Hepatic Phosphatidylcholine Research Articles

In vitro evidence that phospholipid secretion into bile may be coordinated intracellularly by the combined actions of bile salts and the specific phosphatidylcholine transfer protein of liver.

Using model systems, we explored a potential function of hepatic phosphatidylcholine transfer protein to extract biliary-type phosphatidylcholines from intracellular membranes (e.g., smooth endoplasmic reticulum) and deliver them to canalicular plasma membranes where biliary secretion occurs. We measured transfer rates of parinaroyl phosphatidylcholine, a naturally fluorescent phospholipid, from small unilamellar vesicles composed of sn-1 palmitoyl, sn-2 parinaroyl phosphatidylcholine, and egg yolk phosphatidylcholine (molar ratio 75:25) wherein the fluorophore is self-quenched to small unilamellar vesicles composed of phosphatidylethanolamine, sphingomyelin, phosphatidylserine, phosphatidylinositol, and cholesterol (molar ratios 22:22:10:8:38) representing model microsomal and canalicular plasma membranes, respectively. Following addition of phosphatidylcholine transfer protein (purified from bovine liver), fluorescence intensity increased exponentially indicating net phosphatidylcholine transfer from donor to acceptor vesicles. Submicellar concentrations of a wide hydrophobicity range of common and uncommon taurine and glycine conjugated bile salts species (anionic steroid detergent-like molecules), sodium taurofusidate (a conjugated fungal bile salt analog), and sodium dodecyl sulfate and octylglucoside, anionic and nonionic straight chain detergents, respectively, markedly stimulated phosphatidylcholine transfer protein activity. This 40-115-fold effect was most pronounced for the common bile salts and correlated positively with bile salt hydrophobicity. Thermodynamic analysis of net transfer revealed that the rate-limiting step was extraction of phosphatidylcholine molecules from donor vesicles and that bile salts facilitated their capture by enhancing both phosphatidylcholine transfer protein binding as well as perturbing phospholipid packing in vesicle bilayers.(ABSTRACT TRUNCATED AT 250 WORDS)

Hepatic phosphatidylcholines: evidence for synthesis in the rat by extensive reutilization of endogenous acylglycerides.

Studies were performed to compare the extent of fatty acid incorporation into liver phosphatidylcholines (PCs) by acyl remodeling and by de novo synthesis. To this end, isolated rat livers were first perfused with palmitoleic acid (16:1) and [2-3H]glycerol and then with 17:1 fatty acid and nonradiolabeled glycerol that resulted in the formation of new molecular species of radiolabeled PCs containing 16:1 and 17:1 acyl groups. The specific activities of newly formed molecular species of the de novo precursors of acylglyceride synthesis, phosphatidic acids (PAs) and diglycerides (DGs), and the products of synthesis, PCs and triglycerides (TGs), were measured at periods during both the labeling period of perfusion with 16:1 (first 15 min) and the more prolonged chase period with 17:1 (up to 120 min). At the end of the labeling period, the specific activity of all the 16:1-containing PAs, DGs, and 16:1-16:1-16:1 and 16:1-16:1-18:2 TGs were the same and were much higher than any molecular species that did not contain 16:1. The specific activities of these molecular species are indicative of the specific activity of molecular species synthesized exclusively by de novo synthesis (i.e., by acylation of glycerol 3-phosphate) during the labeling period. In contrast, the specific activity of 16:1-16:1 PC was only 2/3 that of the other 16:1-16:1 glycerides, and the specific activities of the other 16:1-containing PCs were only about 1/3 that of the corresponding 16:1-containing PAs, DGs, and 16:1-16:1 TGs. After the labeling period and during the chase period with perfusion of 17:1 and nonradiolabeled glycerol, the specific activities of major 16:1 PCs exceeded the specific activities of their corresponding PAs and DGs and remained considerably higher than these precursors of de novo synthesis for the duration of perfusions. However, during this period, the specific activities of major 16:1 PCs were less than their corresponding molecular species of TGs. During the chase period, new 17:1 molecular species of PCs were formed that were also radiolabeled. The specific activity of 16:1-17:1 PC, the 17:1 PC with the highest specific activity, always exceeded its corresponding PA and DG precursors. During the chase period, non-16:1 and non-17:1 molecular species of PCs that comprised the bulk of hepatic PCs were also radiolabeled and the specific activities of these molecular species progressively increased during this period.(ABSTRACT TRUNCATED AT 400 WORDS)

Hepatic Phosphatidylethanolamine Methyltransferase Activity Is Decreased by Ethanol and Increased by Phosphatidylcholine

Phosphatidylethanolamine N-methyltransferase participates in the synthesis of membrane phosphatidylcholine. Its activity was reported to be decreased in patients with alcoholic cirrhosis, but it is not known whether this is a consequence of the cirrhosis or precedes it. This question was studied in a baboon model of alcohol-induced fibrosis. Phosphatidylethanolamine N-methyltransferase activity was measured in sequential percutaneous needle liver biopsies by the conversion of phosphatidylethanolamine to phosphatidylcholine, using radioactive S-adenosylmethionine as a methyl donor. Chronic alcohol consumption (1-6 years) significantly decreased hepatic phospholipid and phosphatidylcholine levels and reduced phosphatidyl-ethanolamine N-methyltransferase activity even before the development of fibrosis. These effects were prevented or attenuated by supplementing the diet with 2.8 g/1000 kcal of a preparation rich in dilinoleoyl phosphatidylcholine, a highly bioavailable phosphatidylcholine species. There were significant (p < 0.001) correlations between phosphatidylethanolamine N-methyltransferase activity and both hepatic phosphatidylcholine (r = 0.678) and total phospholipid (r = 0.662). 1. Alcohol consumption diminishes phosphatidylethanolamine N-methyltransferase activity prior to the development of cirrhosis and decreases the hepatic content of its product, namely phosphatidylcholine, a key component of cell membranes. This may promote hepatic injury and possibly trigger fibrosis. 2. Phosphatidylcholine administration ameliorates the ethanol-induced decrease in phosphatidylethanolamine N-methyltransferase activity and corrects phospholipid and phosphatidylcholine depletions, thereby possibly contributing to the protection against alcoholic liver injury.

Modulation by dietary oils and clofibric acid of arachidonic acid content in phosphatidylcholine in liver and kidney of rat: Effects on prostaglandin formation in kidney

The manipulation of 20:4( n − 6) contents in phosphatidylcholine of liver and kidney of rats by dietary oils and p-chloropheno-xyisobutyric acid (clofibric acid) as well as the effects on the formation of prostaglandin E 2 in kidney were studied. Three groups of rats were fed diets that contained either safflower oil (SO) or perilla oil (PO) or fish oil (FO) for 1 week. Each dietary group was divided into two groups. One group continued the same diet for another 1 week; the second group continued the same diet and received subcutaneous injections of clofibric acid once a day for 1 week. The content of 20:4( n − 6) in hepatic phosphatidylcholine was markedly lowered by feeding either FO or PO and was further decreased by the administration of clofibric acid. Feeding either FO or PO lowered the content of 20:4( n − 6) in hepatic phosphatidylethanolamine, whereas clofibric acid increased it. The decrease in the level of 20:4( n − 6) in serum phospholipid was produced by feeding either FO or PO and by the administration of clofibric acid as well. There was a high correlation for the levels of 20:4( n − 6) between hepatic phosphatidylcholine and serum phospholipid. The changes brought about by dietary oils and clofibric acid in renal phosphatidylcholine was similar to those observed in liver. The content of 20:4( n − 6) in renal phosphatidylcholine was highly correlated with the level of 20:4( n − 6) in serum phospholipid. Other phospholipids in kidney responded less sensitively to the manipulation by dietary oils and clofibric acid. These results suggest that the level of 20:4( n − 6) in renal phosphatidylcholine is regulated by the level of 20:4( n − 6) in hepatic phosphatidylcholine through the changes in serum level of 20:4( n − 6). Formation of prostaglandin E 2 in kidney slices was dependent on the content of 20:4( n − 6) in renal phosphatidylcholine.

Hepatic and metabolic effects of ethanol: pathogenesis and prevention.

Mechanisms of the hepatotoxicity of ethanol are reviewed, including effects resulting from alcohol dehydrogenase (ADH) mediated excessive hepatic generation of NADH and acetaldehyde. Gastric ADH explains first-pass metabolism by ethanol; its activity is low in alcoholics and in females and is decreased by some commonly used drugs. In addition to ADH, ethanol can be oxidized by liver microsomes: studies over the last 25 years have culiminated in the molecular elucidation of the ethanol-inducible cytochrome P-450 (2E1) which causes metabolic tolerance to ethanol and to various commonly used medications, enhanced degradation of testosterone and vitamin A (with vitamin A depletion) and selective hepatic perivenular toxicity. The latter results from free radical generation and activation of various xenobiotics, causing increased vulnerability of the heavy drinker to the toxicity of industrial solvents, anaesthetic agents, commonly prescribed drugs, over-the-counter analgesics, chemical carcinogens and even nutritional factors such as vitamin A and beta-carotene. Furthermore, induction of the microsomal pathway contributes to increased acetaldehyde generation which promotes GSH depletion and lipid peroxidation and other toxic effects. Nutritional deficits may affect the toxicity of ethanol and acetaldehyde, as illustrated by the depletion in glutathione, ameliorated by S-adenosyl-L-methionine. Other 'supernutrients' include polyenylphosphatidylcholine, shown to correct the alcohol-induced hepatic phosphatidylcholine depletion and to prevent alcoholic cirrhosis in non-human primates.

Mechanisms of Ethanol-Drug-Nutrition Interactions

Mechanisms of the toxicologic manifestations of ethanol abuse are reviewed. Hepatotoxicity of ethanol results from alcohol dehydrogenase-mediated excessive hepatic generation of nicotinamide adenine dinucleotide and acetaldehyde. It is now recognized that acetaldehyde is also produced by an accessory (but inducible) pathway, the microsomal ethanol-oxidizing system, which involves a specific cytochrome P450. It generates oxygen radicals and activates many xenobiotics to toxic metabolites, thereby explaining the increased vulnerability of heavy drinkers to industrial solvents, anesthetics, commonly used drugs, over-the-counter medications and carcinogens. The contribution of gastric alcohol dehydrogenase to the first pass metabolism of ethanol and alcohol-drug interactions is now recognized. Alcohol also alters the degradation of key nutrients, thereby promoting deficiencies as well as toxic interactions with vitamin A and beta-carotene. Conversely, nutritional deficits may affect the toxicity of ethanol and acetaldehyde, as illustrated by the depletion in glutathione, ameliorated by S-adenosyl-L-methionine. Other supernutrients include polyenylphosphatidylcholine, shown to correct the alcohol-induced hepatic phosphatidylcholine depletion and to prevent alcoholic cirrhosis in non-human primates. Thus, a better understanding of the pathology induced by ethanol has now generated improved prospects for therapy.

Selective composition of biliary phosphatidylcholines is affected by secretion rate but not by bile acid hydrophobicity.

Little is known about the mechanisms of: 1) biliary phosphatidylcholine (PC) secretion by the hepatocyte, 2) selectivity for biliary 1-palmitoyl-2-linoleoyl-PC (PLPC) secretion, and 3) exclusion of 1-stearoyl-2-arachidonyl-PC (SAPC) from bile. The experiments were designed to determine, in rats, whether selectivity (for PLPC and against SAPC) is influenced by bile acid hydrophobicity or secretion rate. We examined the effects of bile acid depletion and of ileal infusion of taurocholic acid, tauroursodeoxycholic acid, and taurochenodeoxycholic acid. Compared to bile acid depletion, infusion of each bile acid caused PLPC to decrease from 59% of bile PC to 48%, and SAPC to increase from 2.6% to 5%. Bile acid hydrophobicity had no effect on PC selectivity, but selectivity decreased to a moderate degree as total PC secretion increased. To determine whether selectivity is for preformed molecular species, we used a new method to isotopically label four species of hepatic PC. This was done by intravenous injection of PLPC and SAPC labeled in the linoleate (14C) and arachidonate (3H) moieties. Assuming rapid mixing of each PC species in the hepatocyte as supported by the specific activity data, bile SAPC and SLPC were derived entirely from hepatic preformed SAPC and SLPC; bile PLPC was from both preformed PLPC (55%) and an unlabeled input (45%, probably direct secretion of newly synthesized PLPC). In conclusion, the selective composition of bile PC is not related to bile acid hydrophobicity, but is partially lost as secretion increases within the physiologic range.

Modulation of phosphatidylcholine biosynthesis by peroxisome proliferating fatty acid analogues

The modulation of phosphatidylcholine (PC) and phosphatidylethanolamine (PE) biosynthesis by sulfur-substituted fatty acid analogues has been investigated in rats. We have compared the effects of two non-beta-oxidizable fatty acid analogues, 3-thiadicarboxylic acid and tetradecylthioacetic acid, which induce proliferation of peroxisomes, with those of the analogue tetradecylthiopropionic acid, which is a weak peroxisome proliferator. Repeated administration of 3-thiadicarboxylic acid for seven days resulted in increased hepatic concentrations of both PC and PE, but the PC/PE ratio was decreased. PC synthesis was increased, as evidenced by increased incorporation of [3H]choline into PC and an increased activity of cytidinetriphosphate (CTP): phosphocholine cytidylyltransferase. This was accompanied by a reduction in the pool sizes of choline and phosphocholine. The S-adenosylmethione/S-adenosylhomocysteine ratio (AdoMet/AdoHcy) was marginally affected, indicating no increase in the rate of methylation of PE to PC. Administration of tetradecylthioacetic acid also resulted in increased hepatic phospholipid levels, increased AdoMet/AdoHcy ratios and in slightly elevated activity of CTP:phosphocholine cytidylyltransferase. The most striking effect observed after tetradecylthiopropionic acid treatment was the development of fatty liver. The activity of CTP:phosphocholine cytidylyltransferase and the incorporation of [3H]choline into PC was reduced compared to 3-thiadicarboxylic acid treatment. Although the rate of methylation of PE seemed to be increased at an elevated AdoMet/AdoHcy ratio, this resulted in only minor changes in the hepatic PC and PE levels, and the PC/PE ratio remained unchanged. Furthermore, the hepatic levels of choline and phosphocholine were reduced in these rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Incorporation of dietary 5,11,14-icosatrienoate into various mouse phospholipid classes and tissues

To investigate the basis of acyl specificity in the phosphatidylinositol (PI) lipid class, we fed mice fatty acids lacking the usual methylene interrupted double bonds. Mice were fed 10 wt% diets containing either 2.9 or 16% 5,11,14-icosatrienoate (5,11,14–20:3) as a component of seed oil mixtures, or control oil mixtures in which either 18:1n-9 or 18:3n-3 replaced the 5,11,14–20:3 content of the seed oils, for a 2-week period. 5,11,14–20:3 was found to be maximally incorporated into cardiac and hepatic PI (15–17 area%), and hepatic phosphatidylcholine (13%), but minimally incorporated into neutral lipids and those phospholipids that contain small amounts of 20:4n-6, such as hepatic sphingomyelin and cardiolipin. Within the PI class, there were important differences in the tissue distribution of 5,11,14–20:3: liver>heart>kidney=spleen>thymus=visceral fat. There was a clear selectivity for the incorporation of this fatty acid into PI as compared with other phospholipids. 5,11,14–20:3 was also extensively incorporated into hepatic phosphatidylinositol bisphosphate (PIP 2), a precursor of second messengers. In hepatic PI, 5,11,14–20:3 replaced 20:4n-6, resulting in a 50% reduction in the level of 20:4n-6. By contrast, in phosphatidylcholine and phosphatidylethanolamine lipid classes, 5,11,14–20:3 replaced several polyenes, including 18:2n-6, 20:4n-6, and 20:5n-3. In comparison with dietary 18:3n-3, 5,11,14–20:3 was found to be more effective at decreasing hepatic PI 20:4n-6 levels. Because leukotrienes and prostaglandins cannot be formed from 5,11,14–20:3 due to the lack of an internal Δ8 double bond, and because 20:4n-6 was dramatically reduced in some PI pools, we expect that 5,11,14–20:3 may alter eicosanoid signaling.

Mechanisms of hepatic phosphatidylcholine synthesis in the developing guinea pig: contributions of acyl remodelling and of N-methylation of phosphatidylethanolamine

Hepatic phosphatidylcholine (PC) from the immature fetal guinea pig at day 55 of gestation comprised mainly unsaturated molecular species containing C18:2(n-6) and C22:6(n-3) at the sn-2 position, reflecting placental permeability to essential fatty acids. At both day 55 and term (day 68), [Me-14C]choline was incorporated in utero over 3 h largely into sn-1-C16:0 PC species, with incorporation into sn-1-C18:0 PC species increasing by 18 h of incubation. Comparison of specific radioactivities after 3 h and 18 h suggests PC acyl remodelling by phospholipase A1. No incorporation into C20:4(n-6)-containing PC species could be detected of either [Me-14C]choline in vivo or CDP-[Me-14C]choline in isolated microsomes. The major phosphatidylethanolamine (PE) species were 16:0/22:6 and 18:0/22:6. Although [14C]ethanolamine was initially incorporated mainly into sn-1-C16:0 species, specific-radioactivity analysis suggested differential turnover rather than acyl remodelling. [1,2-14C]Ethanolamine and [Me-14C]methionine incorporation into PC molecular species indicated that both newly synthesized and total PE pools were available for N-methylation. Since the PC pool synthesized from PE included C20:4- and C22:6-containing species, N-methylation may provide a mechanism for supplying essential long-chain fatty acids to developing tissues that can be regulated independently from bulk PC synthesis.

Increased Hepatic Phosphatidylcholine Hydroperoxide and Deteriorated Hepatic Energy Metabolism in Rats with Endotoxemia

Liver dysfunction has been observed during severe nonhepatic infection with gram-negative bacteria; however, the pathogenesis of this clinical manifestation remains obscure. This study was designed to see the effect of endotoxemia on the oxidation product of one of the major classes of lipids associated with hepatocellular membrane, i.e. hepatic phosphatidylcholine hydroperoxide (PCOOH). The relations between PCOOH, hepatic energy charge and hepatic blood flow were also investigated. Cecal ligation and puncture (CLP) for the induction of endotoxemia were performed in rats. Compared to levels before CLP the serum endotoxin and hepatic PCOOH increased significantly (p < 0.05, p < 0.001 and p < 0.01, p < 0.001, respectively), corresponding to a significant fall of hepatic energy charge (p < 0.05, p < 0.005) 12 and 24 h after CLP. After 12 h of CLP α-tocopherol increased significantly (p < 0.05) but declined at 24 h. Hepatic blood flow remained unchanged throughout the experiment. Elevated lipid peroxidation, likely resulting from endotoxemia rather than hepatic hypoperfusion, may be attributable to impaired hepatic energy metabolism.

Cobalt-Vitamin B-12 Deficiency Decreases Methionine Synthase Activity and Phospholipid Methylation in Sheep

Two groups of lambs were fed either a Co-deficient or a Co-sufficient whole barley-based diet for 28 wk to induce a severe Co-vitamin B-12 deficiency. Holo and apo methionine synthase activities were significantly lower in the liver, kidney and spinal cord of Co-deficient animals compared with controls. Neither form of this enzyme in the brain was affected by Co deficiency. The ratio of the tissue concentrations of S-adenosyl methionine to S-adenosyl homocysteine was significantly lower only in the liver of Co-deficient animals, suggesting that the activity of hepatic SAM-dependent methyltransferase enzymes would be impaired. Measurements of tissue concentrations of phosphatidyl choline and phosphatidyl ethanolamine revealed lower concentrations of phosphatidyl choline and a lower phosphatidyl choline:phosphatidyl ethanolamine ratio in both liver and brain of the Co-deficient animals. The latter finding occurred in the absence of changes in either methionine synthase activity or the methylation ratio and may result from impaired availability of hepatic phosphatidyl choline for transport into the brain.

Plasma 1-palmitoyl-2-linoleoyl phosphatidylcholine. Evidence for extensive phospholipase A1 hydrolysis and hepatic metabolism of the products.

1-Palmitoyl-2-linoleoyl phosphatidylcholine (PLPC) labeled in either the choline, glycerol, palmitate, or linoleate component in reconstituted rat high density lipoprotein (rHDL), was administered by vein to rats with bile fistula and taurocholate infusion. PLPC disappeared from plasma in a monoexponential fashion with a half-life of 50 min. A small fraction, about 14%, of PLPC disappearance was due to removal of linoleate from the sn-2 ester bond to form plasma cholesterol esters, presumably by lecithin-cholesterol acyltransferase. Otherwise, nearly all of the PLPC components that disappeared from blood in 1 h were recovered in the liver. The choline, glycerol, and linoleate components appeared predominantly in hepatic phosphatidylcholine (PC). These three components remained together in the liver with similar fractions of each in individual PC molecular species, most notably 1-stearoyl-2-linoleoyl-PC and dilinoleoyl-PC as well as PLPC. However, the palmitate component was spread among hepatic triglyceride, free fatty acid, other phospholipids, and all palmitate-containing molecular species of PC. Less than 2% of any administered PLPC component appeared in 1-stearoyl-2-arachidonyl-PC, the major species by mass in the liver. The palmitate component from plasma PLPC appeared in biliary PC at a more rapid rate than glycerol and linoleate components; the latter components appeared in bile in identical fashion. The results show that about two-thirds of plasma PLPC disappearance is due to phospholipase A1 hydrolysis, probably hepatic lipase. The putative produce, 2-linoleoyl-lysoPC, is efficiently reacylated with a saturated fatty acid in the liver, conserving PC.

Dietary orotic acid accentuates the hepatic response to phenobarbital in rats

In rats treated with phenobarbital for 3 days and simultaneously fed a semisynthetic diet containing 1.0% orotic acid, the extent of the increases in liver microsomal phosphatidylcholine, phosphatidylethanolamine, total RNA, total protein, and cytochrome P-450 were significantly greater than they were in rats treated identically with phenobarbital but without dietary orotic acid. This is attributed primarily to the stimulation of hepatic phosphatidylcholine synthesis by dietary orotic acid. In the absence of phenobarbital, orotic acid was shown to cause some increase in liver smooth endoplasmic reticulum components, but not cytochrome P-450. Orotic acid also decreased the activity of microsomal phosphatidylethanolamine N-methyltransferase, which may have contributed to the increase in the microsomal content of phosphatidylethanolamine. The hypothesis is advanced that phospholipid availability is a limiting factor in the hepatic response to phenobarbital. When more phospholipid is available to provide the structural framework for biogenesis of endoplasmic reticulum, all of the hepatic actions of phenobarbital, including induction of cytochrome P-450, are amplified.

Effect of amitriptyline or phenobarbital on the activities of the enzymes involved in rat liver.

The effect of phenobarbital (PB) or amitriptyline (AMT) on phospholipid metabolism was studied in rat liver. Administration of PB or AMT caused a marked increase in the microsomal phosphatidylcholine (PC) content. In this context, the activities of glycerophosphate acyltransferase (GAT), phosphatidate cytidylyltransferase (PCT), phosphatidate phosphohydrolase (PPH) and choline phosphotransferase (CPT) in the liver were all increased by PB. In contrast, the activities of GAT and PCT were little affected by AMT, while those of PPH and CPT were significantly increased by AMT. These findings suggest that AMT or PB would increase hepatic microsomal PC synthesis through inducing PPH and then CPT.

Sex-related differences in the enhancing effects of perfluoro-octanoic acid on stearoyl-CoA desaturase and its influence on the acyl composition of phospholipid in rat liver. Comparison with clofibric acid and tiadenol.

The effects of the peroxisome proliferators clofibric acid (p-chlorophenoxyisobutyric acid), tiadenol [2,2'-(decamethylenedithio)diethanol] and perfluoro-octanoic acid (PFOA) on hepatic stearoyl-CoA desaturation in male and female rats were compared. Treatment of male rats with the three peroxisome proliferators increased markedly the activity of stearoyl-CoA desaturase. Administration of clofibric acid or tiadenol to female rats increased greatly the hepatic activity of stearoyl-CoA desaturase, the extent of the increases being slightly less pronounced than those of male rats. In contrast with the other two peroxisome proliferators, however, PFOA did not change the activity of stearoyl-CoA desaturase in female rats. Hormonal manipulations revealed that this sex-related difference in the effect of PFOA on stearoyl-CoA desaturase activity is strongly dependent on testosterone. The increase in stearoyl-CoA desaturase activity by peroxisome proliferators was not accompanied by any notable increases in the microsomal content of cytochrome b5 or the activity of NADH: cytochrome b5 reductase. The administration of the peroxisome proliferators greatly altered the acyl composition of hepatic phosphatidylcholine and phosphatidylethanolamine (namely the proportions of C18:1 and C20:3,n-9 fatty acids increased in both phospholipids), and the alterations were partially associated with the increase in stearoyl-CoA desaturase activity.

Effect of Dietary Lipid Composition on Rat Liver Microsomal Phosphatidylcholine Synthesis

Effects of dietary lipid on microsomal membrane fatty acid composition and phosphatidylcholine synthesis were examined with respect to the degree of saturation and chain length of dietary fatty acids. Diets containing 20% fat (wt/wt), primarily in the form of coconut oil, beef tallow, or soybean oil were fed to weanling rats for 28 d. All diets were adequate in essential fatty acids. Feeding the coconut oil diet, containing more than 50% saturated medium-chain fatty acids, increased the long-chain omega 3 fatty acid levels and altered the ratio of omega 6 to omega 3 fatty acids in membrane phosphatidylcholine and phosphatidylethanolamine compared to that in animals fed diets containing a saturated tallow or polyunsaturated soybean oil diet. Change in membrane fatty acid composition in response to the coconut oil diet was accompanied by decreased synthesis of phosphatidylcholine via the cytidine diphosphate choline (CDP-choline) pathway, and increased synthesis via the phosphatidylethanolamine methyltransferase (PEMT) pathway. An overall decrease in phosphatidylcholine production was reflected in a lower ratio of phosphatidylcholine to phosphatidylethanolamine in the membrane. Modulation of hepatic phosphatidylcholine synthesis during early growth thus appears to be in response to the ratio of long-chain omega 6 fatty acids to omega 3 fatty acids in membrane phospholipid rather than levels of specific fatty acids or the relative degree of saturation of fatty acids in the diet.

Hepatic phosphatidylcholine synthesis in deficiency of lysine and threonine: effect of malathion.

Rats fed on a rice diet deficient in lysine and threonine showed increased activities of CDP-Choline pathway enzymes and incorporation of (methyl-3H)-choline into hepatic microsomal phosphatidylcholine, compared to rats fed on the same diet supplemented with lysine and threonine. However, the amount of microsomal phosphatidylcholine was significantly decreased in rats fed a deficient rice diet. These results suggest an enhanced phosphatidylcholine catabolism in rats fed on a rice diet deficient in lysine and threonine. Malathion administration reduced the amount of phosphatidylcholine in all the groups.

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.

Influence of tauroursodeoxycholic and taurodeoxycholic acids on hepatic metabolism and biliary secretion of phosphatidylcholine in the isolated rat liver

Studies were carried out using an isolated rat liver system to define: (1) the contribution of exogenous phosphatidylcholine (PC) to biliary phospholipid secretion; and (2) its hepatic metabolism during perfusion of the livers with conjugated bile salts with different hydrophilic/Hydrophobie properties. A tracer dose of sn-1-palmitoyl-sn-2-[ 14C]linoleoylPC was injected as a bolus into the recirculating liver perfusate, under constant infusion of 0.75 μmol/min of tauroursodeoxycholate or taurodeoxycholate. The effects on bile flow, biliary lipid secretion, 14C disappearance from the perfusate and its appearance in bile, as well as hepatic and biliary biotransformation were determined. With both the bile salts, about 40% of the [ 14C]PC was taken up by the liver from the perfusate over 100 min. During the same period less than 2% of the given radioactivity was secreted into bile. More than 95% of the 14C recovered in bile was located within the identical injected PC molecular species. The biliary secretion of labeled as well as unlabeled PC, however, was significantly higher in livers perfused with taurodeoxycholate than tauroursodeoxycholate, while the reverse was observed with respect to bile flow and total bile salt secretion. The exogenous PC underwent extensive hepatic metabolization which appeared to be influenced by the type of bile salt perfusing the liver. After 2 h perfusion, the liver radioactivity was found, in decreasing order, in PC, triacylglycerol, phosphatidylethanolamine and diacylglycerol. In addition, the specific activity of triacylglycerol was significantly higher in tauroursodeoxycholate than in taurodeoxycholate-perfused livers ( P< 0.025), while the reverse was true for the specific activity of hepatic PC ( P < 0.01). Because taurodeoxycholate and tauroursodeoxycholate showed opposite effects on both biliary lipid secretion and hepatic PC biotransformations, we conclude that the hepatic metabolism of glycerolipids is influenced by the physicochemical properties of bile salts.