A comparison of lipids from liver and hepatoma subcellular membranes

Subcellular fractions of nuclei, mitochondria, endoplasmic reticulum, plasma membrane and cytosol were prepared from liver and hepatoma 72288CTC. Marker enzyme activities, biochemical compositions and electron microscopy were used to establish purity. Hepatoma NADH: cytochrome C reductase and 5′-nucleotidase exhibited abnormal subcellular distributions. The lipids from the subcellular fractions were examined in detail. Mitochondria and plasma membranes were characterized by elevated percentages of diphosphatidylglycrerol and sphingomyelin, respectively, in both tissues. All hepatoma subcellular fractions contained dramatically elevated levels of sphingomyelin and cholesterol, two components that form preferential strong complexes in vitro. The fatty acid composition of hepatoma sphingomyelin differed markedlg from liver and, unlike liver, did not exhibit organelle specific compositions. Some hepatoma lipid classes contained reduced percentages of palmitate while others contained higher levels. Hepatoma phosphatidylcholine and phosphatidylethanolamine from organelles contained lower percentages of long chain polyunsaturated fatty acids than liver. Generally, unique fatty acid profiles exhibited by individual phospholipid classes of liver subcellular fractions were absent or much reduced in the hepatoma. The ratios of oleate to vaccenate were near one for most of the phospholipid classes of most liver fractions, but all hepatoma classes, with few exceptions, contained a much higher percentage of oleate in all subcellular fractions. The hypothesis is proposed that the origin of some acyl moieties for the biosynthesis of various hepatome lipid classes differs from liver sources. The possible changes in acyl pools, sources and compartments for complex lipid biosynthesis could result in change in the quantities of molecular species that could contribute to the abnormal properties of the hepatoma membranes.     

The lipid composition of human liver microsomes

The lipid composition of human liver microsomes isolated from liver biopsy samples obtained at abdominal surgery has been determined. Human liver microsomal phospholipid is composed of 49% phosphatidylcholine, 31% phosphatidylethanolamine, 14% phosphatidylserine+phosphatidylinositol and 6% sphingomyelin, very similar to the phospholipid composition of rat liver microsomes. The fatty acid composition of human liver microsomes is remarkable only for its content of polyunsaturated fatty acids, with 20% of the fatty acids consisting of arachidonic, docosatetraenoic, docosapentaenoic and docosahexaenoic acids. This value contrasts with 33% in rats and 9% in rabbits. The molar cholesterol/phospholipid ratio in human liver microsomes is 0.069, similar to the ratio in rat and rabbit microsomes.     

Hepatoma,host liver,and normal rat liver phospholipids as affected by diet

Individual phospholipid classes derived from hepatoma, host liver, and normal liver of rats maintained on chow and fat free diets were examined in detail and the sphingomyelin and phosphoglyceride structures compared. The concentration of hepatoma spingomyelin was higher while phosphatidylcholine, phosphatidylethanolamine, phosphatidylinositol, phosphatidylserine, and diphosphatidylglycerol were only one-fourth to one-half normal liver concentrations, irrespective of diet. Hepatoma phosphatidylcholine, phosphatidylethanolamine, phosphatidyl-serine, and phosphatidylinositol contained higher percentages of 18:1 and, except phosphatidylinositol, much lower percentages of most polyunsaturated fatty acids than liver. The 1-position of host liver phosphatidylcholine and phosphatidylethanolamine, normal liver phosphatidylcholine and phosphatidylethanolamine, and hepatoma phosphatidylcholine from animals on both diets had the same approximate fatty acid composition, but the percentage of 16:0 in hepatoma phosphatidylethanolamine was reduced dramatically. The low percentage of 16:0 at the 1-position of both phosphatidylethanolamine and triglycerides suggests that the 1-position fatty acids of these two classes may have a similar origin. The fat free diet reduced the percentage of 18:2 in liver diphosphatidylglycerol 3-fold and the decrease was offset by increased percentages of 16:1 and 18:1; whereas the very low percentage of 18:2 in hepatoma diphosphatidylglycerol was offset by increased percentages of 18:0 and 16:0. Liver phosphatidylinositol and phosphatidylcholine from the animals fed the fat free diet contained the highest percentage of 20:3, which replaced 20:4. Hepatoma sphingomyelin contained a much higher concentration of 24:0 and 24:1 than liver. The hepatoma sphingomyelin also contained a C-24 dienoic acid, which was not detected in host and normal liver. Host liver contained a higher percentage of 22:6 than normal liver. The diglycerides derived from host liver PC contained a significantly higher percentage of carbon number 38 than normal liver. Diglycerides derived from hepatoma phosphatidylcholine and phosphatidylethanolamine exhibited a 1-random-2-random distribution of fatty acids, whereas diglycerides from liver phosphatidylcholine and phosphatidylethanolamine showed pairing of specific fatty acids.     

Growth hormone and liver mitochondria: Effects on phospholipid composition and fatty acyl distribution

Effects of growth hormone on phospholipid composition and fatty acyl distribution were studied in liver mitochondria of hypophysectomized rats. After hypophysectomy, only cardiolipin showed a 25% decrease. Its fatty acyl distribution, which consisted mainly of linoleic acid (55–60%) and oleic acid (20%), was unchanged. In phosphatidylcholine and phosphatidylethanolamine fractions the contents of docosahexaenoic and arachidonic acids were decreased with a concomitant increase in linoleic acid content. These changes could be accounted for by small but significant decreases in the activities of Δ⁹-desaturase (sucrose-induced), Δ⁵-desaturase and mitochondrial elongation enzymes. The activities of Δ⁶-desaturase NADH cytochrome b₅ ferri-reductase, cytochrome b₅, NADH cytochrome c reductase and microsomal elongation enzymes remained virtually unchanged. Injection of bovine growth hormone daily for seven days restored cardiolipin and fatty acyl distribution and the enzyme activities. From these and other results, we conclude that growth hormone-dependent increase of respiratory activity of liver mitochondria may be partly mediated by the hormonal effects on membrane lipid distribution.     

Lipid composition of liver mitochondria and microsomes in hyperthyroid rats

Triiodothyronine-induced alteration of the lipid pattern in rat-liver mitochondria and microsomes has been investigated. In mitochondria, a 25% total cholesterol decrease and a 14% phospholipid increase have been detected. In these hyperthyroid rat liver organelles, a strong decrease in the total cholesterol/phospholipid molar ratio occurs. On the contrary, in microsomes from the same animals, a decrease of about 23% has been measured for both total cholesterol and phospholipids; hence, in this fraction, the total cholesterol/phospholipid molar ratio is unaffected by hyperthyroidism. The liver mitochondrial phospholipid composition, unlike the microsomal composition, is altered significantly in hyperthyroid rats; a 7.4% phosphatidylcholine decrease is accompanied by a similar additive percentage increase of both phosphatidylethanolamine and cardiolipin. In regard to total phospholipid fatty acid composition in liver microsomes from hyperthyroid rats, no variation has been observed compared with the control rats, whereas in mitochondria from the same animals, a meaningful linoleic acid decrease with a similar arachidonic acid increase has been found. In addition to fatty acid alteration, the separated mitochondrial phospholipid classes also exhibit some increase in stearic acid. Among phospholipids, cardiolipin changes the most of the esterified fatty acids in hyperthyroid rat liver. In this compound, a strong increase in the percentage of both palmitic and stearic acid and a 32.4% decrease of linoleic acid have been found.     

Mitochondrial and microsomal phospholipid phosphorus metabolism during postnatal growth in rat heart and liver

The relative specific activities of phospholipids of hearts and livers of newborn and older rats were measured 2 hr after intraperitoneal³²P-orthophosphate. There was a close linear correlation between the specific activity of cardiolipin and the net increase in the amount of this phospholipid. In one-day-old animals, the relative specific activities in the heart and liver were highest and exceeded by 7.3- and 3-fold, respectively, the corresponding activities in the adult. The apparent half-lives of cardiolipins were calculated on the basis of the linear correlation that was found between the net increase in cardiolipin and the rate of incorporation. These half-lives were 4.7 days in liver and 6.4 days in heart. Though the changes in the phospholipid composition of the organelles during neonatal development were small, the relative specific activities of the individual phospholipids varied considerably. In addition to cardiolipin, there was a good correlation between the specific activity and the net increase in mitochondrial phosphatidylinositol. At birth, the specific activity of mitochondrial phosphatidylcholine in liver was 2.9 times that in microsomes. During the 12 neonatal hours, the specific activity of microsomal phosphatidylcholine increased 6.3-fold and exceeded the corresponding mitochondrial activity. The relation between the activation of microsomal phosphatidylcholine synthesis and the induction of serum lecithin synthesis in newborn liver is discussed. The finding that at birth the specific activity of mitochondrial phosphatidylcholine unexpectedly was higher than that of microsomal phosphatidylcholine points out the difficulties in interpreting the in vivo evidence for precursor-product relationship.     

The lipid composition of microsomal preparations from lactating bovine mammary tissue

The microsomes isolated from lactating bovine mammary tissue contained 4.3 mg lipid per milligram nitrogen. Phospholipids comprised 83% of the lipids. The neutral lipids were composed of triglycerides (20–30%), diglycerides (5–10%), free fatty acids (15–30%, cholesterol (35–40% and cholesterol esters (10–12%, respectively. Phosphatidylcholine was the predominant phospholipid component (>50%), and the remainder consisted of phosphatidylethanolamine (21–13%), phosphatidylserine (4–6%), phosphatidylinositol (8%), sphingomyelin (9%) and lysophosphatidylcholine (2%) respectively. The composition of the microsomal phospholipids was similar to that of isolated mammary cells and tissue homogenates but quite different from milk and fat globule membrane phospholipids. The triglycerides contained short chain fatty acids but their relative concentrations were lower than in milk triglycerides. The various lipid fractions had a variable proportion of saturated fatty acids, i.e., triglycerides (47.7%), diglycerides (86.7%), free fatty acids (70.6%), phosphatidylcholine (50.6%), phosphatidylethanolamine (50.8%), phosphatidylserine (35.3%), phosphatidylinositol (40.5%) and sphingomyelin (82.3%), respectively. The molecular distribution of fatty acids in the microsomal triglycerides and phosphatidylcholine was similar to that occurring in milk, i.e., the short chain and unsaturated fatty acids were concentrated in the primary positions (sn1 andsn3) of the triglycerides, and the unsaturated acids were preferentially located in positionsn2 of the phosphatidylcholine. The compositional data indicate that mammary microsomes are not the direct source of the phospholipids of the milk fat globule.     

Effect of dietary fat on rat liver microsomal and mitochondrial/lysosomal dolichol,phospholipid and cholesterol

The influence of different fat diets on liver phospholipid, cholesterol and dolichol was studied. Rats were separated into four groups and fed standard laboratory chow (control), a diet containing linolenic acid, a coconut oil diet, or a corn oil-containing diet. After five weeks, microsomes and mitochondrial/lysosomal fractions were prepared from the liver, and lipid compositions were analyzed. No changes in phospholipid content were observed. In control animals, the fatty acid compositions of phosphatidylcholine and phosphatidylethanolamine in the two subfractions were similar. However, these two phospholipids showed different fatty acid patterns, which were altered independently upon dietary treatment. The dietary treatments resulted, in most cases, in decreased cholesterol and dolichol contents and, especially in microsomes, in a decreased level of esterification of both lipids. The fatty acid compositions of cholesteryl esters in the two subfractions showed significant differences and cholesterol was esterified to a large extent with linolenic acid when this fatty acid was supplied in the diet. The same dietary treatment exerted different effects on the cholesterol localized in the two different intracellular compartments. This difference was most pronounced in rats fed the corn oil-containing diet; microsomal cholesteryl esters exhibited increased saturation, whereas cholesteryl esters in the mitochondrial/lysosomal fraction displayed decreased saturation. Dolichyl esters in the two cellular compartments had different fatty acyl compositions, with a considerably higher degree of saturation in microsomes. The various diets influenced the nature of the fatty acid moieties present in the isolated fractions and the effects on the two subfractions were opposite. The diet containing linolenic acid decreased the degree of saturation in microsomal dolichyl esters and increased the degree of saturation in the mitochondrial/lysosomal fraction. The results demonstrate that the fatty acid compositions of both dolichyl and cholesteryl esters display organelle specificity. Both the content of these lipids and their fatty acid compositions are greatly influenced by dietary conditions, and the esterification processes at different cellular locations exhibit independent regulation, regardless of the fatty acid content of the diet.     

Decreases in Phospholipids Containing Adrenic and Arachidonic Acids Occur in the Human Hippocampus over the Adult Lifespan

One of the biggest risk factors for developing Alzheimer's disease is advanced age. Despite several studies examining changes to phospholipids in the hippocampus during the pathogenesis of Alzheimer's disease, little is known regarding changes to phospholipids in this region during normal adult aging. This study examined the phospholipid composition of the mitochondrial and microsomal membranes of the human hippocampus from post‐mortem tissue of neurologically normal subjects aged between 18 and 104 years. Many of the age‐related changes found were in low‐to‐moderately abundant phospholipids in both membrane fractions, with decreases with age being seen in many phospholipids containing either adrenic or arachidonic acid. The most abundant phospholipid of this type was phosphatidylethanolamine 18:0_22:4, which decreased in both the mitochondrial and microsomal membranes by approximately 20 % from ages 20 to 100. Subsequent decreases with age were seen in total adrenic and arachidonic acid in the phospholipids of both membrane fractions, but not in either fatty acid specifically within the phosphatidylethanolamine class. Increases with age were seen in the hippocampus for mitochondrial phosphatidylserine 18:0_22:6. This is the first report of changes to molecular phospholipids of the human hippocampus over the adult lifespan, with this study also providing a comprehensive profile of the phosphatidylcholine, phosphatidylethanolamine and phosphatidylserine phospholipids of the human hippocampus.     

Fatty Acid Profile Analysis of Membrane Phospholipids of Isolated Soybean Lipid Bodies

Phospholipids in isolated soybean lipid bodies at 30‡C underwent degradation by acyl ester and phosphodiester hydrolysis and by phosphatidyl transfer. Under conditions that minimized oxidation, preferential loss of phosphatidylethanolamine and polyunsaturated fatty acids occurred, but acyl ester compositions of all phospholipids reflected enrichment of saturated and/or monounsaturated acids. Such fatty acid changes in phosphatidylinositol, which was degraded less than phosphatidylcholine or phosphatidylethanolamine, and in phosphatidic acid, which accumulated, also suggest that transesterification occurred extensively in lipid bodies. The specific temporal changes in phospholipid and acyl chain composition suggest that several enzymes remained active in isolated lipid bodies.     

Differences in the phospholipid,cholesterol, and fatty acyl composition of 3T3 and SV3T3 plasma membranes

An analysis of the phospholipid, cholesterol, and phospholipid fatty acyl composition of isolated plasma membranes of 3T3 and SV3T3 mouse embryo cells has been performed. The results show that the plasma membrane of SV3T3 cells contain relatively less phosphatidylethanolamine and sphingomyelin and more cholesterol than 3T3 plasma membranes. The fatty acyl composition of individual phospholipid classes as determined by gas liquid chromatography also showed differences between 3T3 and SV3T3 plasma membranes. The plasma membranes of SV40 transformed 3T3 cells contain: (a) a higher percentage of 18∶1 and less 20∶3 and 20∶4 in phosphatidylethanolamine; (b) a higher percentage of 18∶1 in phosphatidylserine; and (c) a higher percentage of 18∶2 and 20∶4 in phosphatidylinositol.     

Effect of pH on the affinity of phospholipids for cholesterol

The ability of multilamellar vesicles of phosphatidylcholine and phosphatidylethanolamine in aqueous phase to prevent access to cholesterol by a nonpolar solvent was examined. Phosphatidylethanolamine vesicles retained less sterol than phosphatidylcholine vesicles. In mixed vesicles, cholesterol was retained in proportion to the amount of phosphatidylcholine. To alter the charge and hydration of head groups, pH was adjusted from 1.2 to 12.5. Above pH 8, both phosphatidylethanolamine and phosphatidylcholine retained sterol in a 1∶1 molar ratio of phospholipid to cholesterol, regardless of acyl side chain composition. Between pH 2.0 and pH 8.0, sterol retention varied with type of head group and side chain. Lipids with 16-carbon saturated side chains retained more sterol than 18-carbon unsaturated or 12-carbon saturated side chains. Between pH 1.1 and 2.0, none of the phosphatidylethanolamines retained sterol, but long chain phosphatidylcholines, saturated or unsaturated, retained sterol in a 1∶1 molar ratio of phospholipid to sterol. Short chain phosphatidylethanolamines and phosphatidylcholines retained 0 to 20% at the low- to mid-pH range. Size of multilamellar vesicles, measured by Doppler effect light scattering analysis, had no bearing on sterol retention. Sonication of vesicles, which increases surface curvature, increases the retention of sterol. Fluorescence polarization indicated that cholesterol does not interact with DPPC or DLPC side chains. The observations can be interpreted in terms of space requirements of head groups, including charge repulsion and hydration. Other factors, such as monovalent cation replacement by protons, juxtaposition of charged groups on vesicle surfaces and length and unsaturation of acyl side chains affect the affinity of phospholipids for cholesterol.     

Dietary ω3 fatty acids and cholesterol modify enterocyte microsomal membrane phospholipids,cholesterol content and phospholipid enzyme activities in diabetic rats

Diabetes-associated changes in intestinal uptake of nutrients are modified by isocaloric variations in the type of dietary lipids, and are associated with alterations in the phospholipid and fatty acyl content of the intestinal brush border membrane. The present study was designed to test the hypothesis that diet- and diabetes-associated changes in enterocyte microsomal membrane phospholipids are due to variations in the activity of two phospholipid metabolizing enzymes, 1,2-diacylglycerol: CDP choline cholinephosphotransferase (CPT) and phosphatidylethanolamine methyltransferase (PEMT). Adult female Wistar rats were fed one of four semisynthetic diets—beef tallow low in cholesterol (BT), beef tallow high in cholesterol (BTC), fish oil low in cholesterol (FO) or fish oil high in cholesterol. In half of the animals, diabetes mellitus was produced by injection of streptozotocin. Jejunal and ileal enterocyte microsomes (EMM) were isolated and analyzed for cholesterol and phospholipids, as well as for CPT and PEMT activities. In control animals, feeding FO reduced EMM total phospholipids including phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylinositol. Feeding FO resulted in a greater than 95% reduction in the activity of CPT. Diabetes was associated with increased jejunal EMM total phospholipids including sphingomyelin (SM) and PE, without associated changes in CPT or PEMT. Dietary cholesterol supplementation did not effect EMM total cholesterol or phospholipid composition in control rats fed BT or FO, but was associated with an increase in EMM cholesterol in diabetic rats fed BT or FO. A decrease in total phospho-lipids due to a decline in SM, PC and PE in diabetic rats fed FO was not associated with changes in the activities of CPT or PEMT in EMM. Thus (i) enterocyte microsomal membrane cholesterol and phospholipid contents are influenced by diabetes, dietary cholesterol and the type of fatty acid in the diet, and (ii) changes in phospholipid composition are not fully explained by alterations in the activities of CPT and PEMT.     

The effects of dietary phospholipids enriched with phosphatidylethanolamine on bile and red cell membrane lipids in humans

The role of phospholipids in biliary cholesterol solubilization and crystallization has only recently begun to be appreciated. Phospholipid vesicles are believed to be the metastable carrier from which cholesterol nucleates. Cholesterol crystallization is influenced by the phospholipid species in bile. Feeding rats and hamsters with diets enriched in phospholipids or their precursors, especially ethanolamine, resulted in reduced cholesterol saturation of bile. Although whole phospholipids are normal dietary constituents, the effects and safety of phospholipid components have not been tested in humans. In the present study, we have evaluated the effects of a dietary phospholipid mixture, enriched with phosphatidylethanolamine, on human bile and red blood cell membrane lipid composition. Five ambulatory volunteers having a chronic indwelling T-tube, with an intact enterohepatic circulation, were investigated. Thirty-six grams of phospholipids (54% phosphatidylethanolamine, 54% linoleyl acyl chains) were added to their daily diet for fourteen days. Biliary nucleation time, cholesterol carriers, as well as plasma, red blood cell membrane, and bile lipid compositions, were monitored. Following phospholipid supplementation, the proportion of linoleyl chains (18:2) in biliary phospholipids increased significantly from 31.1±1.2 to 37.7±5.3%, while that of oleyl chains (18:1) decreased from 11.4±1.6 to 9.6±1.1%. These changes were accompanied by an increase of linoleate and its metabolite, arachidonate, in red cell membranes. Phospholipid feeding did not cause any side effects, and no significant changes in biliary nucleation time, cholesterol, phospholipid, or bile salt concentrations, or in the distribution of cholesterol within micelles or vesicles. We conclude that phospholipid feeding is safe, and can be effective as a vehicle for lecithin fatty acyl chain modulation of bile and lipid membranes. These findings may provide a basis for a controlled modulation of biliary phospholipids to increase cholesterol solubility in bile.     

Stearoyl-coenzyme A desaturase activity in novikoff hepatoma

The activities of microsomal stearoyl-CoA desaturation, NADH-cytochrome b₅ reductase, NADH-cytochrome c reductase, and the content of cytochrome b₅ were similar in livers of normal and host rats. On the other hand, stearoyl-CoA desaturation activity was absent in Novikoff hepatoma. The activities of NADH-cytochrome b₅ and NADH-cytochrome c reductases in the hepatoma microsomes were 4.8% and 2.2%, respectively, of those in normal liver. Furthermore, in hepatoma microsomes, cytochrome b₅ was absent. An active stearoyl-CoA desaturation was reconstituted only on addition of both cytochrome b₅ and the terminal desaturase enzyme to the hepatoma microsomes. These results indicated that a complete absence of cytochrome b₅ and terminal desaturase is responsible for the lack of stearoyl-CoA desaturation in Novikoff hepatoma microsomes.     

Lipid remodeling in mouse liver and plasma resulting from Δ6 fatty acid desaturase inhibition

Electrospray/tandem mass spectrometry was used to quantify lipid remodeling in mouse liver and plasma during inhibition of polyunsaturated fatty acid synthesis by the Δ6 fatty acid desaturase inhibitor, SC-26196. SC-26196 caused increases in linoleic acid and corresponding decreases in arachidonic acid and docosahexaenoic acid in select molecular species of phosphatidylcholine, phosphatidylethanolamine, and cholesterol esters but not in phosphatidylserine, phosphatidylinositol, or triglycerides. For linoleic acid-, arachidonic acid-, and docosahexaenoic acid-containing phospholipid species, this difference was, in part, determined by the fatty acid at the sn-1 position, namely, palmitic or stearic acid. An understanding of phospholipid remodeling mediated by Δ6 desaturase inhibition should aid in clarifying the contribution of arachidonic acid derived via de novo synthesis or obtained directly in the diet during inflammatory responses.     

Lipid composition of morris hepatoma 5123c,and of livers and blood plasma from host and normal rats

The lipid composition of Morris hepatoma 5123c was analyzed together with that of liver and blood plasma from both normal and tumor-bearing rats. The results showed that the liver of tumor-bearing rats contained higher amounts of glycerides, cholesteryl esters, free fatty acids and phospholipids than the liver of normal rats. In the blood plasma of tumor-bearing rats, there was an increase of free cholesterol and triglycerides; this latter difference, however, was not statistically significant. Acyl chain changes in the liver of tumor-bearing rats consisted of an increase of palmitic and oleic acids and a decrease of stearic and arachidonic acids in phosphatidylinositol. Morris hepatoma 5123c contained a lower amount of triglycerides than the livers (both host and normal) and showed a significant decrease of total phospholipids when compared to the host liver. The major acyl chain changes found in Morris hepatoma 5123c compared with both normal and host rat livers were: a) a higher percentage of arachidonic acid together with a lower proportion of palmitic acid in cholesteryl esters; b) an increase of stearic and arachidonic acids and a decrease of palmitic acid in triglycerides; and c) a higher level of palmitic and oleic acids associated with a lower percentage of stearic and C₂₂ polyunsaturated acids in phosphatidylcholine.     

Phospholipid exchange between subcellular organelles of rabbit lung

A soluble protein fraction (PLEP) prepared from rabbit lung can catalyze the exchange of phospholipids between subcellular organelles of the lung and between these subcellular organelles and synthetic liposomes. Phospholipid exchange between microsomes and synthetic liposomes and between mitochondria and synthetic liposomes was stimulated 8-fold and 2.5-fold, respectively, in the presence of the protein fraction. Lung exchange protein could also catalyze phospholipid exchange between subcellular organelles of the liver and synthetic liposomes. Phospholipid transfer between microsomes and lamellar bodies of the lung was stimulated 2-fold by the exchange protein. Both radiolabeled phosphatidylcholine (PC) and phosphatidylinositol (PI) were transferred from³²P-labeled microsomes to lamellar bodies, but the exchange protein exhibited no transfer activity for phosphatidylglycerol (PG) and that for phosphatidylethanolamine (PE) was insignificant compared to the transfer activity for phosphatidylcholine and phosphatidylinositol. While the physiological role of the phospholipid exchange proteins in the lung is unknown, it is possible that they participate in the distribution of the newly synthesized phospholipids from the site of synthesis to lamellar bodies and other membrane compartments of cells.     

Different thermal behavior of neonatal hepatic and cerebral 3-hydroxy-3-methylglutaryl-CoA reductase

The Arrhenius plots of hepatic and cerebral 3-hydroxy-3-methylglutaryl-CoA reductase activity were studied in neonatal chicks fed with a standard diet. Supplementation of the diet with 2% cholesterol from hatching has no effect on the thermal characteristics of the brain enzyme. The Arrhenius plot of brain reductase was practically similar to that found in control chicks. However, hepatic reductase was inhibited by cholesterol feeding. Dietary cholesterol increased the cholesterol/lipidic phosphorus molar ratio in liver microsomes, whereas no significant differences were observed in brain microsomes. These results are in agreement with the hypothesis that activity of hepatic reductase is regulated by the fluidity of microsomal membrane and show that cholesterol feeding does not alter the fluidity of microsomal membranes in neonatal chick brain having, thus, no effect on the thermal behavior of cerebral reductase.     

Analysis of the stearoyl-CoA desaturase system in the Morris hepatoma 7288C and 7288CTC

The microsomal stearoyl-CoA desaturase system was examined in both the Morris hepatoma 7288CTC, maintained in the host Buffalo strain rat, and the Morris hepatoma 7288C, maintained in tissue culture. In vitro examination shows the stearoyl-CoA desaturase system to be similar in the 2 tissues. Both show extremely low overall stearoyl-CoA desaturase activity, having 4% and 8% of normal liver values respectively. Examination of the electron transport system showed both tissues have decreased electron transport components cytochrome b₅ and cytochrome b₅ reductase. Particularly noticeable were the extremely low levels of cytochrome b₅ (2% compared with normal liver). Microsomes from both tissues showed a decreased ability to reduce an artificial electron acceptor, cytochrome c. With the low levels of cytochrome b₅ observed in these tissues, the low levels of overall desaturase activity may be caused by lack of terminal enzyme, lack of sufficient cytochrome b₅, or both. Analysis of the stearoyl-CoA desaturase system in cultured hepatoma cells suggests that these cells are similar to the host-grown tumor in this respect and may be used as a model in further examinations of the stearoyl-CoA desaturase system.