DDT absorption and chylomicron transport in rat

Male rats were fed 100 nM dichlorodiphenyltrichloroethane-¹⁴C in oil by gastric tube. Recovery of dichlorodiphenyltrichloroethane-¹⁴C in thoracic duct lymph was 60% in 12 hr. Lymph dichlorodiphenyltrichloroethane-¹⁴C (97%) occurred in lipoproteins of d<1.006, designated chylomicrons. Mechanical separation of chylomicron triglyceride core (labeled with triglyceride-³H) from chylomicron membrane (labeled with phospholipid-³²P) showed that 97% dichlorodiphenyltrichloroethane-¹⁴C was present in triglyceride core. To investigate possible association of plasma clearance of the two core lipids, rats were pulse injected with chylomicrons, doubly labeled with triglyceride-³H and dichlorodiphenyltrichloroethane-¹⁴C. The decay of dichlorodiphenyltrichloroethane-¹⁴C in sequential serum samples was rapid (T^1/2=2 min) and was independent of triglyceride-³H decay. In tissues removed 14 min after injection of chylomicrons, 30% administered dichlorodiphenyltrichloroethane-¹⁴C was found in liver but only 1% in adipose tissue. In hepatectomized (eviscerated) rats, the decay of serum dichlorodiphenyltrichloroethane-¹⁴C (T^1/2=10 min) was also independent of and more rapid than triglyceride-³H decay. With sucrose density gradients, it was shown that chylomicron dichlorodiphenyltrichloroethane-¹⁴C transferred to higher density serum proteins in vitro and in vivo and to bovine albumin in vitro. Thus, dichlorodiphenyltrichloroethane was transported from intestine largely in the triglyderide phase of chylomicrons; disappearance of chylomicron-dichlorodiphenyltrichloroethane from the systemic circulation was rapid and partly independent of the presence of the liver and of triglyceride hydrolysis; some dichlorodiphenyltrichloroethane was transported from serum chylomicrons to albumin or other plasma proteins before tissue uptake.     

The effect of dietary fats on the plasma lipid composition of sheep

This study reports on the plasma lipid compositions of sheep fed either a control diet (C), a control diet supplemented with tallow (A) or polyunsaturated fatty acid (B) that had been protected against hydrolysis and hydrogenation in the rumen, or a control diet supplemented with maize oil (D). Diet B considerably increased the 18∶2 content of all the major plasma lipid fractions. Although the feeding of diet D also resulted in an increase in the 18∶2 contents within the cholesteryl ester, unesterified fatty acid, and phospholipid fractions the increases were considerably less than those observed with diet B; the levels of 18∶2 within the triglyceride fraction remained similar to that for the sheep which received the control diet. The effect of feeding diet A was confined solely to the triglyceride fraction where the concentrations of 16∶0 and 18∶1 were increased. The lipoproteins of the plasma were separated into very low density lipoproteins (d<1.006), low density lipoproteins (1.006<d<1.063), and high density lipoproteins (1.063<d<1.21), and the distribution of the major lipids between these lipoprotein fractions was investigated. Diet B increased considerably the proportion of triglyceride found in association with the very low density fraction and the concentrations of 18∶2 within all the lipoprotein fractions; these increases in the concentrations of 18∶2 were not confined to any particular lipid fraction of the lipoproteins. In contrast, the increases in the concentrations of 18∶2 produced as a result of feeding diet D were confined to the low and high density lipoproteins. The effect of feeding diet A was confined to fatty acid changes within the triglycerides of the low and very low density lipoproteins.     

Transport of diacylalkylglycerols in chylomicrons and very low density lipoproteins of rat intestinal lymph following intragastric administration of 1,3-dioctadecenoyl-2-hexadecylglycerol

The triacylglycerol (TG) analog 1,3-dioctadecenoyl-2-hexadecyl glycerol was used in the study of the transport of dietary lipids by lipoprotein fractions of rat intestinal lymph. 1,3-Diacyl-2-alkyl glycerols (DAG) are hydrolyzed by pancreatic lipase to form 2-alkyl glycerols and free fatty acids. These hydrolysis products are then absorbed, and DAG are resynthesized within the intestinal mucosa. Intestinal lymph of rats was collected following intragastric administration of 1,3-dioctadecenoyl-2-hexadecyl glycerol. The DAG to TG ratios in very low density lipoprotein (VLDL) and chylomicron fractions were determined as a measure of the incorporation of lipid of dietary origin. The ratio of DAG to TG in the VLDL-2 (S_f 12–100) fraction ranged from0.06 to 0.56 indicating a significant amount of DAG transported relative to TG. The glyceryl ether to TG ratio increased with mean lipoprotein volume from the VLDL-2 fraction to the chylomicron (S_f>400) fraction. The correlation between glyceryl ether to TG ratio and average volume and between the amount of DAG per ml of original lymph and average volume within the chylomicron fraction was 0.99. Thus, the amount of dietary fat transported was correlated with the size of the chylomicrons produced. The glyceryl ether to TG ratio was positively correlated with the average volume of the lipoprotein fractions isolated (chylomicrons, chylomicron rich (S_f>100), VLDL-1 (S_f 100–400) and VLDL-2) (r=0.87). These results suggest that the size of the lipoproteins produced by the intestine is determined by the amount of fat available for transport and that particles of larger diameter are formed by the addition of lipid of dietary origin to existing VLDL. Scientific contribution no. 702, Agricultural Experiment Station, University of Connecticut, Storrs, Connecticut 06268     

Lipid profiles of plasma lipoproteins of fasted and fed normal and choline-deficient rats

Three major density classes of lipoproteins and a residual protein (d>1.21) were isolated by ultracentrifugation from plasma of fasted, fed normal, and choline-deficient rats. Lipid extracts were obtained from total plasma and the various density classes of lipoproteins, and each extract was examined in detail by thin layer and gas chromatographies. The results indicated essentially identical compositions of molecular species of phosphatidyl choline, which suggested their rapid equilibration among the different plasma lipoprotein classes. In contrast, the molecular species of the triacylglycerols and cholesteryl esters showed significant differences among the chylomicrons, very low and low, and high density lipoproteins, which excluded the possibility of their ready equilibration in vivo. Omission of choline from diet resulted in a sharp and statistically significant decrease in all lipid components of the very low and low density lipoproteins within 2 days. After 10 days of choline deficiency, the lipid levels of chylomicrons and very low and low density lipoproteins were ca. one-half the levels found in the choline supplemented animals, and there were discernible distortions in their lipid composition. Reintroduction of choline led to a prompt return to normal levels and lipid composition of both chylomicron and very low and low density lipoprotein fractions. The lack of equilibration of the triacylglycerols among the lipoprotein classes under normal conditions and in choline deficiency demonstrates an as yet unrecognized source of compartmentation of plasma lipids.     

pH gradient electrophoresis and isoelectric focusing of lipoproteins on agarose bead thin layers

A new method of isoelectric focusing (IEF) and pH gradient electrophoresis, using thin layers of agarose gel beads, was devised to investigate chylomicrons and very low density lipoproteins (VLDL). pH gradient stability and cathodal gradient drift were similar to those of polyacrylamide gel IEF, and linearity of gradients was maintained for 23 hr. Chylomicrons and VLDL were detectable without staining. Chylomicrons from human serum and from rat lymph migrated in this system. Rat lymph chylomicrons, obtained by ultracentrifugation, migrated in several discrete bands, and this heterogeneity of rat chylomicrons was confirmed by electron microscopic demonstration of chylomicrons in each band. This new technique has permitted the first measurement of isoelectric points of some lipoproteins in the ultracentrifuged fraction of human serum chylomicrons and the first separation of multiple discrete fractions of ultracentrifuged lymph chylomicrons. This work was presented, in part, at the annual 51st Scientific Sessions of the American Heart Association at Dallas, Texas in November 1978.     

Hydrocarbon transport in chylomicrons and high-density lipoproteins in rat

A lipoprotein system is described that transports gut hydrocarbons of low polarity in chylomicrons of intestinal lymph and plasma to plasma high density lipoproteins (HDL) in rat. Four highly lipophilic aryl and alkyl hydrocarbons [benzo(α)pyrene; 1,1,1-trichloro-2,2-bis(p-chlorophenol)ethane (DDT), hexadecane and octadecane] were selected to give a graded range of polarity. Chylomicrons were labeled doubly with radioisotopes in triacylglycerol and a single hydrocarbon by feeding [³H]-glycerol and [¹⁴C]hydrocarbon. All hydrocarbons were transported in the triacylglycerol oil phase of chylomicrons. Injected chylomicron triacylglycerol and 3 of 4 hydrocarbons were cleared simultaneously from plasma consistent with lipoprotein-lipase dependent hydrocarbon clearance but DDT was cleared more rapidly. HDL was the major plasma acceptor of all labeled hydrocarbons. Plasma chemical fluxes were measured for octadecane and DDT and both showed net fluxes from chylomicrons to HDL. HDL selectively concentrated chylomicron hydrocarbons from chylomicron triacylglycerol. Lipoprotein lipase stimulation by intravenous heparin significantly increased transfer of alkanes from chylomicrons to HDL. These results indicate that (a) chylomicrons transport gut-derived hydrocarbons with a wide range of structure and polarity as triacylglycerol solutes; (b) HDL are a major plasma acceptor of all these hydrocarbons, demonstrating both selective solute uptake from triacylglycerol and net chemical uptake for the 2 hydrocarbons studied and (c) efflux of these chylomicron hydrocarbons from plasma and into HDL is regulated partly by hydrolysis of chylomicron triacylglycerol.     

Carbohydrate content of apolipoprotein B-48 from rat chylomicrons of varying density

Monosaccharide composition was determined in apolipoprotein B-48 (apoB) of chylomicrons of rat mesenteric lymph. Chylomicrons were separated into three fractions based on density. Triglyceride and apolipoprotein content were determined in each. ApoB was isolated and quantified using precipitation with isopropanol. Chylomicrons were collected in lymph under normal conditions, and with Poloxalene 2930 when chylomicron secretion was inhibited. Most of the triglyceride was carried in the least dense fraction, while the highest apoB content was in the most dense fraction under normal conditions. Mannose and galactosamine contents of apoB were similar in all fractions while contents of both glucosamine and galactose were highest in the least dense fraction. When chylomicron secretion was inhibited by Poloxalene, the amount of triglyceride recovered in the least dense fraction was significantly reduced. Despite the inhibition of lipid transport in the least dense fraction of chylomicrons by Poloxalene, there was little change in apoB recoveries and in the relative content of various monosaccharides in the apoB from each of the three fractions as compared to results obtained during lipid absorption under normal conditions. In conclusion, carbohydrate composition of apoB of chylomicrons is heterogeneous and varies with chylomicron density.     

Effect of chronic glucagon administration on lipoprotein composition in normally fed,fasted and cholesterol-fed rats

Male adult Wistar rats received daily (at 9 a.m. and 5 p.m.) 10 μg of zinc-protamine glucagon by subcutaneous injection for 8 days. Plasma cholesterol levels were decreased by 36% in fed rats, 33% in cholesterol-fed rats and by 55% in fasted rats. Lipoproteins were separated into 22 fractions by ultracentrifugation using a density gradient. Glucagon administration decreased the cholesterol content in all lipoproteins except low density lipoprotein (LDL₁) (1.006–1.040) and very low density lipoprotein (VLDL) from cholesterol-fed rats. The main decrease (−57 to −81%) was observed in 1.050–1.100 g/mL lipoproteins (LDL₂ and HDL₂), which contained a large amount of apo E, while HDL₃ cholesterol was not affected. Triacylglycerol levels were decreased only in chylomicrons and VLDL (−70%) of fed and cholesterol-fed rats, while plasma and lipoprotein triacylglycerol levels were not changed in fasted rats treated with glucagon. In normally fed rats glucagon administration increased by 42% the fractional catabolic rate of [¹²⁵I]HDL₂ while the absolute catabolic rate appeared to be unchanged. Glucagon seems to be a potent hypolipidemic agent affecting mainly the apo E-rich lipoproteins. Its chronic administration limits lipoprotein accumulation which occurs upon cholesterol feeding.     

Effects of triton WR 1339 and heparin on the transfer of surface lipids from triglyceride-rich emulsions to high density lipoproteins in rats

The influence of lipolytic mechanisms on the transfer of phospholipids and unesterified cholesterol from artificial emulsions, serving as chylomicron models to other plasma lipoproteins, mainly high density lipoproteins (HDL) were testedin vivo. The emulsions labeled with radioactive lipids were injected into the bloodstream of rats (controls) and the results were compared with those obtained from rats that had previously been treated with Triton WR 1339 or heparin. Plasma clearance and the distribution of cholesteryl esters, phospholipids and unesterified cholesterol in the different plasma lipoprotein fractions were then determined. Whereas virtually no cholesteryl esters were found in d>1.006 g/mL density fraction of the three experimental groups, 2.8±1.3% of the injected phospholipids were in the 1.063–1.210 g/L density fraction of the Triton treated rats, and 12.6±5.4% of the heparin treated rats, as compared to 10.1±1.7% in controls. This indicates that lipolysis directly influences phospholipid transfer to HDL. In contrast, free-cholesterol transfer to HDL, besides being less pronounced than phospholipid transfer, was enhanced by Triton and diminished by heparin, indicating that lipolytic mechanisms were not important determinants in this process. This work is part of a Privatdozent Thesis by the corresponding author.     

Effects of dietary triacylglycerol structure on triacylglycerols of resultant chylomicrons from fish oil- and seal oil-fed rats

We investigated the influence of the intramolecular fatty acid distribution of dietary triacyl-sn-glycerols (TAG) rich in n-3 polyunsaturated fatty acids (PUFA) on the structure of chylomicron TAG. Fish oil and seal oil, comparable in fatty acid compositions but with different contents of major n-3 PUFA esterified at thesn-2 position (20:5n-3, 46.6%, and 5.3%; 22:6n-3, 75.5%, and 3.8%, respectively), were fed to rats. Mesenteric lymph was collected and the chylomicrons were isolated by ultracentrifugation. The fatty acid composition of chylomicrons largely reflected the fatty acid composition of the oils administered. The intramolecular fatty acid distributions of the TAG fed were reflected in the chylomicron TAG as the fraction of the total contents observed in thesn-2 position of 20:5n-3 were 23.6 and 13.3%, and of 22:6n-3 were 30.6 and 5.4% for resultant chylomicrons following fish oil and seal oil administration, respectively. Thus, after seal oil administration, significant higher load of n-3 PUFA was esterified in thesn-1,3 positions of chylomicron TAG compared with fish oil administration (P<0.05).     

Comparison of ultracentrifugation and gel filtration for the isolation of bovine lipoproteins

Lipoproteins from the plasma of three nonlactating Holstein cows were isolated using either preparative ultracentrifugation or gel filtration chromatography. Lipoprotein classes obtained by ultracentrifugation were very low density plus chylomicra, <1.006 g/ml; low density, 1.007–1.039 g/ml; high density₁, 1.040–1.063 g/ml; and high density, 1.064–1.22 g/ml. These lipoprotein classes were individually applied to an agarose gel column to determine at what volume they eluted in comparison to lipoproteins that were separated after applying total bovine lipoproteins to the column. Three major peaks corresponding to very low density lipoproteins plus chylomicra, low density, and high density lipoproteins resulted after gel filtration of total lipoproteins. Very low density lipoproteins plus chylomicra, obtained by ultracentrifugation, eluted as a single peak, as did low density and high density lipoproteins. However, high density₁ lipoproteins eluted as two peaks. The first peak eluted at the same volume as low density lipoproteins, and the second peak eluted at a volume similar to that of the ascending slope of the high density lipoprotein peak. Results from disc polyacrylamide gel electrophoresis, immunoelectrophoresis and double immunodiffusion of lipoprotein fractions, and SDS polyacrylamide gel electrophoresis of their apoproteins, similarly indicated that the lipoproteins present in the 1.040–1.063 g/ml density interval are a mixture of low and high density lipoproteins rather than a unique class of lipoproteins.     

Hypolipidemic activity of 3- and 4-phenyl-piperidine-2,6-diones and selected N-substituted derivatives

Three- and 4-phenyl-piperidine-2,6-dione derivatives were investigated for hypolipidemic activity at 20 mg/kg/day intraperitoneally in rodents. The 3-phenyl compound afforded the best activity and effectiveness in both normal and hyperlipidemia-induced mice. The agent lowered lipids by blocking the de novo hepatic synthesis of cholesterol and triglycerides, specifically at the sites of ATP-dependent citrate lyase, acetyl CoA synthetase,sn-glycerol-3-phosphate acyl transferase and phosphatidylate phosphohydrolase. The agent caused a more rapid clearance of cholesterol by the fecal route. Cholesterol levels of the chylomicrons, very low density lipoprotein and low density lipoprotein (LDL) were reduced, whereas high density lipoprotein cholesterol was significantly elevated after drug administration. Triglyceride content was lowered in the chylomicron and LDL fractions. These modulations of lipid content of serum lipoproteins by the drug suggest a favorable situation for treatment of hyperlipidemic states.     

Interaction of potentially toxic bile acids with human plasma proteins: Binding of lithocholic (3α-hydroxy-5β-cholan-24-oic) acid to lipoproteins and albumin

The binding of lithocholic acid to different plasma fractions was studied. When whole plasma was incubated for 8 hr, approximately 25% of the incubated [¹⁴C]lithocholic acid was bound to the lipoprotein and lipoprotein-free, albumin-rich fractions. An average of 87.6% of the bound-lithocholic acid was present in the lipoprotein-free, albumin-rich fraction, 7.2% in high density lipoproteins, 2.2% in low density lipoproteins, 1.0% in intermediate density lipoproteins and 2.0% in very low density lipoproteins. Expressed as binding per μg protein, considerably less [¹⁴C]lithocholic acid was bound to the lipoprotein-free, albumin-rich fraction, than to the lipoproteins. The binding of [¹⁴C]lithocholic acid after the incubation of the isolated plasma fractions was similar to that found after the incubation of whole plasma. The highest transfer of [¹⁴C]lithocholic acid occurred from the lipoprotein-free, albumin-rich fraction to the lipoprotein fractions. The studies indicate, that, although the largest amount of lithocholic acid is bound to the lipoprotein-free, albumin-rich fraction, per μg protein, the binding of lithocholic acid to lipoporteins is more pronounced and stable than that bound to the lipoprotein-free, albumin-rich fraction. Since lipoproteins, in contrast to albumin, are internalized by most tissues, they may be important carriers into cells of lithocholic acid and other potentially toxic or tumorigenic bile acids. Results of this study were presented at the Annual Meeting of the American Society for Clinical Investigation in Washington, D.C. in May 1988.     

Lipoprotein synthesis. I. rat plasma lipoprotein composition and synthesis from radioactive precursors

The in vivo synthesis of rat plasma lipoproteins was studied by the use of isotopic protein and lipid precursors. Labelled amino acids, palmitic acid and tripalmitin were administered by stomach tube and the radioactivity in the plasma lipoproteins was determined following preparative ultracentrifugal isolation at densities of 1.006, 1.019, 1.063 and 1.21 g/ml. In response to triglyceride feeding, amino acid composition of the high density lipoprotein changed little, but in the low density lipoproteins proportionality in the amino acid pattern was changed as reflected by increases and decreases in certain amino acids. Isotopic amino acids were not incorporated in proportion to the relative abundance with which they occurred in the lipoproteins. Triglyceride feeding markedly stimulated isotope utilization, especially in the low density fractions. Methionine, though only present in small amounts, was extensively utilized and it is suggested that this amino acid may play a significant role in the synthesis of lipoproteins, other than the role of a methyl donor for phosphatidylcholine.     

Apoproteins in association with intralipid incubations in rat and human plasma

Intralipid was incubated with rat and human plasma and examined for changes in lipid and apoprotein composition. Upon incubation in rat plasma, Intralipid acquired an apoprotein complement similar to that found in chylomicrons following plasma incubation or in chylomicrons after alimentary lipemia. Since the apoproteins of lipoproteins probably govern their metabolism, these results suggest that Intralipid and chylomicrons undergo similar metabolic fates. This pattern is characterized by a predominance of Apo E (the arginine-rich apoprotein) and Apo C. Incubation of Intralipid with human plasma showed the uptake of Apo A-I and Apo A-IV as well. Density fractionation of the plasma into separate lipoprotein classes facilitated identification of high density lipoprotein as the major apoprotein donor to the Intralipid. When rat lipoprotein-free plasma (δ>1.21) was incubated with Intralipid, a different apoprotein pattern appeared in the particles of S_f>400 depending on whether the entire Intralipid preparation or only the S_f>400 fraction alone was incubated. The difference consisted of a virtual total absence of the arginine-rich protein on the S_f>400 particles in whole Intralipid incubations. Density fractionation of the S_f<400 particles of Intralipid and recombination of these fractions with the S_f>400 fraction before incubation revealed the major inhibitory fraction to be δ<1.006 (S_f 20–400).     

Effect of peanut oil and randomized peanut oil on cholesterol and oleic acid absorption, transport, and distribution in the lymph of the rat

Peanut oil was shown to be atherogenic in cholesterol-fed rats, rabbits, and monkeys. However, after randomization, a process in which the fatty acids in peanut oil are randomly rearranged, its atherogenicity was significantly reduced in cholesterol-fed rabbits and monkeys. The mechanism for this effect remains unknown. This study was designed to investigate whether the absorption, transport and distribution of dietary cholesterol and oleic acid in the lymph were altered in the presence of peanut oil or randomized peanut oil. Previous investigators collected lymph through the mesenteric duct for 6 h and analyzed lymph for cholesterol. In the present study, lymph fluids were collected at timed intervals for up to 8 h and then at 24 h via the thoracic duct. Cholesterol and oleic acid (fatty acid) were estimated not only in the whole lymph but also in lymph lipoprotein fractions and in major lipid fractions. A 24-h lymph collection will enhance accuracy as short-term fluctuations in lipid absorption will not affect the results. Thoracic duct lymph collection is quantitative compared to mesenteric duct lymph collection, which provides only a fraction of the total lymph. Rats were given a lipid emulsion containing either peanut oil or randomized peanut oil. The emulsion also contained cholesterol, oleic acid, and sodium taurocholate in saline and was given through a duodenal catheter. Results show that absorption, transport, and distribution of cholesterol and oleic acid in the lymph fluids were similar in both dietary groups. These results suggest that the atherogenicity of peanut oil may be due to other events taking place subsequent to the release of cholesterol-containing chylomicrons and very low density lipoprotein by the small intestinal epithelial cells into the blood or may be due to the triglyceride structure itself.     

Biosynthesis of protein relative to the accumulation of fat in the liver of EFA deficient rats

Effect of an essential fatty acid (EFA) deficiency in the rat on the incorporation of leucine-¹⁴C and glucosamine-¹⁴C into serum and liver protein are reported. Weanling male rats of the Sprague-Dawley strain were raised on a fat-free diet for 10–12 weeks and then switched to diets supplemented with 10% corn oil or 10% hydrogenated coconut oil. Leucine-¹⁴C or glucosamine-¹⁴C was injected into the tail veins of the animals of each group. At selected intervals up to 120 min after the injections, the animals were sacrificed and the radioactivity of the liver and serum proteins was measured. The levels of triglyceride (TG) in the serum and the liver were also determined. Less radio-activity was incorporated into the serum β-lipoprotein (β-LP) fraction of the hydrogenated coconut oil than the corn oil fed animals injected with leucine-¹⁴C, but no differences were observed in the incorporation of radioactivity into the liver protein and both albumin and globulin fractions of the serum of these groups of animals. In the similar experiments with glucosamine-¹⁴C less radioactivity was incorporated into the β-LP fraction of the serum and into the smooth endoplasmic reticulum of the liver in the hydrogenated coconut oil (EFA deficient) than the corn oil fed animals. Time course studies also indicated that less radioactivity was incorporated into the β-LP fraction than into the albumin and globulin fractions of the serum of the hydrogenated coconut oil group. These findings suggest that an EFA deficiency results in an impairment of the synthesis or release of lipoprotein.     

Early steps on the in vivo incorporation of 1-14C-linoleic acid into liver lipids from normal and essential fatty acid deficient rats

Essential fatty acid (EFA) deficient rats were injected intraportally with a solution of 1-¹⁴C-linoleic acid during a 1 min period. Livers were quickly frozen, pulverized, and the lipids extracted and fractioned by thin layer chromatography. The incorporation of 1-¹⁴C-linoleic acid into liver lipids was measured. The results were compared with those previously obtained from normal rats. No significant differences were observed in the total radioactivity recovered from lipid extracts. While the distribution of radioactivity into the 1–2 diacylglycerol fraction remained unchanged in both groups of rats, in the EFA deficient rats the 1-¹⁴C-linoleic acid incorporation was actually directed to the phospholipid fractions instead of to the triacylglycerol fractions as was observed in the normal rats.     

Effect of marginal zinc deficiency on the apolipoprotein-B content and size of mesenteric lymph chylomicrons in adult rats

To investigate the mechanisms underlining the impaired intestinal absorption of lipids in zinc deficiency, the apo-B content and chemical composition of chylomicrons from marginally zinc-deficient rats fed 2.8 ppm of dietary zinc (ZD) were compared with those from pair-fed (PF) and ad libitum control (CT) groups fed an adequate level (30.8 ppm) of zinc. Chylomicrons, obtained by cannulating the mesenteric lymph, were isolated by ultracentrifugation at 1.3×10⁶ g/min at 12 C and purified by 2% agarose column chromatography. Apolipoprotein- (apo) B was separated by the method of isopropanol precipitation. The apo-B concentration of chylomicrons was lowered significantly in ZD group. The apo-B contents of chylomicrons in ZD, PF and CT rats, as expressed as % chylomicron protein, were 8.7±0.1, 11.5±0.5 and 10.7±0.7%, respectively. No significant differences were noted between ZD and PF groups in total protein (TP), phospholipid (PL), triglyceride (TG) and cholesterol (CH), although there was a slight decrease in TG and an increase in CH in CT rats compared with ZD and PF groups. The ratio of the core to surface constituents, as determined by TG/(TP+PL), was significantly higher in ZD group relative to the controls, suggesting that chylomicrons from ZD rats were larger. This finding was consistent with the appearance of larger chylomicron particles in the lacteal of the intestinal mucosa following lipid ingestion. These findings suggest that the intestinal synthesis of apo-B may be defective in zinc-deficient rats and may explain in part the impaired absorption of dietary lipids observed in zinc deficiency. Presented in an abstract form at the 70th annual meeting of the Federation of American Societies for Experimental Biology: Fed. Proc. 45, 974, 1986.     

Effect of marginal zinc deficiency on lipoprotein lipase activities in postheparin plasma,skeletal muscle and adipose tissues in the rat

The activities of lipoprotein lipase in postheparin plasma, retroperitoneal adipose and gastrocnemius muscle tissues were determined in the rats fed 2.8 ppm of dietary zinc for eight weeks, as compared with pair-fed and ad libitumfed rats given 30.8 ppm of zinc. The postheparin lipoprotein lipase activity, as determined by using a lipid emulsion labeled with [³H]triolein as the substrate, was significantly lower in the first group of rats, relative to that in the second and third groups. Tissue lipoprotein lipase activities were compared using the lipid emulsion and activator serum obtained from the zinc-deficient rats and the ad libitum-fed rats. The activator sera were devoid of very low density and low density lipoproteins, but enriched in high density lipoproteins. Muscle lipoprotein lipase activities were significantly lower when assayed with the activator serum from the zinc-deficient compared with the activities determined with the activator serum from the ad libitum-fed. Similarly, muscle lipoprotein lipase activities were lower in all groups when [³H]-triolein-labeled chylomicrons from the zinc-deficient were used as the substrate, compared with the activities determined using the chylomicrons from the ad libitum-fed. Lipoprotein lipase activities in the adipose tissues were not affected by the different sources of the activator sera and chylomicrons. The results strongly suggest that the decrease in postheparin lipoprotein lipase activity in zinc deficiency is not due to changes in tissue lipoprotein lipase enzyme per se, but to compositional alterations in with regard to C apolipoproteins, modulators of lipoprotein lipase activity. Presented in abstract form at the 71st annual meeting of the Federation of American Societies for Experimental Biology.Fed. Proc. 46:1472, 1987.