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PubMed1990

Quantitative brain autoradiography of [9,10-3H]palmitic acid incorporation into brain lipids.

The distribution of radioactivity within brain metabolic compartments was examined following the intravenous injection of [9,10-3H]palmitate into awake rats. Brain radioactivity reached a maximum value by 15 min after [9,10-3H]palmitate injection and remained unchanged for at least 4 hr. Regional differences in radioactivity could be determined with high resolution by quantitative autoradiography, at the level of cell layers within the hippocampus and cerebral cortex, and between striosomes of the caudate nucleus. Regional brain radioactivities were converted to normalized regional radioactivities (k) by dividing them by the integrated plasma fatty acid radioactivity (integrated over the time course of the experiment). These values reflected incorporation mainly into brain phospholipids; radioactivity due to nonlipid components was minimal. Indeed, about 85% of brain radioactivity was within lipids between 5 min and 4 hr postinjection, the remainder being equally divided between protein-associated pellet and aqueous-soluble metabolites. The major lipids labeled were phospholipids, particularly phosphatidylcholine, which contained about 75% of phospholipid radioactivity. The results show that [9,10-3H]palmitate can be used to examine incorporation of plasma palmitate into individual brain regions via quantitative autoradiography. Furthermore, the tracer is a rather selective marker for phosphatidylcholine and can be used to examine turnover and synthesis of this phospholipid. [9,10-3H]palmitate has advantages over [U-14C]palmitate for autoradiographic studies of incorporation; following the 14C-tracer, significant label even at 4 hr after injection is in nonlipid compartments (glutamate and aspartate), and the long path length of 14C limits resolution at the cell layer level.

Animals↗
PubMed2005

Assessing the cellular transmembrane electrical potential difference on the hepatic uptake of palmitate.

Understanding the driving forces for the hepatic uptake of endogenous and exogenous substrates in isolated cells and organs is fundamental to describing the underlying hepatic physiology/pharmacology. In this study we investigated whether uptake of plasma protein-bound [3H]-palmitate across the hepatocyte wall is governed by the transmembrane electrical potential difference (PD). Uptake was studied in isolated hepatocytes and isolated perfused rat livers (IPL). Protein-binding and vasoactive properties of the different perfusates were determined using in vitro heptane/buffer partitioning studies and the multiple indicator dilution (MID) technique in the IPL, respectively. Altering hepatocyte PD by perfusate ion substitution resulted in either a substantial depolarization (-14 +/- 1 mV, n = 12, mean +/- S.E., substituting choline for Na+) or hyperpolarization (-46 +/- 3 mV, n = 12, mean +/- S.E., substituting nitrate for Cl-). Perfusate ion substitution also affected the equilibrium binding constant for the palmitate-albumin complex. IPL studies suggested that, other than with gluconate buffer, hepatic [3H]-palmitate extraction was not affected by the buffer used, implying PD was not a determinant of extraction. [3H]-Palmitate extraction was much lower (p < 0.05) when gluconate was substituted for Cl- ion. This work contrasts with that for the extraction of [3H]-alanine where hepatic extraction fraction was significantly reduced during depolarization. Changing the albumin concentration did not affect hepatocyte PD, and [3H]-palmitate clearance into isolated hepatocytes was not affected by the buffers used. MID studies with vascular and extravascular references revealed that, with the gluconate substituted buffer, the extravascular volume possibly increased the diffusional path length thus explaining reduced [3H]-palmitate extraction fraction in the IPL.

Alanine↗
PubMed1996

Role of palmitic acid on the isolation and properties of halorhodopsin.

Purified halorhodopsin was isolated from Halobacterium halobium as previously described (Duschl, A. et al. (1988) J. Biol. Chem. 263, 17016-17022). Two purple bands were eluted from phenyl-Sepharose column, indicating the presence of differently retained halorhodopsin forms; the absorption spectra of the two halorhodopsin bands in the dark were not different. By gas chromatography/mass spectrometry we could identify palmitate (which is only a minor lipid component of archaeal cells) among lipids associated with purple fractions. Typically the palmitate content of the first eluted band was higher than that of the second, indicating a correlation between the palmitate content and the retention time; from one to two fatty acid molecules per halorhodopsin molecule were present depending on the fraction analysed. Very little or no palmitate was released from denatured halorhodopsin. By adding palmitate to buffers used in the phenyl-Sepharose chromatography, only one sharp purple band was collected, corresponding to the less retained halorhodopsin fraction. Pentadecanoic fatty acid could also affect the halorhodopsin chromatography. Chromatography of halorhodopsin in the presence of beta-mercaptoethanol showed only one band, corresponding to the more retrained halorhodopsin form. The two halorhodopsin fractions had different photoreactivity; the less retained halorhodopsin fraction (at higher palmitate content) showed an higher rate of decay of the absorbance at 570 nm upon illumination. By following the decay of the absorbance at 570 nm upon addition of alkali in the dark, we found that the two halorhodopsin fractions had different pKa values of deprotonation.

Bacteriorhodopsins↗
PubMed1980

The in vivo labeling with acetate and palmitate of lung phospholipids from developing and adult rabbits.

The labeling with radiolabeled acetate and palmitate of lung, microsomes isolated from lung, and surfactant phospholipids from adult, 3-day-old, and newborn rabbits was studied. The half-life of phosphatidylcholine from lung and microsomal fractions was shorter when labeled with acetate than when labeled with palmitate. Half-time values similarly measured for phosphatidylglycerol, phosphatidylinositol or phosphatidylethanolamine were not different for the two labels. Acetate and palmitate-labeled phospholipids appeared in the surfactant fraction with similar accumulation curves. The relative specific activities of acetate-labeled phosphatidylcholine from adult, 3-day-old, and newborn rabbits, respectively, were 1.30, 1.86 and 1.77 times those measured for those measured for the palmitate label. Surfactant phosphatidylinositol and phosphatidylethanolamine from 3-day-old animals similarly were labeled preferentially with acetate. However, phosphatidylglycerol purified from the surfactant fraction contained equivalent relative amounts of the acetate and palmitate labels in 3-day-old and adult rabbits. These results suggest that the type II pneumocyte may use acetate preferentially for the synthesis of palmitic acid which then is incorporated into surfactant phospholipids.

Acetates↗
PubMed1981

De novo fatty acid synthesis in the perfused rat lung. Incorporation of palmitate into phospholipids.

1. The incorporation of exogenously derived [14C]palmitate and endogenously synthesized [3H]palmitate (from 3H2O) was measured in the isolated perfused lung. 2. Over 40% of the fatty acid esterified into lung disaturated phosphatidylcholine was derived from de novo synthesis. 3. A major portion of the palmitate synthesized de novo was incorporated in the 2 position of disaturated phosphatidylcholine. 4. Streptozotocin-induced diabetes and the compound 5-(tetradecyloxy)-2-furoic acid markedly inhibited de novo fatty acid synthesis while the incorporation of exogenously supplied palmitate increased into disaturated phosphatidylcholine, primarily in the 2 position. 5. Treatment with insulin resulted in an increase in [14C]glucose incorporation into lung phospholipid, with the largest increase appearing in the glyceride-glycerol fraction of the phosphatidylcholine species. 6. Insulin neither stimulated de novo fatty acid synthesis nor increased exogenous palmitate incorporation. 7. These data show: (1) that de novo fatty acid synthesis in the perfused rat lung is involved in the remodeling reactions in the synthesis of phosphatidylcholine, and (2) that diabetes affects the relative contribution of de novo synthesized and exogenously supplied palmitate available for the esterification of lung phospholipid.

Animals↗
PubMed1995

Effect of dietary triacylglycerol structure on lipoprotein metabolism: a comparison of the effects of dioleoylpalmitoylglycerol in which palmitate is esterified to the 2- or 1(3)-position of the glycerol.

The effect on lipoprotein metabolism of diets enriched in different isomers of dioleoylpalmitoylglycerol was studied. One diet contained fat in which palmitate was esterified to the two outer positions of the glycerol (OOP) and the other in which it was esterified to the middle carbon (OPO). The lipid composition of chylomicrons was similar in rats fed either fat blend. However, triacylglycerol (TAG) in chylomicrons from OPO fed animals was relatively enriched in palmitic acid, at the expense of stearic, oleic and linoleic acids. Silver phase HPLC and 2-positional analysis clearly demonstrated that the identity of the fatty acid in the 2-position was similar in both dietary and chylomicron TAG. No significant differences could be seen in the in vitro hydrolysis of chylomicron TAG from animals fed the two fats labelled with [14C]palmitate. As expected, following hydrolysis, palmitate was released as free fatty acid from chylomicrons isolated from OOP-fed animals but within 2-monoacylglycerol from those fed OPO. The enrichment of chylomicrons with palmitate in animals fed O[14C]PO resulted in increased delivery of [14C]palmitate to the liver. In a further series of experiments Golden Syrian hamsters were fed diets containing the fat blends and either 0.005% or 0.12% (w/w) cholesterol, for 28 days. No differences in fasting plasma lipoprotein concentrations were seen in response to the dietary fats. We conclude that, while these isometric triacylglycerols had transient effects on chylomicron metabolism, no significant longer term effect on plasma concentrations of endogenous lipoproteins could be found.

Animals↗
PubMed1984

Enhanced N-acylation of palmitic acid in sphingomyelin of antibody-stimulated L cells.

Metabolism of sphingomyelin was stimulated in a fibroblast-like transformed cell line, L-929, when the cells were incubated with antibodies of a specific rabbit antiserum. The cells responded with an increased incorporation of [3H] palmitic acid into sphingomyelin. The stimulated uptake of palmitic acid into sphingomyelin can not be explained by simple mass increases in cellular phospholipids but probably represents a selective N-acyl group turnover. Palmitic acid composed only 7.5 percent of the acyl substituents, as assessed by gas-liquid chromatographic analysis, but [3H] palmitic acid was incorporated at a two-fold higher level into the acyl position than into the long chain base precursor (sphingosine). Since it is known that palmitic acid is the predominant fatty acid forming sphingosine, this represents a considerable selection for N-acyl group turnover. Another saturated fatty acid, stearic acid, which was over twice as abundant constituently was incorporated at a much lower rate when the cells were stimulated. Thus palmitic acid was observed to be selectively turning over in a manner suggestive of acylation-reacylation cycles observed with other classes of phospholipids.

Acylation↗
PubMed1991

Differences between human, rat and guinea pig hepatocyte cultures. A comparative study of their rates of beta-oxidation and esterification of palmitate and their sensitivity to R-etomoxir.

Rat hepatocyte cultures have higher rates of beta-oxidation of palmitate and lower rates of esterification to glycerolipid than human or guinea pig hepatocytes. The R-enantiomer of etomoxir (sodium 2-[6-(4-chlorophenoxy)hexyl]oxirane-2-carboxylate), a hypoglycaemic compound and inhibitor of carnitine palmitoyltransferase I, inhibited palmitate beta-oxidation in all three species, but the sensitivity to inhibition was highest in human hepatocytes and lowest in rat hepatocytes. The concentration causing half-maximal inhibition was approximately: 0.1 microM in human; 1 microM in guinea pig and 10 microM in rat hepatocytes. In human and in guinea pig hepatocytes the inhibition of beta-oxidation by R-etomoxir was associated with an increase in the esterification of palmitate but in rat hepatocytes R-etomoxir lowered the total rate of palmitate metabolism. The S-enantiomer of etomoxir had no significant effect on beta-oxidation or esterification of palmitate in any of the three species. It is concluded that there are significant differences between human, rat and guinea pig hepatocytes, not only in the relative partitioning of palmitate between beta-oxidation and esterification, but also in the sensitivity to an inhibitor of carnitine palmitoyltransferase I.

Animals↗
PubMed1987

Palmitate oxidation and some enzymes of energy metabolism in human muscles and cultured muscle cells.

1. Palmitate oxidation rates and activities of creatine kinase, cytochrome c oxidase and citrate synthase were determined in homogenates of three different human muscles and their derived muscle cell cultures. Palmitate oxidation was also assayed in intact cultured cells (myotubes). 2. Biopsies obtained from m. rectus abdominis exhibited a lower palmitate oxidation rate and lower activities of citrate synthase and cytochrome c oxidase than those from m. gluteus and m. quadriceps. In contrast, cell cultures obtained from the three muscles were mutually comparable with regard to these mitochondrial activities. 3. Although cell cultures only reached a low differentiation grade (judged by the total creatine kinase activity and percentage isoenzyme-MM) they are well comparable with the original biopsies with respect to citrate synthase activity and capacity of palmitate oxidation. The activity of cytochrome c oxidase was clearly lower in the cultured cells. 4. Palmitate was more completely oxidized in intact myotubes than in homogenates of myotubes. Apparent Km and Vmax values of palmitate oxidation did not differ significantly in homogenates and intact preparations of myotubes.

Adolescent↗
PubMed1996

Comparative lipoprotein metabolism of myristate, palmitate, and stearate in normolipidemic men.

This project was designed to test the hypothesis that long-chain saturated fatty acids (myristate, palmitate, and stearate) are metabolized differently in human subjects, and that these differences may therefore account for the changes in plasma lipoprotein composition when these fatty acids are altered in the diet. Ethyl esters of each of the stable-isotope-labeled fatty acids (2H3- or 2H4-myristate, 13C16-palmitate, and 13C18-stearate) were fed to five nonhyperlipidemic men. The concentration of each labeled fatty acid was monitored for up to 72 hours as the fatty acids were assimilated into the lipid components (phospholipid [PL], triglyceride [TG], and cholesteryl ester [CE]) of the plasma lipoproteins (TG-rich lipoproteins [TRL], intermediate-density [IDL], low-density [LDL], and high-density lipoprotein [HDL]). Over 95% of the myristate was incorporated into TG, whereas 33% and 9% of the stearate and 18% and 7% of the palmitate were incorporated into PL and CE, respectively. The mean residence times (MRTs) for myristate in TG (8.6 to 9.9 hours) and PL (6.7 to 10.9 hours) in the individual lipoprotein subfractions were significantly shorter than for either palmitate (TG, 12.7 to 15.3 hours; PL, 19.6 to 21.3 hours) or stearate (TG, 10.7 to 15.5 hours; PL, 17.8 to 19.9 hours). The MRTs for stearate were shorter than for palmitate in PL. These data indicate that TG fatty acid in general, and myristate TG in particular, is the most rapidly cleared of the saturated fatty acids. There was a rapid transfer of labeled TG and PL between the lipoproteins. We were unable to detect any significant amount of stearate desaturation or elongation. In conclusion, these data demonstrate that myristate, palmitate, and stearate are metabolized in unique ways, and that it may therefore be inappropriate to continue to regard all "saturated fatty acids" as metabolically similar in clinical studies. Rather, it is important that we elucidate more clearly the specific metabolic pathway of each fatty acid to understand the mechanisms by which it alters plasma lipoprotein concentrations and composition and influences atherogenesis.

Adult↗
PubMed1993

The G protein alpha s subunit incorporates [3H]palmitic acid and mutation of cysteine-3 prevents this modification.

We investigated whether alpha s could be acylated by palmitate by transfecting COS cells with the cDNA for the wild-type, long form of alpha s and metabolically labeling with [3H]palmitate or [35S]methionine. Cells were separated into particulate and soluble fractions and immunoprecipitated with a specific peptide antibody. [3H]Palmitate was incorporated into both endogenous and transfected alpha s. Inhibition of protein synthesis with cycloheximide did not block the radiolabeling of alpha s with [3H]palmitate. Hydroxylamine treatment caused a release of the tritium radiolabel, demonstrating that the incorporation was through a thioester bond. The tritium radiolabel was base-labile and comigrated with [3H]palmitate on thin-layer chromatography. The third residue of the wild-type alpha s was mutated from a cysteine to an alanine by site-directed mutagenesis. This mutant was expressed in COS cells and localized to the particulate fraction as determined by immunoprecipitation of the [35S]methionine-labeled cells. The cysteine-3 mutant did not undergo radiolabeling with [3H]palmitate, indicating that this residue is crucial for the modification.

Animals↗
PubMed1979

Metabolism of palmitate in perfused rat liver. Isolation of subcellular fractions containing triacylglycerol.

1. The metabolism of [1-14C]palmitate in rat liver was studied in a single-pass perfusion system at concentrations of 0.2 or 1 mM. 2. After the perfusion the liver was homogenized and the floating fat was isolated. The incorporation of [1-14C]palmitate into triacylglycerol in this pool increased 9-fold when the palmitate concentration in the medium was increased from 0.2 to 1 mM. In time studies with 1 mM-[1-14C]palmitate 75% of the total accumulation of triacylglycerol occurred in this pool. Our results support the concept that the floating-fat fraction contains the storage pool of triacylglycerol, i.e. the cytoplasmic lipid droplets. 3. In a particulate preparation consisting mainly of mitochondria and microsomal fraction the incorporation of [1-14C]palmitate into triacylglycerol was proportional to the fatty acid concentration. Triacylglycerol in the perfusate medium and in the particulate fraction was in isotopic equilibrium, which indicates that the particulate fraction contained the precursor pool for secreted triacylglycerol, i.e. the pool in endoplasmic reticulum and Golgi apparatus. 4. The oxidation to labelled water-soluble products and to CO2 was increased 14-fold by the 5-fold increase in palmitate concentration.

Animals↗
PubMed1979

Metabolism of palmitate in perfused rat liver. Effect of ethanol in livers from rats fed on a high-fat diet with or without ethanol.

1. Rats were treated for 4 weeks with liquid diets that contained, on the basis of energy content, 35% fat, 18% protein and 47% carbohydrate (high-fat diet) or 35% fat, 18% protein, 11% carbohydrate and 36% ethanol (high-fat/ethanol diet). 2. The livers were perfused with 1mm-[1-(14)C]palmitate and with 0, 10mm- or 80mm-ethanol. The oxidation and esterification of palmitate was measured. Two subcellular pools of triacylglycerol were separated; one contained triacylglycerol from cytoplasmic lipid droplets and the other contained triacylglycerol from the endoplasmic reticulum and Golgi apparatus. 3. In the presence of ethanol, liver from rats fed on the high-fat diet esterified about 70% of the [1-(14)C]palmitate taken up compared with 90% in liver from rats fed chow (containing 11% fat on the basis of energy content). Compared with chow diet the high-fat diet did not potentiate the effect of ethanol on storage of [1-(14)C]palmitate in hepatic triacylglycerol. The relation between the fat content of the diet and the degree of fatty liver induced by by ethanol [Lieber & DeCarli (1970) Am. J. Clin. Nutr.23, 474-478] is discussed. 4. The ethanol-containing diet increased the hepatic content of triacylglycerol 4-fold and the increase was exclusively found in the fraction suggested to contain lipid from cytoplasmic lipid droplets. The ethanol-induced fatty liver, perfused with ethanol, esterified and oxidized palmitate at rates that were quite similar to the rates found in high-fat control livers perfused without ethanol. This suggests that the fatty liver had adapted to the presence of ethanol with respect to palmitate metabolism. 5. O(2) and ethanol uptake by the livers were not affected by the ethanol-containing diet.

Animals↗
PubMed1984

The presence of cysteine in the cytoplasmic domain of the vesicular stomatitis virus glycoprotein is required for palmitate addition.

The transmembrane glycoprotein (G protein) of vesicular stomatitis virus (VSV) is known to contain 1-2 mol of covalently linked fatty acid (palmitate) per mol of protein. G protein is oriented in cellular membranes such that the carboxyl-terminal 29 amino acids protrude into the cytoplasm. We have obtained expression in eukaryotic cells of mutagenized cDNA clones that encode VSV G proteins lacking portions of this cytoplasmic domain. Labeling of these truncated proteins with [3H]palmitate indicated that the palmitate might be linked to an amino acid residue within the first 14 residues on the carboxyl-terminal side of the transmembrane domain. Using oligonucleotide directed mutagenesis, we changed the single codon specifying cysteine in this domain to a codon specifying serine. Expression of this mutant gene results in synthesis of a G protein lacking palmitate. We suggest that linkage of palmitate to G protein is through the cysteine in the cytoplasmic domain and that such a linkage may occur in many viral and cellular glycoproteins. The G protein lacking palmitate is glycosylated and is transported normally to the cell surface.

Amino Acid Sequence↗
PubMed1995

Depalmitoylation of CAAX motif proteins. Protein structural determinants of palmitate turnover rate.

In the present study, we examined the effect of amino acid substitutions on the rate of turnover of palmitate bound to a model "CAAX" motif protein H-Ras. These experiments were designed to shed light on the specificity of the process that removes palmitate from prenylated proteins. H-Ras, protein A-Ras fusion constructs, and constructs with amino acid substitutions in the H-Ras hypervariable region were transfected into COS cells, and the turnover rate of palmitate bound to each expressed protein was measured. We found no evidence for strict sequence specificity for palmitate removal, but found a strong inverse correlation between palmitate turnover rate and the degree of membrane association for any given construct, with slower turnover rates associated with stronger membrane binding. These data support a model in which the palmitate turnover rate is determined by access to a depalmitoylating enzyme and argue against a more complex model in which specific recognition of palmitoylated proteins is required.

Amino Acid Sequence↗
PubMed1986

Direct identification of palmitic acid as the lipid attached to p21ras.

p21v-H-ras, the transforming protein of Harvey murine sarcoma virus, contains a covalently attached lipid. Using thin-layer chromatography, we identified the acyl group as the 16-carbon saturated fatty acid palmitic acid. No myristic acid was detected in fatty acids released from in vivo-labeled p21v-H-ras. The p21v-K-ras protein encoded by Kirsten sarcoma virus was also palmitylated. The processing and acylation of p21v-K-ras however differed from that of p21v-H-ras. Three forms of [3H]palmitic acid-labeled p21ras proteins were detected in Kirsten sarcoma virus-transformed cells. This contrasted with Harvey sarcoma virus, in which two forms of p21v-H-ras contained palmitic acid. Analysis by partial proteolysis of p21v-H-ras labeled with [3H]palmitic acid suggested that all of the lipid found in intact p21v-H-ras was located in the C-terminal region. On sodium dodecyl sulfate-polyacrylamide gels, p21v-H-ras labeled with [3H]palmitic acid migrated slightly ahead of the majority of p21v-H-ras. Of the mature forms of p21v-H-ras, apparently only a subpopulation contains palmitic acid.

Animals↗
PubMed1989

Palmitate uptake by cultured hepatocytes: albumin binding and stagnant layer phenomena.

We compared uptake of palmitate by hepatocyte monolayers with uptake by polyethylene membranes under conditions of identical binding and stirring. Hepatocytes and polyethylene display similar clearances when the fatty acid is free, reflecting partial rate limitation by diffusion across the unstirred water layer. When palmitate is 99.8% bound to albumin, however, hepatocytes clear free fatty acid about seven times faster than does polyethylene. We analyzed the uptake of palmitate by polyethylene at two different pHs to isolate the diffusive resistance of the unstirred layer and to show that codiffusion of bound and free palmitate to the hepatocyte surface accounts for only approximately 20% of the albumin-dependent increment in the clearance of free palmitate. The clearance data are supported by independent measurements of the stagnant layer thickness obtained from indicator dilution data and by an electrochemical method. The findings suggest that hepatocytes facilitate the dissociation of albumin-palmitate complexes. Alternatively, albumin may modulate the uptake capacities of hepatocytes and/or polyethylene.

Analysis of Variance↗
PubMed1992

Uptake of palmitate by hepatocyte suspensions: facilitation by albumin?

Albumin-dependent uptake of unbound [3H]palmitic acid by hepatocytes isolated from female rat livers was studied and the experimental results compared with the predictions of a noncompartmental diffusion-reaction theory for the cellular uptake of protein-bound ligands. The outright theoretical predictions involve values for the parameters of the system, some newly measured (hepatocyte radii and the rate constant for the dissociation of palmitate-albumin complex) and some taken from the literature (diffusion coefficients and the equilibrium association constant for the palmitate-albumin complex). The measured unbound clearance of [3H]palmitic acid, defined as the initial uptake velocity divided by the unbound [3H]palmitic acid concentration in the medium, was enhanced 6.6-fold as the concentration of human serum albumin was increased from approximately 5 to 480 microM. This enhancement factor was predicted by the theory, according to which the enhancement reflects codiffusion of bound ligand across the unstirred layer adjacent to the cell membrane and, therefore, an increased delivery of unbound ligand to the cell surface. In contrast, the absolute magnitude of the unbound clearance was consistent with the theory only for the lowest published value for the equilibrium association constant, 15 microM-1. For higher published values (62 and 94 microM-1), the magnitude of the unbound clearance observed experimentally was severalfold higher than that predicted by the theory. If in fact the association constant exceeds 30 microM-1, the data would imply that an albumin-dependent facilitation mechanism exists which enhances the availability of palmitate to the cell over and above the enhancement predicted by the diffusion-reaction theory.

Animals↗