Quantification,characterization and fatty acid composition of lysophosphatidic acid in different rat tissues

The amount and composition of lysophosphatidate present in different rat tissues have been estimated by an internal standard method in which a synthetic unnatural isomer (1-heptadecanoyl-rac-glycerol-3-phosphate) was added to the total lipid extracts, and the fatty acid composition of purified lysophosphatidate was determined. Lipids from tissues were extracted under acidic conditions, and the lysophosphatidate was purified by solvent partitions followed by thin-layer chromatography in multiple solvent systems. The purified lipid was shown to be 1-acyl-sn-glycerol-3-phosphate by chromatographic and chemical analysis, by its resistance to hydrolysis when treated with phospholipase A₂ and also by its complete conversion to 1-acyl-sn-glycerol when treated with alkaline phosphatase. The fatty acid consituents of this lipid were determined by gas-liquid chromatography of the derived methyl esters. The concentrations (nmol/g of tissue) of lysophosphatidate in various tissues were: 86.2±4.2 in brain, 60.3±6.3 in liver, 46.4±6.5 in kidney, 30.6±5.0 in testis, 22.3 in heart and 19.3 in lung. Mostly (80%) saturated fatty acids were found to be present in this lyso lipid. A significantly high level of stearic acid was present in this lipid from all the tissues (50–60% in liver, kidney, brain and testis, and about 40% in heart and lung) compared to plamitic acid (10–15% in liver, kidney and brain and 25–30% in testis, heart and lung). The fatty acid compositions of phosphatidic acid, the putative product of lysophosphatidate acylation, from different tissues were also determined and palmitate was found to be the major saturated fatty acid. These results suggest that tissue lysophosphatidic acid is not only formed byde novo biosynthesis but is also generated via the breakdown of phospholipids such as phosphoinositides.     

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.     

The dietary regulation of acyltransferase and desaturase activities in microsomal membranes of rat liver

Dietary manipulation produces marked alterations in desaturase activities of rat liver microsomes with no concomitant changes in acyltransferase activities. Desaturation of stearoyl-CoA (Δ9-desaturase), linoleoyl-CoA (Δ6-desaturase), eicosatrienoyl-CoA (Δ5-desaturase) and eicosatrienoyl-phosphatidylcholine (Δ5-desaturase) was elevated in animals fed a corn oil diet and lowered in those fed a coconut oil diet compared to control animals. The Δ5-desaturase activities were also lowered in starved animals and elevated in starved animals refed a fat-free diet. However, no changes in acyl-CoA:1-acylsn-glycero-3-phosphocholine acyltransferase activity were observed in the membranes of animals maintained on any of the dietary regimens studied. These observations suggest that the desaturases of rat liver microsomes are regulated independently of the acyltransferases and that desaturation of eicosatrienoyl-phosphatidylcholine is regulated at the level of the desaturase itself and not by availability of the phospholipid substrate.     

Diacylglycerol acyltransferase activity and triacylglycerol synthesis in germinating castor seed cotyledons

The central importance of storage lipid breakdown in providing carbon and energy during seed germination has been demonstrated by isolating the genes encoding the enzymes involved in FA β-oxidation. In contrast, little is known about the ability of germinating seeds to synthesize TAG. We report that castor cotyledons are capable of TAG synthesis. The rate of incorporation of ricinoleic acid into TAG reached a peak at 7 d after imbibition (DAI) (1.14 nmol/h/mg) and decreased rapidly thereafter, but was sustained at 20 DAI in cotyledons and true leaves. The castor DAG acyltransferase (RcDGAT) mRNA and protein were expressed throughout seed germination at levels considerably enhanced from that in the dormant seed, thus indicating new expression. Significant degradation of the RcDGAT protein was observed after 7 DAI. The DGAT activity was found to be predominantly a function of the level of the intact RcDGAT protein, with the rate of TAG synthesis decreasing as degradation of the RcDGAT protein proceeded. A possible mechanism for the degradation of the RcDGAT protein is discussed. The induction of DGAT mRNA and protein, the capacity for TAG synthesis in vitro and in tissue slices, and the differing TAG composition of dormant seed TAG vs. cotyledonary TAG provide strong circumstantial evidence for active TAG synthesis by cotyledons. However, we have not yet determined the physiological significance of this capability.     

Enhancement of liver microsomal acyl CoA-lysophospholipid acyltransferase activity in pyridoxine-deficient rats

In rats deficient in pyridoxine and essential fatty acids, liver phospholipids contained less arachidonic acid and more oleic and eicosatrienoic acids than those from animals only deficient in essential fatty acids. This pattern persisted after the animals were supplemented with linoleate for 6 days. Liver oleyl and arachidonyl CoA-lysophospholipid acyltransferase activities were significantly higher in pyridoxine-deficient animals. Supplementation with linoleate for 6 days resulted in a marked increase in arachidonyl CoA-acyltransferase activity in pyridoxine-deficient rats but a relatively small increase in the supplemented animals. Differences in fatty acid composition between pyridoxine-deficient and supplemented rats can not be ascribed to lower liver transacylase activity in the deficient animals.     

Increased formation of phosphatidylinositol-4-phosphate in human platelets stimulated with lysophosphatidic acid

Lysophosphatidic acid (LPA, 1-acyl-sn-glycerol 3-phosphate), at a concentration of 1–40 μM, was found to induce the formation of [³H]inositol-labelled phosphatidylinositol-4-phosphate (PIP) without significantly altering the levels of either phosphatidylinositol (PI) or phosphatidylinositol bisphosphate (PIP₂) in washed human platelets. Preincubation of platelets with the cyclooxygenase/lipoxygenase inhibitor, BW755C at 100 μM, did not alter the LPA-induced formation of PIP. Activation of platelets with the phorbol ester, phorbol 12-myristate 13-acetate (PMA), elicited a similar response (induction of PIP formation). The specific protein kinase C (PKC) inhibitor, GF109203X (10 μM), completely blocked the effect of PMA but not the LPA-induced generation of PIP. The present results indicate that LPA can induce PIP formationvia PI-4-kinase activation, through processes which are independent of the eicosanoid/TxA₂ pathway and are not PKC-dependent.     

Decreased lecithin: Cholesterol acyltransferase activity in the plasma of hypercholesterolemic pigs

Lecithincholesterol acyltransferase (LCAT) activity levels were determined, as function of plasma total cholesterol (TC) in 13 normocholesterolemic (TC<85 mg/dL) and in 28 hypercholesterolemic (TC>98 mg/dL) pigs. The normocholesterolemic group consisted of pigs that carried apo-B allelic genes other thanLpb ⁵ and orLpb ⁸. The hypercholesterolemic group consisted ofLpb ^5/x andLpb ^5/8 heterozygous andLpb ^5/5 homozygous animals. The data reported in this study show that the LCAT activity in the plasma of hypercholesterolemic (HC) pigs (79±43 units) was significantly lower (p<0.0005) compared to the normocholesterolemic controls (175±45 units). Furthermore, LCAT activity was positively correlated with TC in the normocholesterolemic group (r=+0.54; p<0.05), whereas it was negatively correlated with TC in the hypercholesterolemic group (r=−0.73; p<0.001). Additional data obtained from incubation experiments suggest that the lower LCAT activity in hypercholesterolemic pigs may be due, at least in part, to inhibition of LCAT activity by components found in the lipoprotein-deficient fractions of the plasma of hypercholesterolemic pigs.     

Increased lysolecithin acyltransferase activity in the plasma of type II hyperlipoproteinenic patients

Lecithin-cholesterol acyltransferase (LCAT) in human plasma has been shown to acylate lysolecethin to lecithin in presence of low density lipoprotein (LDL). To determine the physological importance of LDL in activating lysolecithin acyltransferase (LAT) activity, we assayed the LAT activity in 18 hypercholesterolemic (Type II) patients and 15 control subjects. The enzyme activity was about 60% higher in the patients compared with the controls. On the other hand, the LCAT activity, measured by 2 different procedures, as well as enzyme mass, determined by radioimmuno assay, were comparable in the controls and hypercholesterolemics. The LAT activity was highly correlated with LDL levels in the plasma, but the LCAT activity and the enzyme mass had no correlation with the LDL levels. These results show that the plasma LDL is the rate-limiting activator of the enzyme, and pathological conditions, resulting in higher LDL levels, also cause higher LAT activities.     

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.     

Oleanolic acid and ursolic acid stabilize liposomal membranes

The effects of oleanolic acid (OA) and ursolic acid (UA) on the fluidity and stability of dipalmitoyl phosphatidylcholine (DPPC) liposomal membrane were monitored by measuring the fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene labeled in the liposomal membrane and the leakage of calcein from the probe-encapsulated liposomes. The experiments with the liposomes made of DPPC and OA or UA showed that OA and UA exhibited a moderate fluidity-modulating effect for the liquid-crystalline liposomal membrane, and a strong condensing effect for both crystalline and liquid-crystalline liposomal membranes. Their effects were comparable to those of cholesterol. These results suggest that their fluidity-modulating and condensing effects might have some implications in their biological functions.     

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.     

Effect of peroxyl radicals on lecithin/cholesterol acyltransferase activity in human plasma

The objective of this study was to determine whether subjecting human plasma to oxidant stress reduces the activity of lecithin/cholesterol acyltransferase (LCAT, EC 2.3.1.43). Plasma was incubated for 4h with 2.25–45 mM of 2,2′-azobis(2-amidinopropane)HCl (AAPH), a source of peroxyl radicals. A time- and concentration-dependent reduction of LCAT activity occurred, relative to control samples incubated in the absence of AAPH. Reduction of LCAT activity was disproportionate to elevation of thiobarbituric acid-reactive substances (TBARS) in the plasma. Added ascorbate was able to significantly prevent reduction of LCAT activity, but this effect was unrelated to blockage of TBARS formation by the antioxidant. The results suggest that LCAT activity can be down-modulated by oxidant stress, but not necessarily by lipid peroxidation.     

Metal-lon stimulation and inhibition of lysophospholipase D which generates bioactive lysophosphatidic acid in rat plasma

We found that lysophospholipase D (LPLD) in rat plasma prefers unsaturated to saturated lysophosphatidylcholines as substrates, generating a biologically active lipid, lysophosphatidic acid, but it does not hydrolyze diacyl-phospholipids. In this study, this LPLD required a metal ion for activity, Co²⁺ being the most effective, followed in order by Zn²⁺, Mn²⁺, and Ni²⁺. This metal-ion-stimulated LPLD with unique substrate specificity, which has not been described previously, was susceptible to thiol-blocking reagents and serine esterase inhibitors, but not to a histidine-modifying reagent. Consistent with results using thiol-modifying agents, short-chain fatty aldehydes, secondary products of lipid peroxidation, were found to inhibit LPLD. Addition of dibutylhydroxytoluene or butylhydroxyanisole to the plasma increased the activity of this enzyme, probably in a manner independent of its antioxidant activity, since another antioxidant, propyl gallate, was rather inhibitory. These results suggest that rat plasma contains an active LPLD that differs in some properties from other members of the known phospholipase D family detected in animal tissues and body fluids.     

The influence of purines on plasma lecithin: Cholesterol acyltransferase activity in the rat

The activity of plasma lecithin: cholesterol acyltransferase (LCAT) in the rat is significantly inhibited in vitro by guanine, xanthine, and hypoxanthine. LCAT activity decreases with increase in xanthine concentration. The other two purines, adenine and uric acid, had no significant effect.     

Lecithin: Cholesterol acyltransferase activity in hypercholesterolemic subjects and in hypercholesterolemic subjects treated with clofibrate

The lecithin:cholesterol acyl transfer reaction in the plasma of hypercholesterolemic subjects and of hypercholesterolemic subjects treated with clofibrate was studied. An increased enzyme activity was found in the first group of patients, while lecithin:cholesterol acyl transfer activity tended to normalize in the second group. This increased enzyme activity might be a defense mechanism against the accumulation of cholesterol in the arterial wall.     

Sulfonated polysulfone membranes with antifouling activity

The protein fouling phenomenon was investigated on modified polysulfone ultrafiltration membranes. The sulfonation degree was correlated to the extent of deposition of bovine serum albumin. It was shown that membranes prepared from modified polysulfone were less fouled than their off-charge analogues. The fouling phenomenon was also investigated for membranes prepared from co-cast blends. It was demonstrated that blends containing more than 50 wt.-% of sulfonated derivative behaved like a membrane from pure sulfonated polysulfone. The effect was attributed to surface separation of polymers and formation of surfaces enriched with sulfonic groups.