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At least 19 recordsLinked to original sources
PubMed1978

Composition of proteins of mesenteric lymph chylomicrons in the rat and alterations produced upon exposure of chylomicrons to blood serum and serum proteins.

Protein composition was determined in mesenteric lymph chylomicrons from fat-fed rats. Among the proteins of intermediate molecular weight, apoproteins A-I and the arginine-rich apoprotein accounted for 31% and 4% of the total protein mass, respectively. Apoprotein B and apoprotein A-IV each accounted for about 10% and proteins of low molecular weight (C apoproteins and apoprotein A-II) accounted for most of the remainder. Apoprotein A-I also accounted for more than 30% of the protein mass of mesenteric lymph lipoproteins of density less than 1.006 g/ml ("small chylomicrons") obtained from rats fed glucose. Aproprotein A-I was partially dissociated from chylomicrons during brief ultracentrifugation. Both the arginine-rich apoprotein and the C apoproteins in rat blood serum were transferred to lymph chylomicrons from fat-fed rats during incubation in vitro. Content of arginine-rich apoprotein, determined immunochemically, increased six-fold when chylomicrons were diluted to a final concentration of 500 mg/dl in blood serum. Upon incubation of chylomicrons in equivalent volumes of ultracentrifugal fractions of serum, the increase of the arginine-rich apoprotein was: very low density lipoproteins, 1.5-fold; high density lipoproteins, 1.8-fold; density fraction greater than 1.006 g/ml, 5.0-fold; density fraction greater than 1.21 g/ml, 11-fold. Content of apoprotein A-I, also determined immunochemically, was not altered appreciably by exposure to serum or its ultracentrifugal fractions, whereas content of C apoproteins, estimated from intensity of staining of the low molecular weight protein component in polyacrylamide gel electropherograms, increased in all cases except for the density fraction greater than 1.21 g/ml. The fractional content of apoprotein A-I in the protein of chylomicrons fell after incubation, whereas that of the arginine-rich apoprotein remained constant or rose substantially. The fractional content of apoprotein A-IV in chylomicron-protein tended to follow that of apoprotein A-I, as judged from polyacrylamide gel electropherograms. Transfer of the arginine-rich and C apoproteins to chylomicrons from blood serum was directly related to the volume of serum in which the chylomicrons were diluted and occurred rapidly at room temperature or at 4 degrees C.

Animals↗
PubMed1978

Comparison of the metabolism of chylomicrons and chylomicron remnants by the perfused liver.

1. The hepatic metabolism of chylomicrons and chylomicron remnants was compared after adding approximately equal numbers of each lipoprotein particle to the perfusate of isolated livers. 2. At least 40% of the added remnants were metabolized by the liver compared with less than 3% for chylomicrons. 3. There was significantly more net removal of labelled remnants than of chylomicrons by the liver. 4. A greater proportion of labelled cholesterol than of labelled triacylglycerol fatty acids was transferred to the liver from each lipoprotein. 5. Cholesteryl esters of remnants were hydrolysed to triacylglycerol fatty lipoprotein. 5. Cholesteryl esters of remnants were hydrolysed to triacylglycerol fatty acids of remnants were oxidized to CO2 more extensively than those of chylomicrons. 6. There was greater oxidation of remnant glycerolipic [(1(-14)C]oleate than of glycerolipid [1(-14)C]palmitate. 7. A large fraction of the fatty acids of remnants, but not of chylomicrons, was transferred to phospholipids, which were released by the liver in a lipoprotein of relative density less than 1.006. 8. Label from remnants, but not from chylomicrons, was found in lipoproteins of relative density greater than 1.006, which were not released during perfusion but could be flushed out from the liver at the end of perfusion.

Animals↗
PubMed1979

Quantitation of the transfer of surface phospholipid of chylomicrons to the high density lipoprotein fraction during the catabolism of chylomicrons in the rat.

Small chylomicrons (CM) labeled with cholesterol, cholesterol ester, phospholipid, and, in some cases, protein, were used to study the fate of these constituents as the CM are catabolized in the circulations of the hepatectomized and intact rat. In the hepatectomized animal after (1/2) h, CM are greatly reduced in volume, surface area, and diameter. During this period, the CM lost >92% of the mass of their triacylglycerol, >77% of the mass of their phospholipid, and >39% of their protein. Compared to the injected CM, the chemically altered particles, called CM "remnants," have a reduction in volume of 96% and in surface area of 88%. The labeled cholesterol esters remain with the CM remnants but, strikingly, a major fraction of the labeled phospholipids and labeled soluble apoproteins leave the CM and are found in the high density lipoprotein (HDL) fraction. The chemical composition of this HDL fraction contains relatively more phospholipid and less cholesterol ester than normal rat HDL. Because of the difference in composition of HDL between normal rats and those given CM, we estimate that the HDL phospholipid pool increased by congruent with25% by the infusion of congruent with 4-5 mg of CM phospholipid. Approximately 5 mg of phospholipid is secreted on CM by a fed rat in 1 h. The findings in hepatectomized rats indicate that a major fraction of the phospholipid and a minor fraction of the protein (soluble non-B apoproteins) of newly secreted CM are transferred from the CM to the HDL fraction during remnant formation. The same process probably occurs in intact rats except that the remnant particles are rapidly removed from the plasma by the liver and a smaller fraction of the surface of the CM enters the HDL fraction.

Animals↗
PubMed1979

Metabolic fate of chylomicron phospholipids and apoproteins in the rat.

To study the metabolic fate of chylomicron phospholipid and apoproteins, 15 mg of doubly labeled ([(3)H]leu, [(32)P]phospholipid) rat mesenteric lymph chylomicrons were injected as an intravenous bolus into conscious rats. The specific radioactivity, composition, pool size, and morphology of the plasma lipoproteins were determined after 2-60 min. After injection of chylomicrons, there was a rapid transfer of radioactivity into high density lipoproteins (HDL). At peak specific activity in HDL (2-5 min), 35% of injected apoprotein and 25% of phospholipid radioactivity were recovered in HDL (d 1.063-1.21 g/ml), with smaller recoveries in other lipoproteins and liver. There was an initial rapid rise of (32)P specific activity in HDL and d 1.02-1.063 lipoproteins (low density lipoproteins [LDL]), but whereas LDL specific activity subsequently converged with that of d < 1.02 lipoproteins, HDL specific activity decayed more rapidly than LDL or d < 1.02 lipoproteins. Lipolysis of chylomicrons was associated with a transfer of phospholipid mass into LDL and HDL. At 5 min, 80% of injected triglyceride had been lipolyzed and there was a significant increase in phospholipid mass in LDL and a smaller increase in HDL. At 10 min, the mass of phospholipid in LDL had returned towards control values, and there was a further increase in phospholipid mass in HDL, which suggested phospholipid transfer from LDL to HDL. In donor lymph chylomicrons (3)H-radioactivity was present in apoprotein (apo)B, apoA-I, and apoA-IV, but only radioactivity of apoA-I and apoA-IV were transferred to HDL. Transfer of radioactivity was associated with loss of mass of apoA-I and apoA-IV from the fraction that contained the chylomicron remnants (d < 1.02). With injection of 15 mg chylomicron, there was a small but insignificant increase in the relatively large pool of HDL apoA-I. However, 60 min after injection of 250 mg of human or rat intestinal chylomicrons into the rat, there was a significant increase in HDL apoA-I that resulted from acquisition of a major fraction of the chylomicron apoA-I. After injection of chylomicrons, phospholipid vesicles were observed by negative stain electron microscopy in the LDL and HDL ultracentrifugal fractions, especially in the LDL. Upon addition of an osmotically active compound, cellobiose, vesicles were observed as flattened particles with a double lipid bilayer thickness ( congruent with 100 A). To validate further the identity of these particles, chylomicrons were injected into rats with [(3)H]glucose, and the recipient rats' plasma was fractionated by chromatography on 6% agarose. Trapping of [(3)H]glucose occurred in the void and LDL regions of the column, and vesicular particles were identified in these column fractions by negative stain electron microscopy. Catabolism of chylomicrons is associated with a rapid transfer of phospholipid, apoA-I, and possibly apoA-IV into HDL. Chylomicron phospholipid appears to give rise to vesicles which are probably incorporated into preexisting HDL. Chylomicron surface components may be an important source of plasma HDL.

Animals↗
PubMed1977

The origin of chylomicron phosphatidylcholine in the rat.

This study investigates the pathways of origin of chylomicron phosphatidylcholine (PC) using a lymph- and bile-fistulated rat infused with a stabilized triolein emulsion. [(14)C-glycerol]PC was used to evaluate chylomicron PC generated by lyso PC acyltransferase. The percentage of chylomicron PC derived from the PC infused was directly proportional to the PC concentration in the infusate. When the infusate PC concentration was 10 mM, essentially all the chylomicron PC was derived therefrom at 4-6 h of infusion. Incorporation of the radiolabel was not found to be as great in the lymph subnatant PC as in chylomicron PC, suggesting that chylomicron and lymph subnatant PC might be supplied from different PC precursor pools.(32)P(i) was infused into similarly prepared rats to judge chylomicron PC synthesized from de novo sources. In these experiments it was found that the percentage of chylomicron PC derived from de novo synthesis was inversely related to the PC concentration of the infusate. This suggests that exogenously infused PC inhibits de novo PC synthesis. When [(32)P]rat bile PC was infused with [(14)C-glycerol]potato PC, the bile PC was preferred as a chylomicron precursor despite the greater similarity of the saturated fatty acids of potato PC to those of chylomicron PC. When the saturated fatty acids of bile and chylomicron PC were compared, chylomicron PC was significantly richer in stearate, suggesting extensive enterocyte modification of the saturated fatty acids of bile PC.

Animals↗
PubMed1978

Chylomicron apoprotein alteration after plasma exposure.

Purified rat lymph chylomicrons were incubated with chylomicron-free rat plasma and examined for changes in lipid and apoprotein constituents. Upon incubation there was a five-fold increase in the arginine rich apoprotein and a concomitant reduction in chylomicron Apo A-I to less than one-sixth its preincubation mass. These apoprotein changes were most faithfully reproduced when chylomicrons were incubated with the rat HDL fraction, although incubations of chylomicrons with rat lipoprotein-free plasma showed that arginine-rich apoprotein could readily associate with chylomicrons without concomitant changes in chylomicron lipid constituents. The gain in chylomicron apoprotein paralleled an increased affinity of the incubated chylomicron for heparin, when examined by heparin affinity chromatography. The apoprotein alterations were consistent in incubations in which the triglyceride concentrations varied from 330 mg/dl to 4200 mg/dl, and were not affected by inhibition of the Lecithin:Cholesterol Acyl Transferase (LCAT) reaction in the incubation mixture. The demonstration that in vivo alimentary lipemia chylomicrons have an apoprotein pattern identical to that of chylomicrons following in vitro plasma incubation suggests that these apoprotein alterations occur physiologically in alimentary lipemia.

Animals↗
PubMed1976

Scanning electron microscopic study of chylomicrons incubated with lipoprotein lipase.

The effects of lipolysis on the structure of chylomicrons were studied with the scanning electron microscope using rat chylomicrons incubated with purified bovine milk lipoprotein lipase for 20 minutes at pH 8.1. Since the amount of albumin added to the medium was limited, some of the free fatty acids and partial glycerides formed by lipolysis accumulated in the chylomicrons. Lipolyzed chylomicrons fixed with OSO4 at pH 7.4 appeared in scanning electron micrographs as spheres with multiply idented irregular surfaces, while those fixed at pH 5.5, AS WELL AS CONTROL CHYLOMICRONS FIXED AT BOTH PHs, appeared as spheres with smooth surfaces. Sections of OSO4-fixed specimens, viewed with the transmission electron microscope, showed that the core of lipolyzed chylomicrons fixed at pH 7.4 contained numerous circular electron-lucent areas at the periphery, thus accounting for the indented surfaces observed above, while the core surfaces of the other specimens were circular and smooth. These findings confirm an earlier report that aqueous spaces form in chylomicrons during lipolysis when albumin in the medium is limited, and that the aqueous spaces disappear when specimens are prepared at pH 5.5 for microscopy. Thin sections of specimens that had been prepared for scanning electron microscopy showed that the gold-palladium coating was desposited directly on the indented surface of the lipid core of lipolyzed chylomicrons fixed at pH 7.4. It is concluded that vacuum dehydration during specimen preparation ruptures the outer wall of the aqueous spaces in lipolyzed chylomicrons and thereby exposes the interior of the spaces to gold-palladium coating and viewing with the scanning electron microscope.

Animals↗
PubMed1979

Characteristics of chylomicron binding and lipid uptake by endothelial cells in culture.

Bovine vascular endothelial cells bind chylomicrons via a high affinity membrane receptor site. Subsequent to binding, the chylomicron apoprotein was neither internalized nor degraded by either sparse or confluent (contact-inhibited) cells. However, the adsorption of chylomicrons was associated with interiorization of chylomicron cholesteryl ester and triglyceride and the hydrolysis of these lipids to free cholesterol and unesterified fatty acids by a lysosome-dependent pathway. This pathway was active in both subconfluent and contact-inhibited cells. The chylomicron free cholesterol so produced inhibited endogeneous cholesterol synthesis measured in terms of the incorporation of [1-14C]-acetate into sterol. An excess of high density lipoprotein was 2- to 3-fold more effective in reducing both binding of chylomicrons and interiorization of chylomicron lipid than was low density lipoprotein. Chylomicron binding was not "down-regulated" by preincubation of the cells with low density lipoprotein or chylomicrons. The results are discussed in the context of cholesterol sources for contact-inhibited endothelial cells which do not interiorize low density lipoprotein cholesterol.

Animals↗
PubMedDate not supplied

Chylomicron formation and composition in unanaesthetised rabbits.

Emulsified lipid was infused steadily into the upper small intestine of unanaesthetised rabbits for 6 h and 24 h periods. Lymph was collected from a thoracic duct cannula, the output of infused lipids was measured and the lymph chylomicrons were isolated. Recovery of infused triacylglycerol was 63 +/- 6.4% and recovery of infused radioactive cholesterol was 24 +/- 4.2% during a 24 h period. There was considerable dilution of radioactive exogenous cholesterol with endogenous cholesterol. Provided that absorption was well-established most exogenous lipid was present in the lymph in the form of chylomicrons, and there was a close relationship between lipid content of the lymph and the presence of chylomicrons. During absorption of fats of differing fatty acid composition chylomicrons remained the predominant transport form in the lymph. Chylomicrons obtained during absorption of several fats have been analysed in detail. The protein, phospholipid, free and esterified cholesterol content of these chylomicrons varies within narrow limits. The fatty acid composition of chylomicron triacylglycerols reflects the type of fat in the test meal but the composition of chylomicron phospholipids and cholesteryl esters shows clear discrimination. During absorption of coconut oil there is a strong negative discrimination towards lauric acid both for cholesteryl esters and for phospholipids.

Animals↗
PubMed1976

Chylomicron clearance in normal and hyperlipidemic man.

A method has been developed for measurement of fractional clearance rates of chylomicrons in man. The technique employs constant infusion of emulsified fat into the duodenum at a rate of 200 mg/kh/hr. After 5 hr of infusion, concentrations of triglycerides (TG) in the chylomicron fraction become constant for the subsequent 5 hr. Since the input of chylomicron-TG is known, fractional removal rates can be calculated from steady-state plasma levels. In 21 patients with normal TG levels, clearance rates for chylomicrons were extremely rapid (t1/2 for chylomicron-TG equals 4.5 +/- 2.9 (SD) min). In 30 patients with endogenous hypertriglyceridemia, clearance was generally prolonged (t1/2 equals 23 +/- 5.5 min). This delay in chylomicron clearance could have been due either to a defect in removal of all TG-rich lipoproteinarticles; a generalized defect in clearance capacity for plasma TG was apparently ruled out for most patients by the further observation that reduction of endogenous TG by caloric restriction caused chylomicron removal to return to normal. These studies also showed that endogenous-TG is removed much less efficiently than chylomicron-TG, and in some patients, this discrepancy is particularly marked.

Adult↗
PubMed1977

Electron microscopic studies of the assembly, intracellular transport, and secretion of chylomicrons by rat intestine.

A detailed ultrastructural investigation of the assembly, intracellular transport, and secretion of chylomicrons by rat proximal jejunal intestinal cells was performed in rats fed corn oil. Following fat feeding the smooth endoplasmic reticulum of the absorptive cells becomes laden with triglyceride droplets which are transported through channels of the endoplasmic reticulum to the Golgi apparatus. The Golgi zones become extremely prominent due to the accumulation of osmiophilic droplets, similar in size and configuration to chylomicrons, within proliferated Golgi vesicles. Golgi-derived secretory vesicles, containing nascent chylomicrons, migrate towards the lateral cell membrane. The secretory vesicle membranes fuse with the lateral plasmalemma and nascent chylomicrons are then discharged into the intercellular spaces. Alterations of specific domains of the secretory vesicles were prominent, appearing as coated pits. Coated pits were apparent in the lateral plasmalemma in areas of active chylomicron exocytosis suggesting their derivation from secretory vesicle-membrane fusion. Chylomicrons, within the intercellular spaces, pass through the basement membrane that lines the basal surfaces of the epithelial cells, traverse the cellular elements of the lamina propria, and finally gain access to the lymphatics entering these channels through gaps between adjacent endothelial cells. These observations indicate that nascent chylomicrons accumulate within Golgi vesicles as a pre-requisite to secretion and that secretion occurs by exocytosis resulting in the release of nascent chylomicrons from secretory vesicles.

Animals↗
PubMed1977

Effects of chronic ethanol consumption on the catabolism of chylomicron triacylglycerol and cholesteryl ester in the rat.

Rats were fed for 24 days a liquid diet with ethanol as 36% of calories to produce hyperlipemia and hepatic steatosis. The catabolism of chylomicrons doubly-labeled in the triacylglycerol and cholesteryl ester moieties was studied in conscious rats after ingestion of their usual liquid diets with or without ethanol. A constant intravenous infusion of chylomicrons revealed a defect in chylomicron catabolism after chronic treatment with ethanol. The plasma clearance of chylomicron cholesteryl ester was impaired to a greater extent than clearance of chylomicron triacylglycerol. These findings are consistent with defective catabolism of chylomicron remnants, and suggest that the accumulation of chylomicron remnants in the plasma contributes to the development of increased post-prandial hyperlipemia and chronic hyperlipemia in association with excessive ethanol consumption.

Animals↗
PubMed1976

Retention of lipolytic products in chylomicrons incubated with lipoprotein lipase: electron microscope study.

Early effects of lipolysis on the structure of chylomicrons in vitro were studied in rat chylomicrons incubated with purified bovine mild lipoprotein lipase at pH 8.1. The amount of the albumin added to the incubation medium was limited so that free fatty acids (FFA) and partial glycerides formed during lipolysis would accumulate in the chylomicrons. The structures visualized in lipolyzed chylomicrons was found to be affected by pH during preparation of specimens for microscopy, whether fixed with OsO4 and sectioned, or stained with sodium phosphotungstate and examined as whole mounts. Circular aqueous spaces were present in the triglyceride core of lipolyzed chylomicrons processed at pH 8.1 and 7.4. Sometimes the spaces contained aggregates of osmiophilic material and whorls of bilayered lamellae. The spaces were replaced by lamellar structures having a periodicity of 40 A, in chylomicrons processed at pH 5.5, and the spaces and lamellae were both absent at pH 3.0. The findings indicate that these spaces were lined by a lipid monolayer which formed bilayered lamellae under certain conditions. It is concluded that the monolayer lining the aqueous spaces is an inward extension of the chylomicron surface film produced by the accumulation and movement of lipolytic products, FFA and partial glycerides, in the interfacial plane between core triglyceride and water.

Animals↗
PubMed1978

Metabolism of cholesterol-enriched chylomicrons. Catabolism of triglyceride by lipoprotein lipase of perfused heart and adipose tissues.

The chemical and biochemical properties of cholesterol-enriched and cholesterol-poor chylomicrons from rat lymph have been compared. The enriched particles, prepared from cholesterol-containing lipid dispersions, passed into the duodenum, had four to ten times the cholesteryl ester content of the control chylomicrons but had the same content of total "core" (cholesteryl ester + triglyceride) lipid. Both chylomicron species had the same protein composition, the same phospholipid composition, and the same composition of triglyceride fatty acids. The rate of hydrolysis of chylomicron triglyceride for enriched and control particles was determined using both soluble and membrane-supported lipoprotein lipase (LPL) species from heart and adipose tissues. The lipase that was functional in the isolated perfused heart showed no significant difference in initial catabolic rate with cholesterol-enriched and control chylomicrons. The same result was obtained with this isolated LPL species in vitro. The lipase that was functional in isolated perfused epididymal adipose tissue showed a slightly lower catabolic rate with cholesterol-enriched particles (84% of that obtained with control chylomicrons). The same result was obtained with isolated adipose tissue LPL. It is concluded that cholesteryl ester content of chylomicrons under these conditions neither affects their protein composition nor has a major effect on their rate of reaction with lipoprotein lipase.

Adipose Tissue↗