Effect of puromycin on protein and glycerolipid biosynthesis in isolated mucosal cells

The effect of puromycin on phosphatidylcholine and triacylglycerol synthesis was studied in isolated cells of rat intestinal mucosa using radioactive palmitate, glycerol, 2-hexadecylglycerol, and lysophosphatidylcholine as markers. Puromycin caused a 60–65% inhibition of phosphatidylcholine biosynthesis but did not affect the formation of triacylglycerols. Under comparable conditions protein synthesis was inhibited 90–95% and glycoprotein synthesis 60–70%. The utilization of the various lipid precursors indicated that puromycin inhibited the biosynthesis of phosphatidylcholine via both the CDP-choline and the lysophosphatidylcholine pathways, without interfering with triacylglycerol synthesis from either phosphatidic acid or monoacylglycerol precursors. Since both phosphatidylcholines and proteins are involved in the assembly of chylomicrons, it is suggested that the effect of puromycin on chylomicron formation could be due to an inhibition of the biosynthesis of any one or all three of the membrane components: proteins, glycoproteins, and phosphatidylcholines.     

Role of an acidic compartment in synthesis of disaturated phosphatidylcholine by rat granular pneumocytes

A possible role for an acidic subcellular compartment in biosynthesis of lung surfactant phospholipids was evaluated with granular pneumocytes in primary culture. Incubation with chloroquine (100μm) was used to perturb this compartment. With control cells, incorporation of [9,10-³H]palmitic acid into total lipids and into total phosphatidylcholines increased linearly with time up to 4h. Total incorporation into phosphatidylcholine during a 1h incubation was 999+85pmol of [9,10-³H]palmitic acid, 458±18pmol of [1-¹⁴C]oleic acid and 252±15pmol of [U-¹⁴C]glucose per μg of phosphatidylcholine phosphorus. The cellular content of either disaturated phosphatidylcholine or total phosphatidylcholines did not change during a 2h incubation with chloroquine. In the presence of chloroquine, the specific radioactivity of [³H]palmitic acid in disaturated phosphatidylcholine increased by 40%, and that of disaturated-phosphatidylcholine fatty acids from [U-¹⁴C]glucose increased by 125%. Incorporation of [1-¹⁴C]oleic acid into phosphatidylcholine was decreased by chloroquine by 79% and 33% in the presence or absence of palmitic acid respectively. Chloroquine stimulated phospholipase activity in intact cells, and in sonicated cells at pH4.0, but not at pH8.5. The observations indicate that chloroquine stimulates synthesis of disaturated phosphatidylcholine in granular pneumocytes from fatty acids, both exogenous and synthesized de novo, which can be due to stimulation of acidic phospholipase. This stimulation of acidic phospholipase A activity by chloroquine appears to be coupled to the synthesis of disaturated phosphatidylcholine, thereby enhancing remodelling of phosphatidylcholine synthesized de novo. Our findings, therefore, implicate the involvement of an acidic subcellular compartment in the remodelling pathway of disaturated phosphatidylcholine synthesis by granular pneumocytes.     

On the mechanism of the phospholipase C-mediated attenuation of cardiolipin biosynthesis in H9c2 cardiac myoblast cells

The effect of phospholipase C treatment on cardiolipin biosynthesis was investigated in intact H9c2 cardiac myoblasts. Treatment of cells with phosphatidylcholine-specific Clostridium welchii phospholipase C reduced the pool size of phosphatidylcholine compared with controls whereas the pool size of cardiolipin and phosphatidylglycerol were unaffected. Pulse labeling experiments with [1,3-³H]glycerol and pulse-chase labeling experiments with [1,3-³H]glycerol were performed in cells incubated or pre-incubated in the absence or presence of phospholipase C. In all experiments, radioactivity incorporated into cardiolipin and phosphatidylglycerol were reduced in phospholipase C-treated cells with time compared with controls indicating attenuated de novo biosynthesis of these phospholipids. Addition of 1,2-dioctanoyl-sn-glycerol, a cell permeable 1,2-diacyl-sn-glycerol analog, to cells mimicked the inhibitory effect of phospholipase C on cardiolipin and phosphatidylglycerol biosynthesis from [1,3-³H]glycerol indicating the involvement of 1,2-diacyl-sn-glycerol. The mechanism for the reduction in cardiolipin and phosphatidylglycerol biosynthesis in phospholipase C-treated cells appeared to be a decrease in the activities of phosphatidic acid:cytidine-5triphosphate cytidylyltransferase and phosphatidylglycerolphosphate synthase, mediated by elevated 1,2-diacyl-sn-glycerol levels. Upon removal of phospholipase C from the incubation medium, phosphatidylcholine biosynthesis from [methyl-³H]choline was markedly stimulated. These data suggest that de novo phosphatidylglycerol and cardiolipin biosynthesis may be regulated by 1,2-diacyl-sn-glycerol and support the notion that phosphatidylglycerol and cardiolipin biosynthesis may be coordinated with phosphatidylcholine biosynthesis in H9c2 cardiac myoblast cells.     

Glutamate and CO2 production from glutamine in incubated enterocytes of adult and very old rats

Glutamine is the major fuel for enterocytes and promotes the growth of intestinal mucosa. Although oral glutamine exerts a positive effect on intestinal villus height in very old rats, how glutamine is used by enterocytes is unclear. Adult (8 months) and very old (27 months) female rats were exposed to intermittent glutamine supplementation for 50% of their age lifetime. Treated rats received glutamine added to their drinking water, and control rats received water alone. Jejunal epithelial cells (~ 300×10⁶ cells) were incubated in oxygenated Krebs–Henseleit buffer for 30 min containing [1-¹³C] glutamine (~ 17 M) for analysis of glutamine metabolites by ¹³C nuclear magnetic resonance (¹³C NMR). An aliquot fraction was incubated in the presence of [U-¹⁴C] glutamine to measure produced CO₂. Glutamine pretreatment increased glutamate production and decreased CO₂ production in very old rats. The ratio CO₂/glutamate, which was very high in control very old rats, was similar at both ages after glutamine pretreatment, as if enterocytes from very old rats recovered the metabolic abilities of enterocytes from adult rats. Our results suggest that long-term treatment with glutamine started before advanced age (a) prevented the loss of rat body weight without limiting sarcopenia and (b) had a beneficial effect on enterocytes from very old rats probably by favoring the role of glutamate as a precursor for glutathione, arginine and proline biosynthesis, which was not detected in ¹³C NMR spectra in our experimental conditions.     

Role of endogenous and exogenous cholesterol in liver as the precursor for bile acids in rats

There is considerable evidence suggesting that compartmentalized functional pools of cholesterol in the liver contribute differently to the formation of bile acids as the precursor. The present paper deals with the incorporation of [1-¹⁴C]acetate and of [1,2-³H]cholesterol carried on lipoproteins (LDL and HDL) into biliary bile acids in perfused rat livers and bile-fistula rats. The results showed that endogenous cholesterol synthesized newly from [1-¹⁴C]acetate in the liver was incorporated into both cholic acid and chenodeoxycholic acid in a similar way, while exogenous lipoprotein-[1,2-³H]cholesterol delivered to hepatocytes from hepatic circulation was incorporated into chenodeoxycholic acid at a higher rate.     

Intrahepatic uptake and processing of intravenously injected small unilamellar phospholipid vesicles in rats

Small unilamellar vesicles consisting of sphingomyelin, cholesterol and phosphatidylserine in a molar ratio of 4:5:1 containing [³H]inulin as a marker of the aqueous space or [Me-¹⁴C]choline-labeled sphingomyelin as a marker of the lipid phase were injected intravenously into rats. After separation of the non-parenchymal cells into a Kupffer cell fraction and an endothelial cell fraction by elutriation centrifugation analysis of the radioactivity contents demonstrated that Kupffer cells were actively involved in the uptake of the vesicles whereas endothelial cells did not contribute at all. Uptake by total parenchymal cells was also substantial but, on a per cell base, significantly lower than that by the Kupffer cells. By comparising the fate of the [³H]inulin label and the [¹⁴C]sphingomyelin label it was concluded that release of liposomal lipid degradation products especially occurred from Kupffer cells rather than from parenchymal cells. In both cell types, however, substantial proportions of the ¹⁴C-label accumulated in the phosphatidylcholine fraction, indicating intracellular degradation of sphingomyelin and subsequent phosphatidylcholine synthesis. Treatment of the animals with the lysosomotropic agent chloroquine prior to liposome injection effectively blocked the conversion of the choline-labeled sphingomyelin into phosphatidylcholine in both cell types. This observation indicates that uptake of the vesicles occurred by way of an endocytic mechanism.     

Properties of freshly isolated type II alveolar epithelial cells

The biochemical characteristics of type II alveolar epithelial cells dissociated from adult rabbit lung by instillation of low concentrations of an elastase trypsin mixture are reported. Cells studied immediately (within 4 h) after isolation were found to incorporate the radioactively labelled precursors [U-¹⁴C]glucose, [methyl-³H]choline and [³H]palmitate into cellular phosphatidylcholine at rates 2–10-fold higher than previously reported for cells not subject to short-term cell culture. Secretion of phosphatidylcholine was stimulated by beta-adrenergic agonists. Measurement of specific activities of enzymes of phospholipid biosynthesis in subcellular fractions of isolated lung cells showed a significant enrichment of acyl coenzyme A-lysophosphatidylcholine acyltransferase, an enzyme believed to be involved in pulmonary surfactant phosphatidylcholine remodeling, in the endoplasmic reticulum of type II cells. These observations support the utility of freshly isolated type II cells as a model system for the study of the functions of the alveolar epithelium.     

Glycerolipid biosynthesis in isolated rat intestinal epithelial cells.

Intestinal epithelial cells were prepared from fasted rats by dispersion with collagenase (EC 3.4.24.3). The structural and metabolic integrity of the cells was verified by electron microscopy, a high percentage of Trypan Blue exclusion, a low degree of release of lactate dehydrogenase (EC 1.1.1.27) in the medium, and by the retention of sensitivity to agents known to modify metabolic and transport activity in everted sacs of intestinal mucosa. The isolated intestinal epithelial cells were used to study glycerolipid biosynthesis from glucose, glycerol, 2-monoacylglycerol, and free fatty acids. The cells actively incorporated the labeled precursors into glycerolipids without specific cofactor requirements. Addition of fatty acids stimulated the incorporation of both glucose and glycerol into triacylglycerols and glycerophospholipids, the greatest effect being observed with palmitate. The stimulation of monoacylglycerol acylation appeared to depend on both the nature of the monoacylglycerol and fatty acid supplied. Stereospecific analyses of the diacylglycerols formed from 2-monoacylglycerols and free fatty acids showed that 1,2-diacyl-sn-glycerols (62-70%) were the major and that 2,3-diacyl-sn-glycerols (30-38%) the minor intermediates in triacylglycerol biosynthesis. The data indicate that isolated intestinal epithelial cells exhibit a total capacity of glycerolipid synthesis and a stereochemical course of reaction which is comparable to that observed for triacylglycerol formation in everted sacs of intestinal mucosa, but much less specific than that seen in microsomal preparations of intestinal mucosa.     

Lymphatic absorption and transport of retinol and vitamin D-3 from rat intestine Evidence for different pathways

The lymphatic absorption and transport of retinol and vitamin D-3 from rat intestine has been studied. When rats were cannulated in the intestinal lymph duct and given an intraduodenal bolus of [³H]retinol and ¹⁴C-labelled vitamin D-3, ¹⁴C-labeled vitamin D-3 appeared later in the intestinal lymph than [³H]retinol and the rate of absorption of vitamin D-3 was still maximal at a time when that of retinol had declined. Both vitamins were absorbed via the lymphatic route in association with chylomicrons. Almost all the retinol was esterified, while vitamin D-3 appeared in the chylomicrons as free vitamin D-3. In vitro incubations and in vivo studies using hepatectomized and normal rats showed that the retinyl ester was a relatively nonexchangeable component of the chylomicrons and their remnants. Hence, all the vitamin A followed the remnants in their clearance from plasma. In contrast, significant amounts of vitamin D-3 were transferred from the chylomicrons to other plasma fractions. Therefore, only a fraction of this vitamin may be removed in association with the chylomicron remnants.     

Coupling of ethanol metabolism to lipid biosynthesis: labelling of the glycerol moieties of sn-glycerol-3-phosphate,a phosphatidic acid and a phosphatidylcholine in liver of rats given [1,1-2H2]ethanol

The mechanism behind ethanol-induced fatty liver was investigated by administration of [1,1-²H₂]ethanol to rats and analysis of intermediates in lipid biosynthesis. Phosphatidic acid and phosphatidylcholine were isolated by chromatography on a lipophilic anion exchanger and molecular species were isolated by high-performance liquid chromatography in a non-aqueous system. The glycerol moieties of palmitoyl-linoleoylphosphatidic acid, the corresponding phosphatidylcholine and free sn-glycerol-3-phosphate were analysed by GC/MS of methyl ester t-butyldimethylsilyl derivatives. The deuterium labelling in the glycerol moiety of the phosphatidic acid was 2–3-times higher than in free sn-glycerol-3-phosphate, indicating that a specific pool of sn-glycerol-3-phosphate was used for the synthesis of phosphatidic acid in liver. The results indicate that NADH formed during ethanol oxidation is used in the formation of a pool of sn-glycerol-3-phosphate that gives rise to triacylglycerol and possibly fatty liver.     

Substrate utilization for energy production and lipid synthesis in oligodendrocyte-enriched cultures prepared from rat brain

The metabolism of oligodendrocytes has been studied using cultures of oligodendrocyte-enriched glial cells isolated from cerebra of 5–8-day old rats. Cultures containing 60–80% oligodendrocytes were incubated for 16h with [3-¹⁴C]acetoacetate, d-[3-¹⁴C]3-hydroxybutyrate, [U-¹⁴C]glucose, l-[U-¹⁴C]glutamine and [1-¹⁴C]pyruvate or [2-¹⁴C]pyruvate in the presence or absence of other oxidizable substrates. Labelled CO₂ was collected as an index of oxidative metabolism and the incorporation of label into total lipids, fatty acids and cholesterol was used as an index of the de novo synthesis of lipids. Glucose, acetoacetate, D-3-hydroxybutyrate, pyruvate and l-lactate were measured to determine substrate utilization and product formation under various conditions. Our results indicate that glucose is rapidly converted to lactate and is a relatively poor substrate for oxidative metabolism and lipid synthesis. Ketone bodies were used as an energy source and as precursors for the synthesis of fatty acids and cholesterol. Preferential incorporation of acetoacetate into cholesterol was not observed. Exogenous pyruvate was incorporated into both the glycerol skeleton of complex lipids and into cholesterol and fatty acids. l-Glutamine appeared to be an important substrate for the energy metabolism of these cells.     

RNA synthesis by villus and crypt cell nuclei of rat intestinal mucosa

Rat intestinal mucosa was separated by eversion and vibration to provide a sequence of fractions from predominantly villus cells to predominantly crypt cells. The proportions of these cell types in each fraction were computed from the concentrations of alkaline phosphatase (villus cells) and thymidine kinase (crypt cells) in each population. The isolated mucosal fractions varied from about 90% villus cells to 90% crypt cells. Following injection of the rats with [³H]thymidine, the nuclei were isolated from each mucosal cell fraction and the amount of radioactivity incorporated into DNA was measured as an index of crypt cell abundance. The isolated nuclei were also incubated with ribonucleoside triphosphates and the amount of RNA synthesized was measured. Nuclei labeled with [³H]thymidine were found only in fractions rich in crypt cells, whereas capacity for RNA synthesis remained very active in mucosal fractions consisting predominantly of villus cells. It is concluded that non-dividing villus cells continue to make RNA.     

Further studies on the mechanism of inhibition of intestinal chylomicron transport by Pluronic L-81

This study explored further the hypothesis that intestinal cells have two pathways for producing large triacylglycerol-rich lipoprotein particles. The hydrophobic surfactant Pluronic L-81 (L-81) inhibits formation of chylomicrons (containing triacylglycerol synthesized from dietary fatty acids and monoacylglycerol, through the monoacylglycerol pathway), but not formation of very-low-density lipoproteins. L-81 does not inhibit lymphatic lipid transport during infusion of egg phosphatidylcholine, whose fatty acid is processed through the alpha-glycerol phosphate pathway and is transported in lymph in very-low-density lipoproteins. Thus, the first part of this study tested whether L-81 cannot inhibit the alpha-glycerol phosphate pathway, and thus L-81 can only affect chylomicron lipid secretion. Intestinal lymph fistula rats were infused with a lipid emulsion containing [1-14C]oleic acid, but no monoacylglycerol, to ensure that the oleic acid will be channeled to the alpha-glycerol phosphate pathway. Experimental rats received 1 mg/h of L-81 in their emulsion whereas control rats lacked L-81. Lymphatic triacylglycerol output, measured both chemically and radioactively, was markedly suppressed in the experimental rats as compared to the controls. Thus, these data indicate that the reason why lipid transport was unaffected by L-81 when egg phosphatidylcholine was infused was not because of the pathway used for the resynthesis of triacylglycerol from phosphatidylcholine. In the second part of this study, we measured the appearance time for chylomicron (in control rats) and for very-low-density lipoprotein (in L-81-treated rats). The appearance time is defined as the time between placement of radioactive fatty acid into the intestinal lumen and the appearance of radioactive lipid in the central lacteal. The average appearance time for the control rats was 10.8 min, which was significantly shorter than the 16.2 min in the L-81-treated experimental rats. This difference in appearance time further supports the hypothesis that chylomicron and very-low-density lipoprotein are packaged separately in the enterocytes and only the formation of chylomicron is inhibited by L-81.     

Intestinal apoB synthesis, lipids, and lipoproteins in chylomicron retention disease

Chylomicron retention disease is characterized by fat malabsorption, hypocholesterolemia, normal fasting triglycerides, and marked intestinal steatosis despite the presence of both plasma and intestinal apoprotein B. The defect remains unknown but presumably involves the synthesis or secretion of chylomicrons. The present investigation examines this hypothesis by studying the biosynthesis of chylomicrons in cultured jejunal explants and by defining the quantitative and qualitative abnormalities of plasma lipids and of circulating lipoproteins. Following 2-3 years of a low fat diet supplemented with medium chain triglycerides, six patients with chylomicron retention disease had significantly higher triglyceride (TG) levels coupled with a decrease in both free (FC) and esterified cholesterol (EC) as well as in essential fatty acids and phospholipids (PL) when compared to healthy controls. The low total plasma cholesterol was largely accounted for by low levels of both low density (LDL) and high density lipoprotein (HDL) cholesterol. VLDL and LDL were characterized by a diminished percentage of CE with an increase of TG while HDL contained relatively more FC as well as PL and less CE. The diameter of VLDL was larger whereas those of LDL and HDL were smaller than in normal controls. Jejunal explants, when incubated with [14C]palmitate, were capable of normal biosynthesis of TG, diglycerides, PL, and CE. These lipids, however, except for PL, were retained in the tissue and could not be secreted into the culture medium. Incubation of intestinal biopsies with [3H]leucine and [14C]mannose resulted in normal protein synthesis and reduced glycosylation. The presence of intestinal apoB-48 was confirmed by immunoblot using 2D8 antibodies. These data suggest that the intestinal defect in this disease results from a disorder of the final assembly of chylomicrons or in the mechanism of their exocytosis.     

7 Alpha-hydroxylation of cholesterol by reconstituted systems from rat liver microsomes

A reconstituted system from rat liver microsomes, consisting of partially purified fractions of cytochrome P-450 and NADPH-cytochrome P-450 reductase was shown to catalyze 7α-hydroxylation of cholesterol in the presence of NADPH and a synthetic phosphatidylcholine. The rate of 7α-hydroxylation of added [4-¹⁴C] cholesterol was linear with the concentration of cytochrome P-450 and increased with the concentration of NADPH-cytochrome P-450 reductase up to a certain level and then remained constant. Omission of phosphatidylcholine resulted only in a 20% decrease in cholesterol 7α-hydroxylase activity of the system. The rate of 7α-hydroxylation was 2–3 times higher in reconstituted systems with cytochrome P-450 from cholestyramine-treated rats than in those with cytochrome P-450 from untreated rats.     

Phosphatidylcholine Synthesis in Castor Bean Endosperm : Occurrence of an S-Adenosyl-l-Methionine:Ethanolamine N-Methyltransferase

The methylation steps in the biosynthesis of phosphatidylcholine by castor bean (Ricinus communis L.) endosperm have been studied by pulse-chase labeling. Endosperm halves were incubated with [methyl-¹⁴C]S-adenosyl-l-methionine, [2-¹⁴C]ethanolamine, [¹⁴C]ethanolamine phosphate, or [¹⁴C]serine phosphate. The kinetics of appearance were followed in the free, phospho-, and phosphatidyl-bases. The initial methylation utilized ethanolamine as a substrate to form methylethanolamine, which was then converted to dimethylethanolamine, choline, and phosphomethylethanolamine. Subsequent methylations occurred at the phospho-base and, to a lesser extent, the phosphatidyl-base levels, after which the radioactivity either remained constant or decreased in these compounds and accumulated in phosphatidylcholine. Although the precursors tested did support the synthesis of choline, the kinetics of the labeling make them unlikely to be the major sources of free choline to be utilized for the nucleotide pathway. A model with two pools of choline is proposed, and the implications of these results for the pathways leading to phosphatidylcholine biosynthesis are discussed.     

Fatty acids reverse the cyclic AMP inhibition of triacylglycerol and phosphatidylcholine synthesis in rat hepatocytes.

The influence of cyclic AMP analogues and fatty acids on glycerolipid biosynthesis in monolayer cultures of rat hepatocytes was investigated. Chlorophenylthio-cyclic AMP and adenosine 3':5'-cyclic phosphorothioate inhibited the rate of triacylglycerol synthesis from [1(3)-3H]glycerol, and phosphatidylcholine synthesis from [Me-3H]-choline. Supplementation of the hepatocytes with palmitate (1 mM) reversed chlorophenylthio-cyclic AMP inhibition of triacylglycerol synthesis. Similarly, cyclic AMP analogue-inhibition of phosphatidylcholine synthesis was abolished when the cells were simultaneously incubated with oleate (3 mM). Reactivation of phosphatidylcholine synthesis in chlorophenylthio-cyclic AMP-supplemented cells with oleate was accompanied by conversion of CTP: phosphocholine cytidylyltransferase into the membrane-bound form, since these cells released the enzyme more slowly after treatment with digitonin. The opposing actions of cyclic AMP and fatty acids are discussed in relation to the regulation of glycerolipid biosynthesis during starvation, diabetes and stress.     

Action de la flore microbienne du tractus digestif sur la biosynthese de l'acide cholique chez le rat

Axenic and holoxenic (conventional) rats were fed a diet containing trace amounts of [2,4-³H]cholic and [24-¹⁴C]chenodeoxycholic acids. In the feces of both groups of rats, the percentage of labelled bile acids which were ³H-labelled was slightly different. In the experimental conditions used, the intestinal microflora only slightly modified the synthesis of 12α-hydroxylated bile acids.     

Cholesterol synthesis in rat intestine: effect of fasting and of porphyrogenic chemical allylisopropylacetamide

(a) Administration of allylisopropylacetamide to fasting rats stimulates intestinal sterol synthesis as measured by incorporation of ¹⁴C from [1-¹⁴C]sodium acetate. Stimulatory effect of AIA is confined to the acetate to mevalonate segment of cholesterol biosynthetic pathway.(b) It is also shown that the suppression of sterol synthesis in the ileum of intact rats produced by fasting is of the same order of magnitude as that observed for liver sterol synthesis due to fasting.     

The involvement of parenchymal,Kupffer and endothelial liver cells in the hepatic uptake of intravenously injected liposomes effects of lanthanum and gadolinium salts

¹²⁵I-labeled albumin or poly(vinyl pyrrolidone) encapsulated in intermediate size multilamellar or unilamellar liposomes with 30–40% of cholesterol were injected intravenously into rats. In other experiments liposomes containing phosphatidyl[Me-¹⁴C]choline were injected. 1 h after injection parenchymal or non-parenchymal cells were isolated. Non-parenchymal cells were separated by elutriation centrifugation into a Kupffer cell fraction and an endothelial cell fraction. From the measurements of radioactivities in the various cell fractions it was concluded that the liposomes are almost exclusively taken up by the Kupffer cells. Endothelial cells did not contribute at all and hepatocytes only to a very low extent to total hepatic uptake of the ¹²⁵I-labels. Of the ¹⁴C-label, which orginates from the phosphatidylcholine moiety of the liposomes, much larger proportions were recovered in the hepatocytes. A time-dependence study suggested that besides the involvement of phosphatidylcholine exchange between liposomes and high density lipoprotein, a process of intercellular transfer of lipid label from Kupffer cells to the hepatocytes may be involved in this phenomenon. Lanthanum or gadolinium salts, which effectively block Kupffer cell activity, failed to accomplish an increase in the fraction of liposomal material recovered in the parenchymal cells. This is compatible with the notion that liposomes of the type used in these experiments have no, or at most very limited, access to the liver parenchyma following their intravenous administration to rats.