Chylomicron Triacylglycerol Research Articles

Comparative studies of triacylglycerol structure of very low density lipoproteins and chylomicrons of normolipemic subjects and patients with type II hyperlipoproteinemia

The triacylglycerols of very low density lipoproteins (VLDL) and of chylomicrons were analyzed in the fasting and postabsorptive states from normolipemic subjects and patients with Frederickson's Type II hyperlipoproteinemia, who subsisted on free choice diets, standard diets excluding lard, or were given a breakfast enriched in lard. The VLDL and chylomicrons were obtained by conventional ultracentrifugation, and the triacylglycerols were isolated by thin-layer chromatography (TLC). Representative sn-1,2-, sn-2-3- and sn-1,3-diacylglycerols were generated by partial Grignard degradation of the triacylglycerols and a stereospecific hydrolysis by phospholipase C of the mixed sn-1,2(2,3)-diacyl phosphatidylcholines prepared as intermediates. Representative sn-2-acylglycerols were obtained by hydrolysis with pancreatic lipase. Positional distribution of the fatty acids was established by subtracting in turn the fatty acid composition of the sn-2-position from the fatty acid composition of the sn-1,2- and sn-2,3-diacylglycerols. The molecular association of the fatty acids in the diacylglycerol moieties was determined by gas-liquid chromatography with mass spectrometry (GC/MS) of the tertiary-butyldimethylsilyl (t-BDMS) ethers. The molecular association of the fatty acids in the triacylglycerols was determined by 1-random 2-random 3-random calculation following experimental validation of the distribution. The results confirm a marked asymmetry in the positional distribution of the fatty acids in all triacylglycerol samples, with the palmitic acid predominantly in the sn-1-position, the unsaturated acids about equally divided between the sn-2- and sn-3-positions, and the stearic acid divided about equally between the sn-1- and sn-3-positions. The overall structure of the VLDL and chylomicron triacylglycerols from patients and control subjects was characterized by a non-correlative distribution of fatty acids under all dietary conditions.

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(alpha)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 labelled 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.

Lipoprotein metabolism in the suckling rat: characterization of plasma and lymphatic lipoproteins.

Suckling rat plasma contains (in mg/dl): chylomicrons (85 +/- 12); VLDL (50 +/- 6); LDL (200 +/- 23); HDL1 (125 +/- 20); and HDL2 (220 +/- 10), while lymph contains (in mg/dl): chylomicrons (9650 +/- 850) and VLDL (4570 +/- 435) and smaller amounts of LDL and HDL. The lipid composition of plasma and lymph lipoproteins are similar to those reported for adults, except that LDL and HDL1 have a somewhat higher lipid content. The apoprotein compositions of plasma lipoproteins are similar to those of adult lipoproteins except for the LDL fraction, which contains appreciable quantities of apoproteins other than apoB. Although the LDL fraction was homogeneous by analytical ultracentrifugation and electrophoresis, the apoprotein composition suggests the presence of another class of lipoproteins, perhaps a lipid-rich HDL1. The lipoproteins of lymph showed low levels of apoproteins E and C. The triacylglycerols in chylomicrons and VLDL of both lymph and plasma are rich in medium-chain-length fatty acids, whereas those in LDL and HDL have little or none. Phospholipids in all lipoproteins lack medium-chain-length fatty acids. The cholesteryl esters of the high density lipoproteins are enriched in arachidonic acid, whereas those in chylomicrons, VLDL, and LDL are enriched in linoleic acid, suggesting little or no exchange of cholesteryl esters between these classes of lipoproteins. The fatty acid composition of phosphatidylcholine, sphingomyelin, and lysophosphatidylcholine were relatively constant in all lipoprotein fractions, suggesting ready exchange of these phospholipids. However, the fatty acid composition of phosphatidylethanolamine in plasma chylomicrons and VLDL differed from that in plasma LDL, HDL1, and HDL2. LDL, HDL1, and HDL2 were characterized by analytical ultracentrifugation and shown to have properties similar to that reported for adult lipoproteins. The much higher concentration of triacylglycerol-rich lipoproteins in lymph, compared to plasma, suggests rapid clearance of these lipoproteins from the circulation.

Modulation of lipoprotein lipase activity in cultured rat mesenchymal heart cells and preadipocytes by dibutyryl cyclic amp, cholera toxin and 3-isobutyl- 1-methylxanthine

We have compared the effects of cellular cyclic AMP modulation on the regulation of lipoprotein lipase in cultures of rat epididymal pad preadipocytes and mesenchymal heart cells. Addition of dibutyryl cyclic AMP (dibutyryl cAMP) or 3-isobutyl-1-methylxanthine (IBMX) to preadipocytes grown in serum-containing culture medium resulted in a progressive decrease in lipoprotein lipase activity released into the culture medium so that at 6–8 h enzyme activity ranged between 20 and 30% of that recovered in the control dishes. Similar short-term (6–8 h) studies of the heart cell cultures showed a variable and much less pronounced depression of lipoprotein lipase activity. Thus, following dibutyryl cAMP and IBMX treatment, lipoprotein lipase activity ranged between 70 and 95% of control values. Incubation for 6 h with cholera toxin was followed by a 4-fold rise in the concentration of cellular cyclic AMP in both types of culture, but while in heart cell cultures enzyme activity was unchanged, lipoprotein lipase activity in preadipocytes decreased to 30% of control value. After 24 h incubation with all three effectors, an increase in lipoprotein lipase activity was seen. In the preadipocytes the increase ranged between 50 and 150% above control value, in the heart cell cultures it was 100–250%. 24-h incubation of heart cell cultures with dibutyryl cAMP resulted in a 6-fold increase of heparin-releasable lipoprotein lipase activity while residual activity was doubled. The rise in surface-bound lipoprotein lipase was evidenced also by an increase in the lipolysis of chylomicron triacylglycerol. In the presence of cycloheximide, the dibutyryl cAMP-induced heparin-releasable and residual lipoprotein lipase activity declined at the same rate as the basal activity. The reason for the difference in response of cultured preadipocytes and heart cells to the effectors during the first 8 h of incubation has not been elucidated, but could be related to a possible absence of hormone-sensitive lipase in the heart cells, and hence in a difference in intracellular metabolism of triacylglycerol. On the other hand, a common mechanism can be postulated for the long-term effect of cyclic AMP on the induction of lipoprotein lipase activity in both types of cultures. It probably involves mRNA and protein synthesis, which culminates in an increase in enzyme activity.

Hydrolysis of chylomicron retinyl esters by an acid hydrolase of rat liver

Rat liver homogenate hydrolyzed chylomicron remnant retinyl esters with pH optima at pH 4.5, 6 and 8. The retinyl ester hydrolysis at pH 4.5 was correlated to the acid phosphatase activity in different subcellular fractions, and thus the highest activity was found in highly purified lysosomes. These fractions could also catalyze the reverse reaction i.e., the esterification of retinol when incubated with excess palmitic acid. Purified rat liver lysosomes hydrolyzed chylomicron retinyl and cholesteryl esters with comparable rates, both being much lower than the hydrolysis of chylomicron triacylglycerols.

Influence of exercise on chylomicron triacylglycerol metabolism

Triacylglycerides (TG), circulating in chylomicrons, represent a potentially rich source of plasma substrates available for tissue uptake. Chylomicron TG are first hydrolyzed by the action of lipoprotein lipase prior to tissue uptake of the TG-derived fatty acids. Although the removal of these TG by fat cells appears to favor a storage function, uptake by skeletal muscle accounts for a significant portion of the TG removed from plasma. Further, uptake of TG-derived fatty acids by skeletal muscle is increased during exercise. This could contribute to an increased rate constant for TG removal from the plasma during exercise. A significant portion of the TG-derived fatty acids that enter muscle during exercise remain in the non-esterified fatty acid pool of the muscle where they could provide a substrate for beta-oxidation. Indirect estimates of the contribution that TG-derived fatty acids may make to the total energy costs during exercise in a fasting condition may be minor. However, appropriate direct measurements during a postprandial condition, where total plasma TG turnover is very large, have not been made.

Influence of exercise on chylomicron triacylglycerol metabolism

TERJUNG, RONALD L., BRENDA G. MACKIE, GARY A. DUDLEY, and HANNA KACIUBA-USCILKO. Influence of exercise on chylomicron triacylglycerol metabolism: plasma turnover and muscle uptake. Med. Sci. Sports Exerc., Vol. 15, No. 4, pp. 340–347, 1983. Triacylglycerides (TG), circulating in chylomicrons, represent a potentially rich source of plasma substrates available for tissue uptake. Chylomicron TG are first hydrolyzed by the action of lipoprotein lipase prior to tissue uptake of the TG-derived fatty acids: Although the removal of these TG by fat cells appears to favor a storage function, uptake by skeletal muscle accounts for a significant portion of the TG removed from plasma. Further, uptake of TG-derived fatty acids by skeletal muscle is increased during exercise. This could contribute to an increased rate constant for TG removal from the plasma during exercise. A significant portion of the TG-derived fatty acids that enter muscle during exercise remain in the non-esterified fatty acid pool of the muscle where they could provide a substrate for beta-oxidation. Indirect estimates of the contribution that TG-derived fatty acids may make to the total energy costs during exercise in a fasting condition may be minor. However, appropriate direct measurements during a postprandial condition, where total plasma TG turnover is very large, have not been made.

A re-examination of the fate of glyceride-glycerol in neutral lipid absorption and transport

Conventional ideas concerning the unidirectional movement of triacylglycerol from intestinal lumen to lymph with sn-2-monoacylglycerol being the major glyceride-glycerol precursor were challenged by our finding that steady state specific activities of radiolabeled triacylglycerol (glyceryl moiety) in the intestinal mucosa and lumen were greatly reduced as compared to the specific activity of intraduodenally infused triacylglycerol. Investigation of the point at which the radiolabel was diluted was performed in mesenteric lymph duct-cannulated rats with a duodenal cannula through which trioleoyl[3H]glycerol was constantly infused. Both within the bowel lumen and in the intestinal mucosa, monoacylglycerol, diacylglycerol, and triacylglycerol specific activities were 31% or less of the specific activity of the infusate; chylomicron triacylglycerol specific activity was 75%. Efflux of neutral lipid from the mucosa into the bowel lumen was directly demonstrated by finding that when 3H glucose was injected intraperitoneally during triolein infusion, luminal triacylglycerol had a higher specific activity than was present in the mucosa. We conclude that there are two pools of mucosal triacylglycerol. One is rapidly transported and derives most of its glyceride-glycerol from luminal monoacylglycerol. The second is slowly transported; it derives its glyceride-glycerol mainly from endogenous sources and may efflux back into the bowel lumen.

Fate of milk 125I-labeled lipoprotein lipase in cells in culture comparison of lipoprotein lipase- and non-lipoprotein lipase-synthesizing cells

Radioiodinated lipoprotein lipase, isolated from bovine milk ( 125I-labeled milk lipoprotein lipase) was shown to retain full hydrolytic activity towards its native substrate, i.e., chylomicron triacylglycerol. The 125I-labeled enzyme interacted with various cells in culture by being bound to the cellular surface, internalized and degraded. Cellular binding of the labeled enzyme occurred in the presence or absence of substrate and was related to enzyme concentration. Heparin reduced cellular binding by 50% but inhibited uptake and degradation more extensively. Cellular uptake was not affected by chloroquine or NH 4Cl, but degradation of the labeled enzyme was blocked. Uptake and degradation were not inhibited by mannose 6-phosphate. The interaction between the exogenous enzyme and cells which do not synthesize lipoprotein lipase, i.e., fibroblasts and endothelial cells, resulted in a high ratio of surface binding to degradation. In heart cell cultures and preadipocyte cultures, which produce lipoprotein lipase, the ratio of enzyme catabolized to that bound was high at all time points examined. Since in the intact organism lipoprotein lipase acts at the luminal surface of vascular endothelium, it seems expedient that these cells are able to bind the enzyme, but will catabolize it only slowly. The rapid and extensive degradation of the 125I-labeled lipoprotein lipase in heart cells and preadipocytes may be related to the metabolism of the endogenously produced lipoprotein lipase.

Chylomicron catabolism differs between Hooded and albino laboratory rats.

To extend previous reports that some aspects of lipid metabolism are different between Hooded and albino strains of laboratory rats, thoracic duct lymph chylomicrons were collected and their composition and metabolism were compared in this study. Chylomicrons from Hooded rats had more core components and fewer surface components than albino rats. After adding 1% cholesterol to the diet the ratio of cholesterol:phospholipid was higher in Hooded rat chylomicrons. Nascent lymph chylomicrons from Hooded rats contained less apolipoprotein A-IV, but after incubation with serum there was a gain of apolipoprotein A-IV and a loss of A-I from both types of chylomicron. The ratio of apoC to apoE was higher in Hooded than in albino rats. The metabolism of injected chylomicrons was slower in Hooded rats. Twenty minutes after a single injection, only 1-3% of chylomicron triacylglycerol and 8-13% of chylomicron cholesteryl ester remained in the plasma of albino rats, compared with 6-12% of triacylglycerol and 30-33% of cholesteryl ester in Hooded rats. At 30 min in the Hooded rats, the uptake of injected chylomicron cholesterol into the liver was decreased whereas uptake into other tissues, notably adipose tissue and muscle, was increased. During the steady intravenous injection of chylomicrons from the same strain, the fractional clearance rate of chylomicron triaclyglycerol was about three-fold faster in albino rats than in Hooded rats, and the fractional clearance rate of chylomicron cholesteryl ester was 70% faster in albino rats. When chylomicrons were allowed to circulate for 30 min after injection into functionally hepatectomized rats, with or without heparin, the remnant formation and chemical composition were similar in Hooded and albino rats. Hence the deficiency in chylomicron metabolism in Hooded rats was not due to an absolute impairment in the formation of remnants by the action of lipoprotein lipase in peripheral tissues, although our findings were most likely accounted for by some relative impairment of remnant formation in the Hooded strain. The deficiency in chylomicron metabolism correlated with an increased cholesterolemic response in Hooded rats compared with albino rats when 1% cholesterol was added to the diet.-Jeffery, F., and T. G. Redgrave. Chylomicron catabolism differs between Hooded and albino laboratory rats.

Uptake of chylomicron remnants causes cholesterol accumulation in cultured human arterial smooth muscle cellS

Chylomicron remnants ( S f > 100) were prepared by treating human chylomicrons ( S f > 400) with human post heparin plasma. Chylomicron remnants recovered after 70–80% of chylomicron triacylglycerol was hydrolyzed, suppressed LDL-receptor activity and increased cell cholesterol esterification to the same extent as did LDL when added to cultured human arterial smooth muscle cells at an equal cholesterol concentration. Cell cholesterol mass increased 36% after incubation with 25 μg LDL cholesterol/ml and 35% with 25 μg chylomicron-remnant cholesterol/ml. Addition of 30 μM chloroquine plus LDL or chylomicron remnants further increased cholesterol content of cells (74% and 87%, respectively) and caused a significant rise in cell esterified cholesterol (344% and 369%, respectively). Cholesterol content per unit of apolipoprotein B mass of remnants was 2–3-fold higher than that of LDL. Therefore, if lipoprotein particles were added at equivalent apolipoprotein B mass, chylomicron remnants increased cell cholesterol content and cholesterol esterification and suppressed LDL receptor activity significantly more than did LDL. This suggests that an additional determinant, presumably apolipoprotein E, is important for receptor recognition of chylomicron remnants. These results may be relevant to the delivery of chylomicron-derived cholesterol to arterial cells proposed as a feature of atherogenesis.

Milk lipid absorption and chylomicron production in the suckling rat

Structural correlates of milk lipid absorption and chylomicron production were studied in 10-day-old suckled rats. The gastric and duodenal contents and duodenal mucosae were examined with the light and electron microscopes. In the gastric lumen the milk lipid globule cores were smooth, circular and uniformly electron opaque. Many membranes and lamellar structures with a trilaminar and multilamellar appearance were adherent to the peripheries of the cores. In the central duodenal lumen the milk lipid globule cores were also smooth, circular and uniformly electron opaque. Very few milk lipid globules in the duodenal lumen showed adherent membranes or lamellae. Membrane fragments and lamellae were present in the lumen separate from the milk lipid globules. In the duodenal lumen between villi the milk lipid globules had multiple electron lucent indentations of the core. It is believed that the irregular peripheries of the milk lipid globule cores are the result of lipolysis within the duodenal lumen acting at the milk lipid globule surface. This lipolysis of triacylglycerol would produce amphiphilic lipids which may result in the electron lucent spaces at the milk lipid globule periphery. The absorptive epithelial cells along the length of the duodenal villus varied in structure relative to their position at the tip, middle, or base of the villus. Typical mid-villus epithelial cells contained lipid droplets averaging 0.3-micrometer diameter in the smooth and rough endoplasmic reticulum and in Golgi complexes in the apical cytoplasm. Villus tip and villus base cells contained large lipid droplets between 7-16 micrometers. Only a few 0.3-micrometer lipid droplets were present within these cells. These large lipid droplets appeared to be accumulations of triacylglycerol present in the apical cytoplasm associated with lamellar and membranous structures. Numerous chylomicrons were present between epithelial cells located in the middle region of the villus while significantly fewer chylomicrons were seen between epithelial cells at the tip and base of the villus. These observations suggest that the cells at the middle of the duodenal villus of suckling rats were more efficient in the production of chylomicron triacylglycerol derived from incoming milk triacylglycerol than cells at the tip and base of the villus.

Hydrolysis of chylomicron triacylglycerol by endothelium-bound lipoprotein lipase. Effect of decreased apoprotein C-II/C-III ratio.

Chylomicrons with a decreased ratio of C-II/C-III apoproteins on their surface produced by the addition of apoproteins C-III-0 or C-III-3 to intact rat lymph chylomicrons. These chylomicrons inhibited the activity of soluble lipoprotein lipase in vitro, but had no effect on the activity of the endothelium-bound enzyme in the perfused heart.

Antibodies to lipoprotein lipase application to perfused heart

An antibody was prepared against purified rat heart lipoprotein lipase. 1. 1. This antibody showed marked species specificity. It inhibited almost totally the lipoprotein lipase activity from all rat tissues examined (i.e., heart, adipose, postheparin plasma, and mammary gland), while having no effect on the activity of lipoprotein lipase partially purified from rabbit, guinea pig and bovine heart and from bovine milk. The antibody also had no effect on the hepatic lipase activity of rat postheparin plasma. 2. 2. After antibody to rat heart lipoprotein lipase was recirculated for 5 min through isolated rat hearts, little or no lipoprotein lipase activity could be detected in the perfusate during 0–20 s of a subsequent non-recirculating perfusion with buffer containing 1 unit heparin/ml. 3. 3. Following recirculation of antibody to lipoprotein lipase for 10 min and a non-recirculating perfusion with buffer for 2 min, the hearts no longer oxidized any significant amounts of 14C-labelled palmitate chylomicron triacylglycerol fatty acid to 14CO 2 during a 15-min perfusion. The data give compelling evidence that the functional fraction of lipoprotein lipase in hearts is at the endothelial cell surface accessible to lipoprotein lipase antibody.

Catabolism of chylomicron triacylglycerol and cholesteryl ester in genetically obese rats

The catabolism of chylomicrons was investigated in genetically obese rats and their nonobese littermates, and was compared with catabolism in older Sprague-Dawley rats with body weights similar to the obese rats and their younger controls. Labeled thoracic-duct lymph was collected from donor rats and the catabolism of the labeled chylomicrons was studied after a single intravenous injection or during steady intravenous infusion in unanesthetized, nonfasting, recipient rats. In the genetically obese rats clearances from the plasma of chylomicron triacylglycerol and cholesteryl ester were less than in their nonobese littermates. Fractional clearance rates were reduced for both triacylglycerol and cholesteryl ester but triacylglycerol turnover rate (mg min(-1)) was greater than controls. Chylomicron triacylglycerol clearance was more efficient than cholesteryl ester clearance so that radioactivity remaining in the plasma was relatively depleted in triacylglycerol. The large-bodied old Sprague-Dawley rats showed no reduction in clearance of chylomicron radioactivity in comparison with younger controls. These results suggest that hyperlipidemia in genetically obese rats may be due in part to an accumulation of chylomicron remnants in the plasma. Flotation characteristics of plasma lipoproteins in the obese rats were consistent with this interpretation. However, separate experiments showed that genetically obese, fasting rats also accumulated more triacylglycerol in the plasma after injection of Triton WR 1339. The enlarged plasma triacylglycerol pool appears to derive from a mixture of hepatic and intestinal triacylglycerol-rich lipoproteins which, together, overload their common removal mechanism. Addition of cholesterol to the diets of the obese rats exacerbated their hyperlipemia and hepatic steatosis whereas their nonobese littermates and the large-bodied Sprague-Dawley rats were unaffected.

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.

Effect of albumin on products formed from chylomicron triacylglycerol by lipoprotein lipase in vitro

The effect of albumin and Ca 2+ on the action of purified bovine milk lipoprotein lipase on chylomicron triacylglycerol in vitro was studied with rat lymph chylomicrons containing triacylglycerol labeled with [ 14C]oleic acid and [ 14H]glycerol. Lipoprotein lipase hydrolyzed chylomicron triacylglycerol to mostly glycerol and fatty acids when incubated in medium containing sufficient albumin to bind all of the fatty acids formed. There was, however, transient accumulation of monoacylglycerol during the first 5 min of incubation. It is suggested that the latter represented monoacylglycerol undergoing isomerization within chylomicrons before being hydrolyzed to glycerol and fatty acid. Hydrolysis of monoacylglycerol was markedly inhibited (67%) when the incubation mixture contained a 9-fold excess of albumin. This effect on hydrolysis probably resulted from binding of monoacylglycerol to albumin in the medium, thus making the monoacylglycerol inaccessible to the enzyme. When the incubation mixture contained limited albumin, hydrolysis of all acylglycerols was reduced, and diacylglycerol and monoacylglycerol accumulated. Although some of the monoacylglycerol was released to the medium, the molar ratio of monoacylglycerol to albumin in the medium never exceeded 0.35. In contrast, the molar ratio of fatty acids to albumin in the medium was sometimes as high as 7. Ca 2+ was a poor substitute for albumin as an acceptor for fatty acids produced by the action of lipoprotein lipase on chylomicrons. The rate of hydrolysis of chylomicron triacylglycerol by lipoprotein lipase in vitro was related directly to the amount of enzyme added up to 1.7 μg (34 pmol) of enzyme per μmol of triacylglycerol. It was calculated that a nearly maximal rate was obtained when 43 molecules of enzyme were added per chylomicron.

Clearance of chylomicron triacylglycerol and cholesteryl ester from the plasma of streptozotocin-induced diabetic and hypercholesterolemic hypothyroid rats

Labeled chylomicrons in thoracic duct lymph were collected after test meals containing 14C cholesterol and 2- 3H glyceryl trioleate and were given by intravenous injection to groups of control rats, rats made diabetic by treatment with streptozotocin, and rats made hypothyroid and hypercholesterolemic by a diet containing cholesterol, peanut oil, cholic acid, and thiouracil. In the diabetic rats clearances from the plasma of chylomicron triacylglycerol and cholesteryl ester were impaired. A large variability in triacylglycerol clearance in diabetic rats was ascribed to variability in plasma triacylglycerol concentrations. Adipose tissue lipoprotein lipase activity was not impaired in the female diabetic rats used in this study. In the hypothyroid hypercholesterolemic rats chylomicron cholesteryl ester clearance from the plasma was impaired but chylomicron triacylglycerol was cleared efficiently, and adipose tissue lipoprotein lipase activity was similar to or greater than activity in controls. Ten minutes after intravenous injection most plasma radioactivity was recovered in lipoproteins of density <1.006 g/ml in all groups of rats, but relatively more was recovered at this density in both treatment groups. We suggest that chylomicron remnants accumulate in the plasma and contribute to the development of hyperlipemia in both treatment groups, but that the remnants formed in the diabetic rat are less depleted of triacylglycerol than the remnants formed in the hypothyroid hypercholesterolemic rat. It is suggested that factors other than measured lipoprotein lipase activities of adipose tissues may be important in determining the initial extent of hydrolysis of chylomicron triacylglycerol. We propose that the hypercholesterolemic hypothyroid rat is a useful model for the experimental production of the remnants of triacylglycerol-rich primary lipoproteins.

Lipoprotein lipase and uptake of triacylglycerol, cholesterol and phosphatidylcholine from chylomicrons by mammary and adipose tissue of lactating rats in vivo

The relationship between lipoprotein lipase activity and uptake of triacyl-glycerol, cholesterol and phosphatidylcholine from chylomicrons was studied in mammary gland and adipose tissue of rats lactating 6–7 days. 60% of triacyl-glycerol [ 14C]oleic acid, 13% of [ 3H]cholesterol and 8% of [ 32P]phosphatidyl-choline in chylomicrons injected intravenously were taken up within 11 min by mammary gland whereas negligible amounts were taken up by adipose tissue. Non-suckling for 44 h decreased markedly uptake of all lipids by mammary gland and retarded clearance of chylomicrons from blood, while it increased significantly uptake of triacylglycerol fatty acids and cholesterol by adipose tissue. Non-suckling also decreased lipoprotein lipase activity in mammary gland from 7.7 to 0.4 units/g, while it increased activity in adipose tissue from 0.1 to 2.7 units/g. These findings indicate that lipoprotein lipase is involved in uptake of chylomicron triacylglycerol and cholesterol by mammary gland and adipose tissue, and also in uptake of chylomicron phosphatidylcholine by mammary gland. They also show that reciprocal changes in lipoprotein lipase activity in mammary gland and adipose tissue, as occur during lactation, result in diversion of chylomicron lipids from one tissue to the other.

The inhibition in vivo of lipoprotein lipase (clearing-factor lipase) activity by triton WR-1339

1. Lipoprotein lipase activity was measured in heart homogenates and in heparin-releasable and non-releasable fractions of isolated perfused rat hearts, after the intravenous injection of Triton WR-1339. 2. In homogenates of hearts from starved, rats, lipoprotein lipase activity was significantly inhibited (P less than 0.001) 2h after the injection of Triton. This inhibition was restricted exclusively to the heparin-releasable fraction. Maximum inhibition occurred 30 min after the injection and corresponded to about 60% of the lipoprotein lipase activity that could be released from the heart during 30 s perfusion with heparin. 3. Hearts of Triton-treated starved rats were unable to take up and utilize 14C-labelled chylomicron triacylglycerol fatty acids, even though about 40% of heparin-releasable activity remained in the hearts. 4. It is concluded that Triton selectively inhibits the functional lipoprotein lipase, i.e. the enzyme directly involved in the hydrolysis of circulating plasma triacylglycerols. 5. Lipoprotein lipase activities measured in homogenates of soleus muscle of starved rats and adipose tissue of fed rats were decreased by 25 and 39% respectively after Triton injection. It is concluded that, by analogy with the heart, these Triton-inhibitable activities correspond to the functional lipoprotein lipase.