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Metabolic fate of non-esterified fatty acids in isolated hepatocytes from newborn and young pigs. Evidence for a limited capacity for oxidation and increased capacity for esterification.

In hepatocytes isolated from 48 h-old starved of suckling newborn pigs or from 15-day-old starved piglets, the rate of ketogenesis from oleate or from octanoate is very low. This is not due to an inappropriate fatty acid uptake by the isolated liver cells, but results from a limited capacity for fatty acid oxidation. Some 80-95% of oleate taken up is converted into esterified fats, whatever the age or the nutritional conditions. Three lines of indirect evidences suggest that fatty acid oxidation is not controlled primarily by malonyl-CoA concentration in newborn pig liver. Firstly, the addition of glucagon does not increase fatty acid oxidation or ketogenesis. Secondly, the rate of lipogenesis is very low in isolated hepatocytes from newborn pigs. Thirdly, the rates of oxidation and ketogenesis from octanoate are also decreased in isolated hepatocytes from newborn and young piglets. The huge rate of esterification of fatty acids in the liver of the newborn pigs probably represents a species-specific difference in intrahepatic fatty acid metabolism.

Age Factors↗

Carrot cells detoxify N-phosphonoacetyl-L-aspartate by esterification.

Unlike bacterial and mammalian cells, carrot cells are able to tolerate N-phosphonoacetyl-L-aspartate (PALA), a potential inhibitor of pyrimidine biosynthesis, by detoxifying the compound. Anion-exchange chromatography showed that detoxified PALA was less negatively charged than PALA, and allowed detoxified PALA to be isolated. Incubation of detoxified PALA with a low-specificity carboxylic-ester hydrolase fully restored the ability to inhibit aspartate transcarbamoylase, the target enzyme, indicating that the detoxification involves the formation of carboxylic ester. G.1.c. analysis of the alcohol products of enzymic hydrolysis, and of their ratio to PALA, showed that the detoxification produced a mixture of mono- and di-carboxylic esters and of methyl and ethyl esters. The detoxification mechanism showed considerable specificity towards PALA, since the analogous carboxy groups of succinate were not modified in the same way. Succinate was depleted much more slowly, no succinate esters could be detected, and the presence of a 10-fold excess of succinate did not inhibit the esterification rate of PALA. The possible significance of these results is discussed.

Aspartate Carbamoyltransferase↗

Pregnenolone esterification in Saccharomyces cerevisiae. A potential detoxification mechanism.

While studying the effect of steroids on the growth of the yeast Saccharomyces cerevisiae, we found that pregnenolone was converted into the acetate ester. This reaction was identified as a transfer of the acetyl group from acetyl-CoA to the 3beta-hydroxyl group of pregnenolone. The corresponding enzyme, acetyl-CoA:pregnenolone acetyltransferase (APAT) is specific for Delta5- or Delta4-3beta-hydroxysteroids and short-chain acyl-CoAs. The apparent Km for pregnenolone is approximately 0.5 microm. The protein associated with APAT activity was partially purified and finally isolated from an SDS/polyacrylamide gel. Tryptic peptides were generated and N-terminally sequenced. Two peptide sequences allowed the identification of an open reading frame (YGR177c, in the S. cerevisiae genome database) translating into a 62-kDa protein of hitherto unknown function. This protein encoded by a gene known as ATF2 displays 37% identity with an alcohol acetyltransferase encoded by the yeast gene ATF1. Disruption of ATF2 led to the complete elimination of APAT activity and consequently abolished the esterification of pregnenolone. In addition, a toxic effect of pregnenolone linked to the disruption of ATF2 was observed. Pregnenolone toxicity is more pronounced when the atf2-Delta mutation is introduced in a yeast strain devoid of the ATP-binding cassette transporters, PDR5 and SNQ2. Our results suggest that Atf2p (APAT) plays an active role in the detoxification of 3beta-hydroxysteroids in association with the efflux pumps Pdr5p and Snq2p.

ATP-Binding Cassette Transporters↗

Scavenger receptor, class B, type I-dependent stimulation of cholesterol esterification by high density lipoproteins, low density lipoproteins, and nonlipoprotein cholesterol.

Scavenger receptor, class B, type I (SR-BI) is a cell surface glycoprotein that mediates selective uptake and efflux of sterols from high density lipoproteins (HDL) to cells. A Chinese hamster ovary cell line that is deficient in functional LDL receptors, but has high expression levels of recombinant SR-BI (ldlA7-SR-BI), was used to examine the effect of SR-BI on the trafficking of sterols between lipoproteins and cells. To monitor the fate of sterols transported by SR-BI into cells, we measured the incorporation of [14C]oleate into cholesterol esters by acyl-CoA:cholesteryl acyltransferase in the endoplasmic reticulum. We show that incubation of ldlA7-SRBI cells with either LDL or HDL resulted in an equally dramatic increase in the formation of [14C]oleate-labeled cholesterol esters. The lipoprotein-stimulated, SR-BI-dependent increase in cholesterol esterification was inhibited by chloroquine. The uptake of sterols and their incorporation into cholesterol esters by SR-BI from LDL was largely a selective process. The addition of free cholesterol to ldlA7-SRBI cells also stimulated cholesterol ester formation in a chloroquine-sensitive fashion. We also show that SR-BI mediates the efflux of endogenously synthesized sterols from the cell membrane. From these studies we conclude that, in the absence of the LDL receptor, overexpression of SR-BI can mediate significant transport of sterols between lipoproteins and the endoplasmic reticulum of cells.

Acids↗

A novel methyltransferase catalyzes the methyl esterification of trans-aconitate in Escherichia coli.

We have identified a new type of S-adenosyl-L-methionine-dependent methyltransferase in the cytosol of Escherichia coli that is expressed in early stationary phase under the control of the RpoS sigma factor. This enzyme catalyzes the monomethyl esterification of trans-aconitate at high affinity (Km = 0.32 mM) and cis-aconitate, isocitrate, and citrate at lower velocities and affinities. We have purified the enzyme to homogeneity by gel-filtration, anion-exchange, and hydrophobic chromatography. The N-terminal amino acid sequence was found to match that expected for the o252 open reading frame at 34.57 min on the E. coli genomic sequence whose deduced amino acid sequence contains the signature sequence motifs of the major class of S-adenosyl-L-methionine-dependent methyltransferases. Overexpression of the o252 gene resulted in an overexpression of the methyltransferase activity, and we have now designated it tam for trans-aconitate methyltransferase. We have generated a knock-out strain of E. coli lacking this activity, and we find that its growth and stationary phase survival are similar to that of the parent strain. We demonstrate the endogenous formation of trans-aconitate methyl ester in extracts of wild type but not tam- mutant cells indicating that trans-aconitate is present in E. coli. Since trans-aconitate does not appear to be a metabolic intermediate in these cells but forms spontaneously from the key citric acid cycle intermediate cis-aconitate, we suggest that its methylation may limit its potential interference in normal metabolic pathways. We have detected trans-aconitate methyltransferase activity in extracts of the yeast Saccharomyces cerevisiae, whereas no activity has been found in extracts of Caenorhabditis elegans or mouse brain.

Amino Acid Sequence↗

High-density lipoprotein subclasses and esterification rate of cholesterol in children: effect of gender and age.

Since the development of coronary heart disease (CAD) is affected by a specific pattern of plasma high density lipoprotein (HDL) effects it may be useful to know whether this occurs already in childhood. In this study we evaluated particle size distribution of HDL by gradient gel electrophoresis and the determination of cholesterol esterification rate (FER(HDL)) in plasma depleted of apo B lipoproteins in 221 children (108 boys and 113 girls) aged 4 months to 20 years. Total plasma- (TC), low-density lipoprotein- (LDL-C) and HDL- (HDL-C) cholesterol, HDL unesterified cholesterol (HDL-UC) and plasma triglycerides (TG) were also measured. There were no significant gender and age differences with respect to the plasma TC, LDL-TC and TG but concentration of HDL-TC increased with age. Post-pubertal girls had significantly higher relative concentrations of HDL2b compared to boys (30.4% vs 17.2%), while HDL3b,c was lower in post-pubertal girls (8.7% vs. 16.5%). FER(HDL) correlated inversely with HDL2b and positively with HDL3b,c particles and was significantly higher in boys of the post-pubertal group compared to girls (16.9%/h vs 12.5%/h). While in girls there was a positive correlation between age and HDL-C, HDL-UC and the relative concentration of HDL2b no significant correlation were observed in boys. In girls the increase in TC showed a significant correlation with a simultaneous increase in HDL-C, HDL-UC and HDL2b. In boys TC correlated significantly with changes in TG only. When HDL2b and HDL3b,c cholesterol levels are calculated from HDL-C concentration and per cent distribution the differences between males and females are further emphasized. These data indicate that HDL particle size distribution is age- and gender-dependent.

Adolescent↗

Dietary fat and hypercholesteremia in the Cebus monkey. II. Esterification and disappearance of cholesterol-4-C14.

A series of studies of cholesterol metabolism in the Cebus monkey were carried out in an attempt to understand the mechanisms responsible for the great differences in serum cholesterol levels when different dietary fats were used. Three groups of monkeys, one fed diets including 45 per cent of calories as corn oil, a second corn oil plus cholesterol (0.1 gm./100 calories), and a third lard plus cholesterol for 5 months (mean serum cholesterol values were 237, 268, and 601 mg. per cent, respectively) were injected with emulsions of cholesterol-4-C(14). The mean biological half-lives for the disappearance of serum radiocholesterol were 8.8, 8.4, and 6.6 days respectively. Esterification of radiocholesterol as measured by equilibration of specific activities of serum-free cholesterol and total cholesterol was delayed in the monkeys fed lard plus cholesterol. When cholesterol-4-C-(14)-stearate was given intravenously to a series of monkeys, an erratic non-exponential biological decay curve resulted. Specific activity for free serum cholesterol was greater than that for total cholesterol within 1 hour after the injection. After 7 months on experimental diets including corn oil with added cholesterol and lard with added cholesterol the levels of lipides in most tissues were not different for the two dietary groups, nor were they appreciably elevated above previous control figures for monkeys not fed cholesterol. Total lipide levels in the adrenals of monkeys fed corn oil were twice those of monkeys fed lard. Monkeys were fasted before and after intragastric administration of cholesterol-4-C(14) in small formula meals including various fats and fatty acids. The disappearance of total cholesterol from the serum consisted of a rapid followed by a slow exponential function. The type of fat and fatty acid appeared to influence the rate of disappearance of radiocholesterol. There was a broad range of apparent activity of the different fats and fatty acids in promoting cholesterol absorption.

Animals↗

DNA adducts formed from the probable proximate carcinogen, N-hydroxy-3,2' -dimethyl-4-aminobiphenyl, by acid catalysis or S-acetyl coenzyme A-dependent enzymatic esterification.

The arylamine carcinogen 3,2'-dimethyl-4-aminobiphenyl (DMABP) has been proposed to be metabolically activated to DNA-binding derivatives through the formation of an N-hydroxy intermediate. In this study, the subsequent activation of N-hydroxy-DMABP through acid catalysis or enzymatic esterification was examined. [Ring-3H]N-hydroxy-DMABP was reacted with calf thymus DNA at pH 4.6 for 15 min to yield 370 arylamine residues per 10(6) nucleotides, while at pH 7.4 the binding was only two residues per 10(6) nucleotides. The DNA modified under acidic conditions was enzymatically hydrolyzed and analyzed by h.p.l.c. which indicated the presence of three major adducts. The products were identified by spectral and chemical properties as N-(deoxyguanosin-8-yl)-DMABP (60-70%), 5-(deoxyguanosin-N2-yl)-DMABP (2-3%) and N-(deoxyadenosin-8-yl)-DMABP (1-3%). The same adducts have previously been detected in the liver and colon of rats administered DMABP or its hydroxamic acid. Incubation of rat hepatic or intestinal cytosol at pH 7.4 for 15 min with [ring-3H]N-hydroxy-DMABP in the presence of S-acetyl coenzyme A (AcCoA) and calf thymus DNA resulted in DNA binding at levels of 30-80 arylamine residues per 10(6) nucleotides. H.p.l.c. analysis of the DNA modified in the presence of AcCoA indicated the formation of the same adducts detected in the acid-catalyzed reactions. When arylhydroxamic acid N,O-acyltransferase assays were conducted with rat liver cytosol and N-acetyl-N-hydroxy-DMABP as the substrate, binding to nucleic acids was not observed. Similarly, 3'-phosphoadenosine-5'-phosphosulfate-dependent sulfotransferase-mediated DNA binding could not be demonstrated. These data indicate that in a suitable acidic environment, N-hydroxy-DMABP will react with DNA to yield the same adducts found in vivo. Under neutral conditions, however, N-hydroxy-DMABP appears to undergo AcCoA-dependent transacetylation to an electrophilic acetoxy ester which will spontaneously react with DNA.

Acetyl Coenzyme A↗

One-step esterification of benzoylecgonine with dimethylformamide-dipropylacetal or dimethylformamide-diisopropylacetal in the presence of pyridine.

A simple procedure was developed to derivatize benzoylecgonine extracted from urine for subsequent confirmation by gas chromatography-mass spectrometry. The compound was esterified with dimethylformamide-dipropylacetal (DMF-DPA) or dimethylformamide-diisopropylacetal (DMF-DIPA) to the corresponding propyl and isopropyl esters. The optimum reaction condition was found to be heating the reaction mixture in the presence of pyridine at 100 degrees C for 30 min. The procedure is a one-step esterification followed by evaporation of excess reagents. When benzoylecgonine was extracted from urine using a solid-phase extraction technique and derivatized with this procedure, the compound was detected at a level as low as 10 ng/mL. Quantitation was linear over the concentration range 10-8000 ng/mL.

Acetals↗

Pectin with low molecular weight and high degree of esterification increases absorption of 58Fe in growing rats.

Effects of pectins with different degrees of esterification (DE) and molecular weights (MW) on iron bioavailability were investigated in healthy growing rats by following erythrocyte incorporation of a dose of 58Fe. Rats were fed a control diet for 8 d and then deprived of food for 16 h. Two hours after the start of feeding iron-deficient diets, with or without pectin (80 g/kg diet), a dose of FeSO4 rich in 58Fe (60.28%) was intubated into the stomach; rats were then allowed to feed for an additional 4 h before withdrawal of food for 10 h. Rats were then fed iron-adequate diets for 9 d. The pectins differed in DE and MW, respectively, as follows: P-A (73%, 860,000), P-B (75%, 89,000), P-C (22%, 1,260,000) and P-D (24%, 114,000). Rats fed pectin-free diet with free access to food or restricted to the same quantity consumed by a respective pectin group served as controls. Iron absorption was 48% in the control group and 57% in rats fed P-B. Rats fed P-B had higher (P 2 < or = 0.05) serum iron, transferrin saturation, hematocrit and liver and spleen iron than the control group or the group fed P-C. These indices, except for transferrin saturation, were also higher In rats fed P-A and P-D compared with those fed P-C and controls, but to a lesser extent than in rats fed P-B. The data indicate that bioavailability of dietary non-heme iron was enhanced when pectin of low MW and high DE was added to the diet. This improvement was not evident with pectins having high MW and/or low DE.

Absorption↗

Oxidation and esterification of geometrical and positional isomers of octadecenoic acids in perfused rat liver.

The metabolic fate of the geometrical isomers of 6-, 9-, and 11-octadecenoic acids was studied in isolated perfused rat livers. Although the ketogenicities of these monounsaturated fatty acids generally decreased as the double bond was moved away from the carboxyl group, a dependence on the geometrical difference was found only for 9-octadecenoate, the rate being significantly lower in the cis-isomer. Very low density lipoprotein lipid secretion was distinctly and specifically decreased when trans-9-octadecenoic acid was perfused, but no such difference was observed with other isomers. Various trans-isomers were actively incorporated into the hepatic lipids and secreted apparently at the expense of preexisting endogenous cis-octadecenoate. The trans-isomers were all incorporated exclusively at the 1-position of hepatic phosphatidylcholine. Concentrations of the cis-octadecenoate in the glycerolipids secreted and remaining in the liver were characteristically modified depending on the location of the ethylenic bond of the cis-octadecenoate. Thus, both the location of the double bond in the acyl-chain and the geometrical configuration specifically influence the rate of oxidation and esterification of octadecenoic acid in perfused rat liver.

Animals↗

Initial cholesterol esterification rate in hyperlipoproteinaemia: effects of triglyceride-rich lipoproteins.

The initial cholesterol esterification rate (LCAT activity) was determined in ninety-four hyperlipidaemic subjects. LCAT activity was elevated in hypertriglyceridaemia, whereas patients with hypercholesterolaemia had normal activities. In hypertriglyceridaemic subjects LCAT activity correlated with the concentrations of d less than 1.006 lipoproteins, plasma triglycerides, cholesterol and cholesterol esters and phospholipid levels. Addition of d less than 1.006 lipoprotein to normal plasma resulted in a dose dependent stimulation of enzyme activity with a sigmoidal response curve. When the d less than 1.006 lipoproteins were removed from hypertriglyceridaemic plasma by ultracentrifugation, the enzyme activity in the residual d greater than 1.006 fraction dropped, but still was higher than in normal plasma and correlated with the amount of d less than 1.006 lipoproteins originally present. Thus, high LCAT activity in hypertriglyceridaemia cannot be explained solely by the presence of an increased d less than 1.006 lipoprotein concentration. An increase of enzyme concentration or changes in concentration or composition of other lipoproteins (high density lipoproteins) may contribute to the high LCAT activity in hypertriglyceridaemia.

Adult↗

The influence of esterification of carboxyl groups of ribonuclease-A on its structure and immunological activity.

The effect of modification of carboxyl groups of Ribonuclease-Aa on the enzymatic activity and the antigenic structure of the protein has been studied. Modification of four of the eleven free carboxyl groups of the protein by esterification in anhydrous methanol/0.1 M hydrochloric acid resulted in nearly 80% loss in enzymatic activity but had very little influence on the antigenic structure of the protein. Further increases in the modification of the carboxyl groups caused a progressive loss in immunological activity, and the fully methylated RNase A exhibited nearly 30% immunological activity. Concomitant with this change in the antigenic structure of the protein, the ability of the molecule to complement with RNase-S-protein increased, clearly indicating the unfolding of the peptide "tail" from the residues for orthobenzoquinone reaction and the loss in immunological activity of the more etion of these derivatives as compared with the compact native conformation. The fact that even the fully methylated RNase-A retains nearly 30% of its immunological activity suggested that the modified protein contained antibody recognizable residual native structure, which presumably accommodates some antigenic determinants.

Animals↗

Esterification of 9-fluorenylmethoxycarbonyl-glycosylated serine and cysteine derivatives with an hydroxymethyl resin.

Esterification of glycosylated serine and cysteine derivatives with a 4-alkoxybenzyl alcohol (Wang) resin is described. The classical methods of ester bond formation (symmetrical anhydride, 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate [TBTU]/4-dimethylaminopyridine [DMAP] with or without 1-hydroxybenzotriazole [HOBT], pentafluorophenyl [Pfp] esters gave high percentages of racemization of the glycosylated serine or cysteine residues. To reduce the D-amino acid content, we found that the best results were obtained with the highly efficient MSNT reagent (2,4,6-mesitylenesulfonyl-3-nitro-1,2,4-triazolide), which gave a high yield of substitution of the resin and the lowest percentage of racemization. A difference in behavior was observed between the two amino acids. The glycosylated cysteine derivative always gave lower racemization than the analogous glycosylated serine.

Alanine↗

Overexpression of carnitine palmitoyltransferase I in skeletal muscle in vivo increases fatty acid oxidation and reduces triacylglycerol esterification.

A key regulatory point in the control of fatty acid (FA) oxidation is thought to be transport of FAs across the mitochondrial membrane by carnitine palmitoyltransferase I (CPT I). To investigate the role of CPT I in FA metabolism, we used in vivo electrotransfer (IVE) to locally overexpress CPT I in muscle of rodents. A vector expressing the human muscle isoform of CPT I was electrotransferred into the right lateral muscles of the distal hindlimb [tibialis cranialis (TC) and extensor digitorum longus (EDL)] of rats, and a control vector expressing GFP was electrotransferred into the left muscles. Initial studies showed that CPT I protein expression peaked 7 days after IVE (+104%, P<0.01). This was associated with an increase in maximal CPT I activity (+30%, P < 0.001) and a similar increase in palmitoyl-CoA oxidation (+24%; P<0.001) in isolated mitochondria from the TC. Importantly, oxidation of the medium-chain FA octanoyl-CoA and CPT I sensitivity to inhibition by malonyl-CoA were not altered by CPT I overexpression. FA oxidation in isolated EDL muscle strips was increased with CPT I overexpression (+28%, P<0.01), whereas FA incorporation into the muscle triacylglycerol (TAG) pool was reduced (-17%, P<0.01). As a result, intramyocellular TAG content was decreased with CPT I overexpression in both the TC (-25%, P<0.05) and the EDL (-45%, P<0.05). These studies demonstrate that acute overexpression of CPT I in muscle leads to a repartitioning of FAs away from esterification and toward oxidation and highlight the importance of CPT I in regulating muscle FA metabolism.

Animals↗

Quantitative studies in vitro on uptake and esterification of palmitate into human and rat jejunal mucosa.

Quantitative incorporation and esterification of (9-10) 3H palmitate into jejunal mucosa was studied in vitro comparatively on human biopsies and on everted rings of rat intestine following various incubation times. This procedure is fitted for investigation of the glycophosphate pathway. Quantitatively the total uptake of palmitate was similar in man and rat. But the distribution among the various esterified fractions was different in animal and man and different again in coeliac disease. Notwithstanding incorporation of palmitate into triglyceride the fatty acid composition of mucosal triglyceride showed little or no modification.

Animals↗

Cholesterol esterification rates in very low density lipoprotein- and low density lipoprotein-depleted plasma. Relation to high density lipoprotein subspecies, sex, hyperlipidemia, and coronary artery disease.

The fractional rate of cholesterol esterification in very low density lipoprotein- and low density lipoprotein-depleted plasma (FERHDL) was studied in normolipidemic subjects and in individuals with hyperlipidemia and proven coronary artery disease (CAD). The FERHDL was significantly higher than the FER in whole plasma and was significantly higher in normal men than in normal women. In addition, men and women with primary hyperlipidemia had significantly higher FERHDL values relative to their sex-matched controls. The most significant increases in FERHDL values, however, were observed in individuals with CAD. In all patient groups, FERHDL was positively correlated with plasma triglyceride concentration. In addition, FERHDL was negatively related to plasma high density lipoprotein (HDL) cholesterol concentration in all groups except in men with CAD and in normolipidemic women. The gradient gel electrophoretic pattern of HDL from individuals with either low or high FERHDL values indicated an inverse relation between this activity and the relative amount of HDL2b particles. FERHDL likely reflects the metabolic properties of the heterogeneous population of HDL particles in the plasma and may be a function of the relative content of larger and smaller HDL particles. It appears to be a sensitive and reliable functional measure of the particle size distribution in the HDL pool and one of potential clinical value in the assessment of risk for CAD.

Adult↗

Metabolism of arachidonic acid in ionophore-stimulated neutrophils. Esterification of a hydroxylated metabolite into phospholipids.

[14C]Arachidonic acid incubated with human neutrophils was esterified into phospholipids and triglycerides. Stimulation of these labeled neutrophils with ionophore A23187 (2 microM) results in release of [14C]arachidonate from phospholipid and its metabolism to prostaglandin E2 and 5-hydroxy-6,8,11,14-eicosatetraenoic acid (5-HETE), a lipoxygenase product. The released arachidonate is also metabolized to a polar lipid of unknown composition here disignated compound A. 5-HETE was found to be released into the medium and then taken up again by the cells. To determine its metabolic fate, [14C]5-HETE was prepared biosynthetically, purified, and incubated with stimulated, unlabeled neutrophils. Most of the radioactivity entered the cells and was esterified into phospholipids and triglycerides. The radiolabeled complex lipids were saponified, and the released fatty acids cochromatographed with authentic 5-HETE. The esterification of 5-HETE, a hydroxylated fatty acid, into membrane phospholipids may be an example of a more generalized mechanism for altering membrane characteristics.

Arachidonic Acids↗