Cholesterol-induced microsomal changes modulate desaturase activities

The effect of 1% dietary cholesterol and 0.5% cholate on the rat liver microsomal composition and fatty acid desaturase activities was studied over various periods of time. The cholesterol content of liver microsomes increased as well as that of phosphatidylcholine. Cholesterol/phosphatidylcholine and phosphatidylcholine/phosphatidylethanolamine ratios were also elevated. Phosphatidylinositol decreased, but it recovered its original values at the end of the experimental period. Phosphatidylserine and sphingomyelin slightly decreased with time. Fatty acid composition changes were expressed by a saturated acid decrease and monounsaturated acid increase. Arachidonic acid content was also reduced. A similar pattern appeared in the main phospholipids: phosphatidylcholine and phosphatidylethanolamine. Delta 9-Desaturase activity was enhanced as early as 48 h after cholesterol administration, whereas delta 5- and delta 6-desaturases were depressed during the same period and this enzymatic behaviour remained after 21 days of diet administration. The microsomal membrane was rigidized, as demonstrated by the increase of the fluorescence anisotropy of 1,6-diphenyl-1,3,5-hexatriene.     

Effect of long-term feeding olive and sunflower oils on fatty acid composition and desaturation activities of liver microsomes

Changes in microsomal fatty acid composition, delta 9- and delta 6-desaturase activities and cholesterol and phosphorus liver content were studied in dogs fed olive and sunflower oil diets. No changes were observed in the saturated fatty acids between dietary groups. The level of monounsaturated fatty acids was more elevated in animals fed the OO diet, because of its high relative content in this diet although the in vitro delta 9-desaturase activity was similar in microsomes from the two groups. The proportion of arachidonic acid was similar in SO and OO fed animals. This similar level occurred despite a significant increase in the level of linoleic acid in membrane lipids as a result of feeding the SO supplement. The in vitro delta 6-desaturase activity in liver microsomes showed no differences between dogs fed the two diets. Thus, the higher desaturation presented in vivo by microsomes from OO group may be related to the inhibition by linoleic acid of delta 6-desaturase in dogs fed the SO diet. The polyunsaturated fatty acids (PUFA) from the n-3 series were higher in microsomal phosphatidylcholine and phosphatidylethanolamine from animals fed the OO supplemented diet. The cholesterol/phosphorus molar ratio was higher in the SO group in which the unsaturation index was only slightly affected in phospholipids.     

Influence of dietary fat on the lipid composition of rat brain synaptosomal and microsomal membranes.

The modulation of rat brain microsomal and synaptosomal membrane lipid by diet fat was examined. Brain synaptosomal and microsomal membrane composition was compared for rats fed on diets containing either soya-bean oil (SBO), SBO plus choline, SBO lecithin, sunflower oil (SFO), chow or low-erucic acid rape-seed oil (LER) for 24 days. Cholesterol and phosphatidylcholine levels in both membranes were altered by diet. Diet fat also affected the microsomal content of sphingomyelin. Change in membrane phosphatidylcholine level was related to the relative balance of omega-6, omega-3 and monounsaturated fatty acids within the diets fed. The highest phosphatidylcholine levels appeared in membranes of animals fed on SBO lecithin and the lowest in those fed on LER. Microsomal membrane cholesterol and sphingomyelin content increased by feeding on SBO lecithin. In both synaptosomal and microsomal membranes a highly significant correlation was observed between membrane phosphatidylcholine and cholesterol content. The fatty acyl composition of phospholipids from both membranes also altered with diet and age. Alteration in fatty acid composition was observed in response to dietary levels of omega-6, omega-3 and monounsaturated fatty acids, but the unsaturation index of each phospholipid remained constant for all diet treatments. These changes in lipid composition suggest that dietary fat may be a significant modulator in vivo of the physicobiochemical properties of brain synaptosomal and microsomal membranes.     

Fatty acid composition and properties of the liver microsomal membrane of rats fed diets enriched with cholesterol.

Male rats were fed diets containing olive (OO) or evening primrose (EPO) oil (10% w/w), with or without added cholesterol (1% w/w). After 6-week feeding, the lipid and fatty acid compositions, fluidity, and fatty acid desaturating and cholesterol biosynthesis/esterification related enzymes of liver microsomes were determined. Both the OO and EPO diets, without added cholesterol, increased the contents of oleic and arachidonic acids, respectively, of rat liver microsomes. The results were consistent with the increases in delta 9 and delta 6 desaturation of n-6 essential fatty acids and the lower microviscosity in the EPO group. Dietary cholesterol led to an increase in the cholesterol content of liver microsomes as well as that of phosphatidylcholine (PC). The cholesterol/phospholipid and PC/PE (phosphatidylethanolamine) ratios were also elevated. Fatty acid composition changes were expressed as the accumulation of monounsaturated fatty acids, with accompanying milder depletion of saturated fatty acids in rat liver microsomes. In addition, the arachidonic acid content was lowered, with a concomitant increase in linoleic acid, which led to a significant decrease in the 20:4/18:2 ratio in comparison to in animals fed the cholesterol-free diets. Cholesterol feeding also increased delta 9 desaturase activity as well as membrane microviscosity, whereas it decreased delta 6 and delta 5 desaturase activities. There was a very strong correlation between fluidity and the unsaturation index reduction in the membrane. Furthermore, the activity of hydroxymethylglutaryl-CoA reductase increased and the activity of acyl-CoA:cholesterol acyltransferase decreased in liver microsomes from both cholesterol-fed groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

High-cholesterol diets induce changes in lipid composition of rat erythrocyte membrane including decrease in cholesterol,increase in alpha-tocopherol and changes in fatty acids of phospholipids

Effects of high dietary cholesterol on erythrocyte membrane lipids were studied. Feeding rats with a diet containing 0.5% cholesterol and 0.15% sodium cholate for two weeks induced changes in erythrocyte membrane lipids including a decrease in cholesterol, an increase in alpha-tocopherol (alpha-Toc) and changes in the fatty acid composition of phospholipids. Oleic acid and linoleic acid increased, while arachidonic acid decreased in phosphatidylcholine. Saturated fatty acids decreased and unsaturated fatty acids increased in phosphatidylethanolamine. Almost the same changes in membrane lipids were also noted after six weeks of feeding rats with the diet. A diet containing 0.5% cholesterol but without sodium cholate caused a decrease in erythrocyte cholesterol and an increase in erythrocyte alpha-Toc after two weeks of feeding, as compared to the basal diet, indicating that high dietary cholesterol, but not sodium cholate, was responsible for these changes in the erythrocyte membrane.     

High-cholesterol Diets Induce Changes in Lipid Composition of Rat Erythrocyte Membrane Including Decrease in Cholesterol,Increase in α-Tocopherol and Changes in Fatty Acids of Phospholipids

Effects of high dietary cholesterol on erythrocyte membrane lipids were studied. Feeding rats with a diet containing 0.5% cholesterol and 0.15% sodium cholate for two weeks induced changes in erythrocyte membrane lipids including a decrease in cholesterol, an increase in α-tocopherol (α-Toc) and changes in the fatty acid composition of phospholipids. Oleic acid and linoleic acid increased, while arachidonic acid decreased in phosphatidylcholine. Saturated fatty acids decreased and unsaturated fatty acids increased in phosphatidylethanolamine. Almost the same changes in membrane lipids were also noted after six weeks of feeding rats with the diet. A diet containing 0.5% cholesterol but without sodium cholate caused a decrease in erythrocyte cholesterol and an increase in erythrocyte α-Toc after two weeks of feeding, as compared to the basal diet, indicating that high dietary cholesterol, but not sodium cholate, was responsible for these changes in the erythrocyte membrane.     

Influence of dietary lipids on microsomal membranes from chick breast muscle

1. The effect of different dietary fat intake on the lipid composition and fluidity of microsomal membranes as well as in the enzymatic activity of the Ca2+-ATPase from chick breast muscle was investigated. 2. When a standard diet was supplemented with 10% sunflower seed oil, an increase in the relative amounts of unsaturated fatty acids and membrane fluidity and a decrease in the cholesterol content was observed. 3. The presence of 6% cholesterol in the diet does not modify the fatty acid composition and the fluidity of the membrane but increased, in a low extension, the cholesterol content. 4. The provision of the sunflower seed oil-rich diet supplemented with cholesterol just 48 hr before death promoted an increase in the relative amounts of unsaturated fatty acids and cholesterol content whereas the membrane fluidity decreased in a significant extent. 5. Despite that dietary lipids gave rise in some cases to changes in lipid composition and in the physical state of the microsomal membrane, neither the Ca2+ uptake capacity nor the ATPase activity were significantly affected.     

Lipid composition of liver microsomes in rats fed a high monounsaturated fatty acid diet

The fatty acid and cholesterol contents of tissue membranes are the determinants of membrane stability and functionality. This study was designed to evaluate the influence of a high monounsaturated fatty acid diet on the fatty acid composition of rat liver microsomes and on their cholesterol and lipid phosphorus content. Weanling animals were fed for 5 weeks with high fat diets containing olive oil or corn oil. Saturated fatty acids were increased and oleic acid decreased in microsomal total phospholipids and in the three major phosphoglycerides, phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylinositol (PI), of rats fed corn oil as compared to the olive oil group. The percentage of linoleic acid was higher in the corn oil group, but only for total phospholipids and PC. Linoleic and alpha-linolenic metabolites were significantly increased in total phospholipids of olive oil-fed animals with respect to those fed corn oil. These changes were responsible for the low unsaturation index found in microsomal phospholipids of the corn oil group. The diet did not affect the microsome cholesterol or the lipid phosphorus content. These results show that, in olive oil-fed rats, the cholesterol content and the degree of unsaturation of liver microsomes was similar to that observed in weanling animals; this probably suggests an adequate maintenance of functionality of membranes in olive oil-fed animals.     

Fatty acid desaturation and microsomal lipid fatty acid composition in experimental hypothyroidism.

We have studied the influence of experimental hypothyroidism in the rat on the synthesis of unsaturated fatty acids and on liver microsomal lipid fatty acid composition. Hypothyroid rats demonstrated an 80% decrease in delta 9 (stearate) desaturation and a 43% decrease in delta 6 (linoleate) desaturation. Liver microsomal fatty acid composition was altered in the hypothyroid animals with a significantly decreased proportion of arachidonate and increased proportions of linoleate, eicosa-8,11,14-trienoate, eicosapentaenoate and docosahexaenoate. The bulk of these changes occurred in both of the two major phospholipid components, phosphatidylcholine and phosphatidylethanolamine. All of the changes were corrected by treatment of the hypothyroid rat with 25 micrograms of tri-iodothyronine/100 g body wt. twice daily. The diminished delta 9 desaturation did not lead to any changes in fatty acid composition. The increased linoleate and decreased arachidonate levels may be due to the diminished delta 6 desaturase activity, the rate-controlling step in the conversion of linoleate into arachidonate. The increases in the proportions of the other polyunsaturated fatty acid components cannot be explained by changes in the synthesis of unsaturated fatty acids, but are probably due to diminished utilization of these fatty acids.     

In vivo phospholipid modification induces changes in microsomal delta 5-desaturase activity.

The phospholipid composition of rat-liver microsomes was modified by feeding weaning rats a choline-free diet. After 21 days, the phosphatidylcholine content decreased with a concomitant increment of phosphatidylserine and cholesterol. The bulk fluidity of the membrane decreased. Under these conditions, the delta 5-desaturase activity was diminished as well as the arachidonic-acid content of the membrane lipids.     

Fatty acid desaturase activities are modulated by phytosterol incorporation in microsomes

The effect of phytosterol-rich diets (3% beta-sitosterol + 2% campesterol) on rat liver microsomal fatty acid desaturases, membrane dynamics and lipid composition was investigated. After a 21 day period, phytosterol was incorporated into microsomes and the membrane fluidity decreased. There were no changes in either the phospholipid composition or in the total sterol content. However, the phytosterol/cholesterol ratio increased. In the animals fed phytosterols, the delta 5-, delta 6- and delta 9-fatty acid desaturases were significantly more active than in control animals. The changes in the lipid fatty acid composition were consistent with those of the desaturase activities. Hence, it is suggested that: (1) dietary phytosterol modulates desaturase activities; (2) phytosterols make the membrane more rigid but do not induce changes in the relative phospholipid composition; (3) delta 9-, delta 5- and delta 6-desaturase activities increase when the membrane becomes more rigid without changes in the phospholipid composition.     

Influence of dietary cholesterol on polyunsaturated fatty acid composition, fluidity and membrane-bound enzymes in liver microsomes of rats fed olive and fish oil.

Male rats were fed diets containing olive or marine fish oils (10% w/w) with or without added cholesterol (1% w/w). After six weeks of feeding, the major fatty acid composition, fluidity, fatty acid desaturating and cholesterol biosynthesis/esterification related enzymes of liver microsomes were determined. Both olive oil and marine fish oil diets, without added cholesterol, enriched content of oleic and docosahexaenoic acids, respectively, of rat liver microsomes. The results were consistent with reduction in delta 6 and delta 5 desaturation of n-6 essential fatty acids and higher fluidity in the marine origin oil group. Inclusion of cholesterol into diets resulted in decreased membrane arachidonic acid content, with concomitant increase in linoleic acid content. Cholesterol feeding also decreased delta 6 and delta 5 desaturase activities, as well as membrane fluidity. Furthermore, the activity of acyl-CoA:cholesterol acyltransferase decreased, whereas the activity of hydroxymethylglutaryl-CoA reductase increased, in liver microsomes from both cholesterol-fat groups.     

Changes in lipid composition of hepatocyte plasma membrane induced by overfeeding in duck

This experiment was carried out to examine the influence of overfeeding ducks with corn on the lipid composition of hepatocyte plasma membrane. Seventy-day-old male Mule ducks (Cairina moschata × Anas platyrhynchos) were overfed with corn for 12.5 days in order to induce fatty livers. The cholesterol and phospholipid contents were approximately 50% higher in hepatocyte plasma membranes from fatty livers compared to those of lean livers obtained from non-overfed ducks. However, the cholesterol/phospholipids molar ratio did not differ between both groups. Overfeeding induced a significant change in phospholipid composition of hepatocyte plasma membrane with a decrease in phosphatidylcholine proportion and conversely an increase in phosphatidylethanolamine. The fatty acid profile of phospholipids was also altered. In fatty hepatocyte plasma membrane, the overall proportion of polyunsaturated fatty acids (PUFA) was decreased and this was due to the decrease of some of, but not all, the PUFA. In addition, the proportions of oleic acid and n-9 series unsaturated fatty acids were higher in fatty than in lean liver membranes. This study provides evidence that overfeeding with a carbohydrate-rich corn-based diet induces a de novo hepatic lipogenesis in Mule duck which predominates over dietary lipid intake to change the lipid composition of the hepatocyte plasma membrane.     

Copper deficiency-induced changes in the fatty acid composition of mitochondrial and microsomal membranes of rat liver

The effects of copper deficiency on the fatty acid composition of mitochondrial and microsomal phospholipids in rat liver were studied. Copper deficiency was induced by a milk powder diet. To evaluate the effect of the milk diet on the fatty acid pattern of mitochondrial and microsomal phospholipids, one group of rats was fed Cusupplemented powdered milk. A decrease in the relative proportion of linoleic acid and an increase in the level of oleic and docosahexaenoic acids in membrane phospholipids were found in this group. However, no changes in the fatty acid pattern characteristic of essential fatty acid deficiency were observed. Dietary copper deficiency produced a significant decrease in the relative amounts of linoleic and arachidonic acids, as well as an increase in the docosahexaenoic acid content in both mitochondrial and microsomal membranes compared to the nondeficient controls. The disproportionate quantities of polyunsaturated fatty acids are discussed with a view to the disturbances of membrane function in copper deficiency.     

Effect of distal small bowel resection on ACAT activity and microsomal lipid composition in rat small intestine

1. The acyl-CoA:cholesterol acyltransferase (ACAT) activity and lipid composition of intestinal microsomal membrane were investigated 6 weeks after both 50 and 75% distal small bowel resection (DSBR). 2. No changes in both microsomal ACAT activity and cholesteryl ester levels were found, while microsomal non-esterified cholesterol content was increased after the surgical operation. 3. The total phospholipid content of the microsomes did not change as a result of DSBR. 4. The microsomal phospholipid fatty acid composition showed a significant increase in saturated fatty acids together with no changes in both total monounsaturated and total polyunsaturated fatty acids after resection. 5. An increase in the levels of linoleic acid accompanied by a decrease in arachidonic acid was found in remnant intestine of resected rats.     

Ethynylestradiol alters lipid composition and phosphatidylethanolamine metabolism in red blood cells

Treatment of female rats with ethinylestradiol at a dose of 60 micrograms/rat, daily for 21 days, produced marked changes in red blood cell lipids. Cholesterol was decreased by 22% and total phospholipids were increased by 13%, resulting in a 31% decrease in the cholesterol to phospholipid ratio. The mass distribution of phosphatidylcholine and phosphatidylethanolamine relative to total phospholipids was unchanged. Whereas control red cells incorporated preferentially fatty acids in phosphatidylcholine, ethinylestradiol stimulated their incorporation specifically in phosphatidylethanolamine, where increases occurred with palmitic acid (+75%), oleic acid (+68%) and arachidonic acid (+31%). Incorporation in phosphatidylcholine was unaffected with any of the 3 fatty acids. The stimulation of fatty acid incorporation in phosphatidylethanolamine is likely to reflect an estrogen-dependent increase in turnover rate of fatty acids in this phospholipid. Such alterations in lipid composition and fatty acid incorporation in red cell phospholipids may have significant effects on membrane function.     

Mineralocorticoids modify rat liver delta 6 desaturase activity and other parameters of lipid metabolism

The changes in linoleyl-CoA desaturase activity of rat liver microsomes were studied after a single intraperitoneal injection of 11-deoxycorticosterone or aldosterone at physiological doses. Fatty acid of plasma and different liver fractions, and physical properties of microsomal membranes were also investigated. It was found that the specific activity of delta 6 desaturase decreased 4-fold 24 hr after the injection of aldosterone or deoxycorticosterone. Pretreatment of the rats with aldosterone led to a significant decrease in the percent distribution of palmitic, arachidonic, docosapentaenoic and docosahexenoic acids, with a concomitant increase in palmitoleic, oleic and linoleic acids in plasma and liver homogenates, microsomes and cytosol fractions. A similar pattern was observed after deoxycorticosterone administration. The changes resulted in a decreased unsaturation index of all fractions studied and were well-correlated with the increase observed in fluorescence depolarization of the hydrophobic probe 1,6-diphenylhexatriene in liver microsomal membranes. The interlipid and lipid/protein ratios in microsomes remained constant after hormonal treatment. These results are consistent with the idea that the inhibition of delta 6 desaturase activity and the alterations in fatty acid composition induced by mineralocorticoids, are solely responsible for the measured decrease in liver microsomal membrane fluidity.     

Effect of cholesterol deficiency on the composition of phospholipids and fatty acids of the housefly, Musca domestica

The housefly larvae were grown in the aseptic diet containing 0.56 μmole cholesterol/g wet weight of diet (control) and 0.05 μmole cholesterol/g wet weight of diet (deficient). The effects of cholesterol deficiency upon the phospholipid composition and fatty acids of the total phospholipid and triglyceride fractions from the lipid extract of the various larval tissues, whole larva, and in both sexes of adults 4 days after eclosion were examined. The total sterol and phospholipid contents (expressed relative to the wet weight of the insect) of the control and deficient insects at the larval and adult stages were analysed and molar ratios compared. The results suggest that cholesterol deficiency reduced the free sterol content of the larvae and adult insects to approximately 25% of the content of the control insects. However, cholesterol deficiency did not effect the phospholipid content during larval and adult stages when compared to that of control insects. Though the larvae reared on the cholesterol deficient diets did not show a profound alteration in the phospholipid composition, a marked increase in the ratio of phosphatidylcholine to phosphatidylethanolamine of the larval fat body and composite gut fraction were noticed. The cholesterol deficiency induced significant changes in the fatty acid composition of the phospholipid fraction of the insect. The ratio of unsaturated fatty acids to saturated fatty acids of the phospholipid fractions decreased significantly due to cholesterol deficiency in the whole larvae and in both sexes of adult flies. The data indicates that cholesterol deficient insects compensated for the lack of cholesterol by increasing saturated fatty acids preferentially in the phospholipid fraction of the lipids for the maintenance of proper membrane fluidity.     

In vitro modification of cholesterol content of rat liver microsomes. Effects upon membrane 'fluidity' and activities of glucose-6-phosphatase and fatty acid desaturation systems

The cholesterol content of rat liver microsomal membranes was modified in vitro by incubating microsomes and cytosol with liposomes prepared by sonication of microsomal lipids and cholesterol. In this way, the cholesterol to phospholipid molar ratio was increased from 0.11-0.13 in untreated microsomes to a maximal of 0.8 in treated ones. Cholesterol incorporation in microsomes produced an increase in the diphenyl-hexatriene steady-state fluorescence anisotropy and a decrease in the efficiency of pyrene-excimer formation which indicated a decrease in the rotational and translational mobility, respectively, of these probes in the membranes lipid phase. Cholesterol incorporation in microsomes did not affect significantly the glucose-6-phosphatase activity in 0.1% Triton X-100 totally disrupted microsomes, but diminished the glucose-6-phosphatase activity of 'intact' microsomes. This indicates that possibly the glucose 6-phosphate translocation across the microsomal membrane is impeded by an increase in the membrane apparent 'microviscosity'. Cholesterol incorporation in microsomes decreased NADH-cytochrome c reductase without affecting NADH-ferricyanide reductase activity. The delta 9 desaturation reaction rate was enhanced by cholesterol incorporation at low but not at high palmitic acid substrate concentration. delta 5 and delta 6 desaturase reaction-rates were increased both at low and high fatty acid substrate concentrations. These results suggest that a mechanism involving fatty acid desaturase enzymes, might exist to self-regulate the microsomal membrane lipid phase 'fluidity' in the rat liver.     

Interactive effects of dietary (n-3) polyunsaturated fatty acids and chronic ethanol intoxication on synaptic membrane lipid composition and fluidity in rats.

The influence of dietary polyunsaturated fatty acids on fatty acid composition, cholesterol and phospholipid content as well as 'fluidity' (assessed by fluorescence polarization of 1,6-diphenyl-1,3,5-hexatriene (DPH) probes) of brain synaptic plasma membranes (SPM) and their interactions with chronic ethanol effects were studied in rats fed for two generations with diets either devoid of (n-3) fatty acids (sunflower oil diet), rich in alpha-linolenic acid (soya oil diet) or in long chain (n-3) fatty acids (sunflower + cod liver oil diet). Results were compared with rats fed standard lab chow. Sunflower oil led to an increase in the (n-6)/(n-3) ratio in the membranes with an increase of the 'fluidity' at membrane apolar level; sunflower + cod liver oil decreased the (n-6)/(n-3) ratio without affecting membrane 'fluidity' while no difference was seen between the SPM of rats fed soya oil and standard diet. After 3 weeks alcohol intoxication in rat fed the standard diet: oleic alpha-linoleic acids and cholesterol levels were increased, arachidonic acid and the double bond index/saturated fatty acids were decreased and there was a decrease of 'fluidity' in the lipid core of the SPM. Soya oil almost totally abolished these usually observed changes in the SPM fatty acids composition but increased oleic acid and cholesterol without any change in fluidity. Sunflower oil led to the same general alterations of fatty acid as seen with standard diet but to a greater extent, with decrease of the 'fluidity" at the apolar level and in the region probed by TMA-DPH. When sunflower oil was supplemented with cod liver oil, oleic and alpha-linoleic acids were increased while the 'fluidity' of the apolar core of SPM was decreased. So, the small changes in fatty acid pattern seem able to modulate neural properties i.e. the responses to a neurotoxic like ethanol. A structurally specific role of PUFA is demonstrated by the pernicious effects of the alpha-linolenic acid deficient diet which are not totally prevented by the supply of long chain (n-3) PUFA.