. Hence, DGAT activity may be an indicator of coordinated genetic expression of gene-products in the entire glycerolipid synthetic pathway for a given genotype. In any case, results of this investigation demonstrated that genotypic differences in DGAT activity contributed to expression of genetic variation in oil content among soybean gemplasm.     

On the Role of DGAT1 in Seed Glycerolipid Metabolic Network and Critical Stages of Plant Development in Arabidopsis

Studies on the model plant Arabidopsis thaliana have uncovered the identities of most enzymatic components involved in seed storage lipid biosynthesis. However, much remains to be learned on how pathway interactions operate in the seed metabolic network. In this study, we dissected seed glycerolipid molecular compositional changes in the Arabidopsis mutant deficient in diacylglycerol acyltransferase 1 (DGAT1). Our results indicate that metabolic adjustments occurred in both phosphatidylcholine synthesis and deacylation in developing seeds. Ultrastructural changes of perturbed oil and protein bodies were also evident in cotyledon parenchyma cells. To unmask the physiological and developmental role associated with DGAT1-mediated neutral lipid biosynthesis, we attempted to combine dgat1 mutation with lpcat2 that harbors a defect in lysophosphatidylcholine acyltransferase 2 (LPCAT2). Disruption in both DGAT1 and LPCAT2 led to an apparent defect in pollen development that manifested as pollen sterility. Collectively, our results highlight a role of DGAT1 in both storage lipid synthesis and plant development.     

Both histidine residues of the conserved HHXXXDG motif are essential for wax ester synthase/acyl‐CoA:diacylglycerol acyltransferase catalysis

Bacterial acyltransferases of the wax ester synthase/diacylglycerol acyltransferase (WS/DGAT) family possess a highly conserved HHXXXDG motif. In this study, we describe the first experimental evidence that this motif is part of the active site of WS/DGAT from the Acinetobacter baylyi strain ADP1 and that it is crucial for enzymatic activity. The second histidine residue of this motif (H₁₃₃) turned out to be essential for the catalytic activity. In addition, the replacement of the first histidine (His₁₃₂) also led to explicitly decreased activity. A complete loss of activity was only observed upon substitution of both histidine residues by leucine, revealing that both are necessary for maximal activity. In contrast, the replacement of Asp₁₃₇ and Gly₁₃₈ against alanine had only little effect on enzyme activity, thus demonstrating that they are not essential for WS/DGAT catalysis although belonging to the highly conserved motif. One peculiarity of WS/DGAT enzymes is their little substrate specificity regarding hydrophobic compounds. In this study, we demonstrated the inability of WS/DGAT to accept polar compounds as substrates.     

Diacylglycerol acyltransferase: A key mediator of plant triacylglycerol synthesis

Many plants deposit TAG in seeds and fruits as the major form of storage lipid. TAG production is of tremendous socioeconomic value in food, nutraceutical, and industrial applications, and thus numerous conventional and molecular genetic strategies have been explored in attempts to increase TAG content and modify the FA composition of plant seed oils. Much research has focused on the acyl-CoA-dependent reaction catalyzed by diacylglycerol acyltransferase (DGAT), which is an integral endoplasmic reticulum protein and has also been shown to be present in oil bodies and plastids. DGAT enzymes exhibit diverse biochemical properties among different plant species, many of which are summarized here. In addition to catalyzing a critical step in TAG biosynthesis, there is evidence that DGAT has roles in lipid metabolism associated with germination and leaf senescence. TAG can also be formed in plants via two different acyl-CoA-independent pathways, catalyzed by phospholipid: diacylglycerol acyltransferase and diacylglycerol transacylase. The current understanding of the terminal step in TAG formation in plants and the development of molecular genetic approaches aimed at altering TAG yield and FA composition of TAG are discussed.     

Multifunctional Acyltransferases from <Emphasis Type="Italic">Tetrahymena thermophila</Emphasis>

Multifunctional acyltransferases are able to catalyze the esterification of various acyl-acceptors with activated fatty acids. Here we describe the identification of four proteins from Tetrahymena thermophila that share certain properties with mammalian acyltransferases regarding their predicted transmembrane structure, their molecular mass and the typical acyltransferase motif. Expression of the Tetrahymena sequences results in production of triacylglycerols and wax esters in recombinant yeast when appropriate substrates are provided. The in vitro characterization shows, that these enzymes are capable of esterifying different acyl-acceptors including fatty alcohols, diols, diacylglycerols and isoprenols with acyl-CoA thioesters. Based on these catalytic activities and the sequence similarities of the Tetrahymena proteins with acyl-CoA:diacylglycerol acyltransferase 2 (DGAT2) family members, we conclude that we identified a new group of DGAT2-related multifunctional acyltransferases from protozoan organisms.     

Identification of Diacylglycerol Acyltransferase Inhibitors from <Emphasis Type="Italic">Rosa centifolia</Emphasis> Petals

Diacylglycerol acyltransferase (DGAT) catalyzes the final step of triacylglycerol (TAG) synthesis, and is considered as a potential target to control hypertriglyceridemia or other metabolic disorders. In this study, we found that the extract of rose petals suppressed TAG synthesis in cultured cells, and that the extract showed DGAT inhibitory action in a dose-dependent manner. Fractionation of the rose extract revealed that the DGAT inhibitory substances in the extract were ellagitannins; among them rugosin B, and D, and eusupinin A inhibited DGAT activity by 96, 82, and 84% respectively, at 10 μM. These substances did not inhibit the activities of other hepatic microsomal enzymes, glucose-6-phosphatase and HMG-CoA reductase, or pancreatic lipase, suggesting that ellagitannins inhibit DGAT preferentially. In an oral fat load test using mice, postprandial plasma TAG increase was suppressed by rose extract; TAG levels 2 h after the fat load were significantly lower in mice administered a fat emulsion containing rose extract than in control mice (446.3 ± 33.1 vs 345.3 ± 25.0 mg/dL, control vs rose extract group; P < 0.05). These results suggest that rose ellagitannins or rose extract could be beneficial in controlling lipid metabolism and used to improve metabolic disorders.     

A Novel Assay of DGAT Activity Based on High Temperature GC/MS of Triacylglycerol

Diacylglycerol acyltransferase (DGAT) catalyzes the final step in the acyl-CoA-dependent biosynthesis of triacylglycerol (TAG), a high-energy compound composed of three fatty acids esterified to a glycerol backbone. In vitro DGAT assays, which are usually conducted with radiolabeled substrate using microsomal fractions, have been useful in identifying compounds and genetic modifications that affect DGAT activity. Here, we describe a high-temperature gas chromatography (GC)/mass spectrometry (MS)-based method for monitoring molecular species of TAG produced by the catalytic action of microsomal DGAT. This method circumvents the need for radiolabeled or modified substrates, and only requires a simple lipid extraction prior to GC. The utility of the method is demonstrated using a recombinant type-1 Brassica napus DGAT produced in a strain of Saccharomyces cerevisae that is deficient in TAG synthesis. The GC/MS-based assay of DGAT activity was strongly correlated with the typical in vitro assay of the enzyme using [1-¹⁴C] acyl-CoA as an acyl donor. In addition to determining DGAT activity, the method is also useful for determining substrate specificity and selectivity properties of the enzyme.     

Cyclic AMP increases incorporation of exogenous fatty acids into triacylglycerols in hamster fibroblasts

Incorporation of various exogenous saturated or unsaturated [¹⁴C]labeled fatty acids (palmitic, stearic, oleic, linoleic and arachidonic) into triacylglycerols by hamster fibroblasts was markedly enhanced (two-to fourfold) in the presence of theophylline or dibutyryl cyclic adenosinemonophosphate (dbcAMP). This effect was observed for short-term (1–6 hr) as well as long-term (15–24 hr) preincubation with dbcAMP. In the presence of sodium fluoride, a phosphoprotein phosphatase inhibitor, measurement of diacylglycerol acyltransferase (DGAT) activity in cells pretreated with dbcAMP pointed out a marked increase (3X) in specific activity. The results suggest that DGAT activity in fibroblasts could be activated by a cAMP-dependent phosphorylation process.     

Generation and Characterization of a Soybean Line with a Vernonia galamensis Diacylglycerol Acyltransferase-1 Gene and a myo-Inositol 1-Phosphate Synthase Knockout Mutation

Soybean (Glycine max) meal is an important protein source. Soybean meal with lower phytate and oligosaccharides improves meal quality. A single recessive mutation in soybean myo-inositol 1-phosphate synthase (Gm-lpa-TW75-1) confers a seed phenotype with low phytate and increased inorganic phosphate. The mutant was crossed with high oil lines expressing a diacylglycerol acyltransferase1 (DGAT) gene from Vernonia galamensis (VgD). Gm-lpa-TW75-1 X VgD, designated GV, has 21%, and 22% oil and 41% and 43% protein from field and greenhouse seed production, respectively. No significant differences were found in mineral concentrations except for Fe which was 229 μg/g dry mass for GV followed by 174.3 for VgD and 162 for Gm-lpa-TW75-1. Phosphate (Pi) is higher in Gm-lpa-TW75-1 as expected at 5 mg/g, followed by GV at 1.6 mg/g whereas Jack, VgD, and Taiwan75 have about 0.3 mg/g. The Gm-lpa-TW75-1 line has the lowest phytate concentration at 1.4 mg/g followed by GV with 1.8 mg/g compared to Taiwan75, VgD, and Jack with 2.5 mg/g. This work describes a high oil and protein soybean line, GV, with increased Pi and lower phytate which will increase the nutritional value for human and animal feed.     

Biotransformation of glycidol by the unspecific wax ester synthase/acyl‐CoA:diacylglycerol acyltransferase of Acinetobacter baylyi ADP1

Glycidol was biologically derivatized by the unspecific wax ester synthase/acyl coenzyme A (acyl‐CoA): diacylglycerol acyltransferase (WS/DGAT) from Acinetobacter baylyi ADP1 into glycidyl acyl ester. Catalysis of in vitro conversion of glycidol to glycidyl acyl ester by the WS/DGAT from A. baylyi was verified by (i) a radiometric assay, (ii) thin‐layer chromatography and (iii) also by ESI‐MS. A specific activity of 50 nmol·mg^–1·min^–1 was obtained when 10 mM glycidol and 5 µM palmitoyl‐CoA were used. In vivo synthesized glycidyl acyl esters in recombinant E. coli were detected and quantified by staining with the epoxide‐specific reagent 4‐(4‐nitrobenzyl)‐pyridine. Of glycidyl acyl esters, 1.5 mg/L was obtained from the culture in the presence of 10 mM glycidol and 10 mM oleate.     

Modification of phospholipid polar head group with monomethylethanolamine and dimethylethanolamine decreases cholesteryl ester and triacylglycerol synthesis in cultured human fibroblasts

Modification of the phospholipid polar head group was achieved by supplementation of the growth medium of cultured human fibroblasts with the choline analogues monomethylethanolamine (ME) or dimethylethanolamine (DE) at a concentration of 80–200 μg/mL for 48 hr. The maximum concentration of phosphatidylmonomethylethanolamine (PME) or phosphatidyldimethylethanolemine (PDE) reached without affecting the phospholipid/protein ratio was about 45% of total phospholipids. Incorporation of oleic acid into cholesteryl esters and triacylglycerols was markedly inhibited after supplementation with ME or DE, and accounted for 60% and 40% of controls, respectively, at 200 μg/mL, whereas incorporation into phospholipids was not affected. AcylCoA:cholesterol acyltransferase (ACAT) and diacylglycerol acyltransferase (DGAT) activities measured on cell-free extracts appeared to be decreased also by phospholipid polar head group modification, whereas the overall phospholipid acyltransferase activity remained unchanged. The intracellular content of cholesteryl esters and triacylglycerols, determined by the isotopic equilibrium method with radioactive cholesterol and glycerol, was found to be diminished to 50–60% and 40–50% of controls, respectively, after supplementation with the choline analogues. The study showed that modification of the phospholipid polar head group affects the activity of membrane-bound enzymes involved in the metabolism of neutral lipids.