Chylomicron Triglyceride Research Articles

Human plasma albumin transports [13C]docosahexaenoic acid in two lipid forms to blood cells

Docosahexaenoic acid (22:6) decreases blood platelet function and is highly concentrated in the brain where its depletion leads to functional impairments. Because the platelets and blood brain barrier capillary endothelium cannot hydrolyze the complex lipids for fatty acid (FA) uptake, nonesterified FA (NEFA) bound to albumin are assumed to be the delivery route of FA to these cells. The supply of 13C-labeled 22:6 to blood cells by plasma albumin was studied in humans after a single ingestion of this FA esterified in a triglyceride (TG). The 22:6 13C/12C ratio, measured by gas chromatography combustion-isotope ratio mass spectrometry was measured in lipid classes from albumin, platelets, leukocytes, and erythrocytes (taken as a tentative index of the brain uptake). Nonesterified [13C]22:6 bound to albumin was rapidly produced after ingestion, as a result of the hydrolysis of very low density lipoprotein (VLDL) plus chylomicron TG. We found that albumin carried another source of 22:6, lyso-phosphatidylcholines (lyso-PC), in which [13C]22:6 accumulated while the nonesterified [13C]22:6 reached its minimal plasma concentrations. Computation of the relative contribution of NEFA and lyso-PC for the [13C]22:6 delivery to platelets and erythrocytes showed that the [13C]22:6 supply to platelets occurred uniquely through NEFA, whereas this pool was weakly involved in the delivery to erythrocytes. In contrast, lyso-PC was uniquely concerned with the 22:6 delivery to erythrocytes and represented the major part of this supply. We conclude that plasma albumin carries 22:6 in two lipid forms that are involved differently in the delivery of this FA to target cells.

Formula Containing Randomized Fats with Palmitic Acid (16:0) in the 2-Position Increases 16:0 in the 2-Position of Plasma and Chylomicron Triglycerides in Formula-Fed Piglets to Levels Approaching Those of Piglets Fed Sow’s Milk

Human and pig milk fat contains a high proportion of palmitic acid (16:0) which is largely esterified to the 2-position of the triglycerides. In contrast, the 16:0 in most nonmilk fats and in infant formulas is mainly esterified at the triglyceride 1,3 positions. Gastric and pancreatic lipases hydrolyze fatty acids from the dietary triglyceride 1- and 3-positions to produce unesterified fatty acids and 2-monoglycerides which are absorbed and re-esterified. In this study, we determined whether formula with chemically randomized oils, which equally distributes 16:0 among all the positions of triglycerides, influences growth or the distribution of fatty acids in plasma and liver lipid of formula-fed piglets compared with piglets fed formula with native oils or sow’s milk. After feeding from birth to 18 d, piglets fed formula with palm olein randomized with canola oil (co-randomized) had higher weight gain per liter of formula intake and higher 16:0 in the chylomicron triglyceride 2-position than piglets fed formula with randomized or native palm olein oil blended with canola oil. The fatty acid distribution of formula triglycerides is an important determinant of pathways of 16:0 absorption, and consequently of plasma lipid fatty acids in formula-fed piglets.

Delayed postprandial retinyl palmitate and squalene removal in a patient heterozygous for apolipoprotein A-I FIN mutation (Leu 159 → Arg) and low HDL cholesterol level without coronary artery disease

A low HDL cholesterol level is frequently but not consistently associated with inefficient postprandial fat clearance. We studied triglycerides, retinyl palmitate and squalene and apolipoprotein B-48 after a fat loading test in one subject heterozygous for a novel point mutation of apolipoprotein A-I (A-I FIN, Leu 159 → Arg) and low HDL cholesterol level without coronary artery disease, and in 16 healthy controls with the same apolipoprotein E phenotype, 3 3 , as the proband. HDL cholesterol and apolipoprotein A-I levels were 0.32 mmol/l and 57 mg/dl in the proband, and 1.29 ± 0.12 mmol/l (mean ± S.E.) and 126 ± 4 mg/dl in the controls. The peak concentration for triglycerides in plasma, chylomicrons and VLDL occurred at 4 h both in the case and controls. However, the peak concentrations for retinyl palmitate and squalene in chylomicrons and VLDL were delayed to 12 h in the proband compared with 4 and 9 h in the controls. The peak of apolipoprotein B-48 occurred at 6 h in the proband and at 4 h in the controls, so that triglycerides, apolipoprotein B-48 and retinyl palmitate and squalene peaked differently. After 24 h, retinyl palmitate, squalene, and apolipoprotein B-48 had returned to the baseline levels. The results show for the first time an impaired postprandial lipoprotein removal in a case heterozygote with moderately low HDL cholesterol due to an apolipoprotein A-I mutation not associated with coronary artery disease.

Down-regulation of tissue lipoprotein lipase expression in experimental chronic renal failure

Chronic renal failure (CRF) is associated with hypertriglyceridemia, impaired clearance of very low density lipoproteins (VLDL) and chylomicrons and their remnants as well as triglyceride-enrichment of various lipoproteins. These abnormalities are indicative of depressed lipoprotein lipase (LPL)-mediated hydrolysis of triglycerides in VLD and chylomicrons. In fact, impaired post-heparin lipolytic activity and decreased adipose tissue LPL activity has been previously demonstrated in CRF. The reduction in LPL activity in CRF has been attributed to PTH-induced insulin resistance and the presence of excess lipase inhibitors in uremic plasma. However, the effect of CRF on gene expression of LPL has not been elucidated and was studied here. Heparin-releasable, detergent-extractable and total LPL activities, as well as LPL mRNA of the heart, soleus muscle and fat body were determined in male Sprague-Dawley rats at baseline and on weeks 1, 3 and 6 following 5/6 nephrectomy (CRF group) or sham operation (control group). The CRF group exhibited a marked and steady rise in plasma triglycerides along with a steady decline in LPL activities and mRNA levels of all tissues studied. In contrast, the study parameters remained virtually unchanged throughout the study period in the control group. A strong inverse correlation was found between plasma triglycerides and LPL activity in the study animals. LPL activity was directly related to LPL mRNA. We conclude that CRF results in marked down-regulation of LPL expression that can contribute to dyslipidemia and altered energy metabolism in uremia. The effect of depressed LPL expression is compounded by the previously demonstrated elevations in uremic plasma of Apo C-III and pre-beta-HDL, which are potent inhibitors of LPL.

Effects of pancreas transplantation on distribution and composition of plasma lipoproteins

In type I (insulin-dependent) diabetic patients, peripheral hyperinsulinemia due to subcutaneous insulin treatment is associated with increased high-density lipoprotein (HDL) cholesterol, and also with an altered surface composition of HDL. Pancreas grafts also release insulin into the systemic rather than into the portal venous system, giving rise to pronounced peripheral hyperinsulinemia. We hypothesized that if peripheral hyperinsulinemia is responsible for high HDL cholesterol and/or altered surface composition of HDL in diabetic subjects, similar changes in the lipid profile should be present in pancreas-kidney transplant recipients (PKT-R). Using zonal ultracentrifugation, we isolated HDL 2, HDL 3, very-low-density lipoprotein (VLDL), intermediate-density lipoprotein (IDL), and low-density lipoprotein (LDL) from fasting plasma of 14 type I diabetic PKT-R, eight nondiabetic kidney transplant recipients (KT-R), and 14 healthy control subjects and determined the level and composition of the above lipoproteins. HDL 2 cholesterol was increased in PKT-R as compared with KT-R and healthy controls (both P < .05), whereas HDL 3 cholesterol was unchanged. However, an altered lipoprotein surface composition was evident in PKT-R: HDL 2, HDL 3, and LDL were enriched in unesterified cholesterol ([UC] PKT-R v KT-R, P = .13, P < .005, and P < .05, respectively; PKT-R v controls, all P < .005); HDL 2 was enriched in phospholipids; and LDL was depleted of phospholipid. KT-R, in contrast, showed no changes in lipoprotein surface composition but a substantial triglyceride enrichment of HDL 2 as compared with PKT-R and healthy controls (both P < .05). LDL size as determined by gradient gel electrophoresis was increased in PKT-R compared with controls ( P < .005). The plasma concentration of cholesteryl ester (CE) transfer protein (CETP), involved also in phospholipid transfer, was increased in both transplant groups compared with healthy controls (both P < .05). Insulin concentrations in fasting plasma were directly related to CETP levels and to the weight-percentage of UC in HDL 3, and inversely to the weight-percentage of phospholipids in LDL (all P < .05). We explain the increase in HDL 2 cholesterol and LDL size in PKT-R by their high lipoprotein lipase (LPL) activity conferring an excellent capacity to clear chylomicron triglycerides. Effective handling of postprandial triglycerides, high HDL 2 cholesterol, and predominance of LDL pattern A, respectively, are established indicators of a low risk of atherosclerosis. However, it is presently unclear what effects the compositional changes on the surface of HDL and LDL may have on cardiovascular risk in clinically stable PKT-R.

Chylomicron fatty acid composition and serum lipid concentrations in subjects fed carpenin or palm oil/palm kernel oil as the major dietary fat

This study, part of a larger study of dietary fats with different saturated fatty acid content, was performed to investigate the effect of caprenin on chylomicron fatty acid composition and postprandial serum lipid concentrations. Caprenin is a triglyceride made of caprylic acid (8:0), capric acid (10:0) and behenic acid (22:0). Thirty subjects were standardized for 3 weeks on a diet providing palm oil/palm kernel oil ( PO PKO ) as the major fat in a 38 energy % fat diet. Then, 15 subjects continued eating PO PKO while 15 switched to caprenin. After 5 weeks subjects consumed a meal with 40 g of test fat; blood was sampled at 0, 2, 4 and 6 hours. Amounts of 22:0 but not 8:0 and 10:0 varied (P<0.05 for time effect) over the 6 hour period in caprenin subjects; proportions of these fatty acids were much lower in chylomicrons than in the caprenin meal while proportions of palmitic acid (16:0) and oleic acid (18:1) were at least 4-fold higher in chylomicron lipids than in the meal. Postprandial concentrations of triglycerides in chylomicrons and serum and of cholesterol in plasma did not differ between dietary groups. These findings indicate 1) very low uptake of 8:0, 10:0 and 22:0 into chylomicrons, 2) a postprandial lipemia after caprenin comparable to that produced by other dietary fats as opposed to a fat-free meal, 3) considerable contribution of endogenously-derived fatty acids to chylomicron lipids and 4) equal effects of saturated fatty acids on pre- and postprandial concentrations of plasma cholesterol.

Quantification of lipoprotein lipase (LPL) by dissociation-enhanced lanthanide fluorescence immunoassay Comparison of immunoreactivity of LPL mass and enzyme activity of LPL

Lipoprotein lipase (LPL) hydrolyses triglycerides in chylomicrons and in very low density lipoproteins. In this study, a new sensitive enzyme immunoassay, the dissociation-enhanced lanthanide fluorescence immunoassay (DELFIA), for the quantification of immunoreactive LPL mass in biological specimens was developed. In the indirect sandwich DELFIA assay polyclonal anti-human or anti-bovine LPL IgGs were used as capture antibodies, monoclonal antibody (mAb) 5D2 and Eu 3+-labelled goat anti-mouse IgG were used as detection antibodies. In the direct sandwich DELFIA assay, mAb 5D2 was used as capture and Eu 3+-labelled mAb 5D2 as detection antibodies. Both, purified bovine and human LPL proteins served as standards in the indirect and the direct DELFIA assay. Standard curves were linear between 0.1 and 1000 ng LPL/ml, assuring the sensitivity of the DELFIAs within this range. Mean values for immunoreactive LPL mass in normal individuals were found to be 40.3 ± 14.4 ng/ml preheparin plasma and 334.1 ± 71 ng/ml postheparin plasma. In patients affected with type I hyperlipoproteinemia 82.4 ± 29.3 ng/ml (postheparin plasma) were determined. Coefficients of inter- and intra-assay variation were 4.3% and 6.2% on average. The correlation coefficient between the indirect and the direct DELFIA technique was 0.9694. The correlation coefficient between immunoreactive LPL mass (estimated by DELFIA) and LPL activity (estimated by the LPL activity assay) was 0.9345. Our data are consistent with the concept that LPL is active as a dimer. Dissociation of the LPL dimer into monomers is tightly coupled to both loss of immunoreactivity and enzyme activity of LPL.

Chylomicron metabolism in rats: lipolysis, recirculation of triglyceride-derived fatty acids in plasma FFA, and fate of core lipids as analyzed by compartmental modelling

Chylomicrons labeled in vivo with [14C]oleic acid (primarily in triglycerides (TG), providing a tracer for lipolysis) and [3H]retinol (primarily in ester form, providing a tracer for the corelipids) were injected into rats. Disappearance of the two labels from plasma and appearance of label in plasma free fatty acids (FFA) were analyzed by compartmental modelling. Both core and TG label distributed into an apparent volume 10-15% larger than the blood volume. Part of this probably represents margination to endothelial-binding-lipolysis sites. An open two-compartmental model for plasma FFA was derived from experiments where unesterified oleic acid complexed to albumin was injected. Applying this model revealed that most of the oleic acid from chylomicron triglycerides mixes with the FFA. The disappearance of chylomicron core label required a model in which the label transfers into a second compartment before it leaves the blood. The rate constant for the transformation was high and predicted that, on average, chylomicron spent less than 2 min in the first compartment. The rate out from the second compartment predicted that about 60% of the core label left blood while, on average, chylomicron retained more than half of its triglyceride molecules, i.e., after rather limited lipolysis. The mechanism by which the core label leaves blood is not clear. Modelling showed that under the assumption that the process is shared by chylomicron triglycerides, about half of them go out by this pathway. Comparing fed and fasted rats, the main differences were in the turnover of FFA and in the extent to which chylomicron TG label reappeared in the FFA. This study indicates that a large fraction of the triglycerides in chylomicrons leave plasma together with the core lipids and that most of the fatty acids from chylomicron triglycerides mix into the same metabolic compartments as do plasma free fatty acids.

ESTIMATES OF CONSERVATION OF 2-PALMITIC ACID DURING DIGESTION AND ABSORPTION OF HUMAN MILK BY BREAST-FED INFANTS. ▴ 1880

Human milk fatty acids contain 20-25% palmitic acid (16:0) with about 70% of the 16:0 esterified at the sn-2 (center) position of the triglyceride (TG). Endogenous lipases hydrolyse fatty acids from the TG 1,3, but not sn-2 position, to give 2-monoglycerides(MG) and unesterified fatty acids. The positioning of 16:0 at the 2 position of human milk TG may facilitate 16:0 absorption as a 2-MG; unesterified 16:0 tends to form insoluble soaps with minerals, eg Ca2+, Mg2+ the intestine. However, the milk enzyme bile salt stimulated lipase can hydrolyze milk TG to glycerol and unesterified fatty acids in vitro. Thus, the pathway for absorption of 16:0 (2-MG or free fatty acid) is uncertain. These studies, therefore, compared the enrichment of 16:0 at the sn-2 position of the plasma chylomicron (CM) TG from full-term infants who were exclusively breast-fed or fed formula with synthesized TG (Betapol) containing 22% 16:0 with 31% 16:0 in sn-2 fatty acids, to 4 mths of age. Blood (3mL) was collected and CM isolated from plasma at d = 1.006g/mL by ultra-centrifugation(2 hours, 435,000g). Lipids were extracted, TG purified and 2-MG prepared using pancreatic lipase. TG and 2-MG fatty acids were analyzed by capillary GLC. Infants fed formula with Betapol had 15.0% and breast-fed infants had 26.3% 16:0 in the CM TG sn-2 fatty acids. As the formula contained no bile salt stimulated lipase, the enrichment of 16:0 at the sn-2 position indicates the monoacylglycerol pathway (re-esterification of absorbed 2 MG) accounted for synthesis of 50% of the CM TG. The recovery ofsn-2 16:0 in the CM TG of breast-fed infants was also 50% of the amount in the milk. This suggests 16:0 is not quantitatively released from the 2 position of human milk, in vivo, in breast-fed infants. Supported by MRC Canada.

Lipoprotein lipase and lipolysis: central roles in lipoprotein metabolism and atherogenesis

Although it has been known for over 50 years that lipoprotein lipase (LPL) hydrolyzes triglyceride in chylomicrons, during the past half decade there has been a reinterest in the physiologic and pathophysiologic actions of this enzyme. In part, this has coincided with clinical studies implicating increased postprandial lipemia as a risk factor for atherosclerosis development. In addition, the recent creation of genetically altered mice with hypertriglyceridemia has focused the interest of geneticists and physiologists on the pathophysiology of triglyceride metabolism. As reviewed in this article, it is apparent that the lipolysis reaction is only partially understood. Several factors other than LPL are critical modulators of this process, in part, because the reaction requires the lipoproteins to interact with the arterial or capillary wall. Among the factors that affect this are the apolipoprotein composition of the particles, the size of the lipoproteins, and how LPL is displayed along the endothelial luminal surface. Zilversmit's observation that LPL activity is found in greater amounts in atherosclerotic than normal arteries has led to a large number of experiments linking LPL with atherogenesis. In medium and large arteries LPL is found on the luminal endothelial surface and in macrophage-rich areas within the plaque. LPL actions in both of these locations probably have major effects on the biology of the blood vessel. Possible atherogenic actions for this LPL based on in vitro experiments are reviewed.

A frequently occurring mutation in the lipoprotein lipase gene (Asn291Ser) results in altered postprandial chylomicron triglyceride and retinyl palmitate response in normolipidemic carriers.

An Asn291Ser mutation in exon 6 of the lipoprotein lipase gene (LPL) frequently occurs in Caucasians (2-4%) and results in a partial catalytic defect. Although this mutation may be associated with low HDL cholesterol and elevated triglyceride levels, some carriers are normolipidemic and may have LPL activity in the normal range in the fasting state. To assess in vivo the influence of dietary stress on the function of this mutation, we have performed oral fat load studies on three unrelated normolipidemic Asn291Ser carriers and compared these results to five healthy controls and to a subject with a clear 50% reduction in LPL activity compared with controls. The Asn291Ser carriers exhibited a more pronounced postprandial response compared with non-carriers as evidenced by higher chylomicron triglyceride (TG) and chylomicron retinyl palmitate peaks (P = 0.03 and P = 0.02, respectively). Significantly higher area under response curves were also seen for both chylomicron triglycerides (P = 0.02) and chylomicron retinyl palmitate (P = 0.01) when compared with non-carriers. These results provide further in vivo evidence for the functional effects of this common mutation despite normal fasting lipid levels. These data suggest that even though subjects with this mutation may be normolipidemic in the fasting state, environmental stress such as an oral fat load may unmask the catalytic defect and result in significant disturbances in postprandial chylomicron metabolism.

Intravenous lipid emulsions: removal mechanisms as compared to chylomicrons.

We have compared the metabolism of chylomicrons and a labeled emulsion, similar to those used for parenteral nutrition. Both were labeled in their triglyceride moieties and by a core label. It is known that chylomicron triglycerides are cleared by two processes: removal of triglycerides from the particles through lipolysis and removal of whole or partly lipolyzed particles. It has been proposed that emulsion droplets are cleared by the same pathways. After intravenous injection to postprandial rats, triglycerides were cleared less rapidly from the emulsion than from the chylomicrons (half-lives of 6.4 and 4.0 min), whereas the core labels were cleared at the same rate (half-lives around 7.5 min). This suggests that there was less lipolysis of the emulsion droplets which was further supported by the finding that less label appeared in the plasma free fatty acids (FFA). In adipose tissue of fed rats given chylomicrons, the ratio between fatty acid and core label was above 6, showing that fatty acids had been taken up after lipoprotein lipase-mediated hydrolysis. In contrast, for rats given emulsion, that ratio was only 1.2 showing that nearly as much emulsion droplets as emulsion-derived fatty acids were present in the tissue. In the liver the ratio was 0.55 after chylomicrons but 0.93 after emulsion. In further support of more lipolysis, fatty acids were oxidized more rapidly from chylomicrons than from emulsion. These data suggest that a large fraction of the emulsion droplets was removed from plasma with little or no preceding lipolysis. A substantial proportion, more than 50%, of this uptake occurred in extrahepatic tissues.

Use of oral and intravenous fat tolerance tests to assess plasma chylomicron clearance in dogs

The reduced clearance of chylomicrons from plasma results in an exaggerated post prandial lipaemia and fasting hypertriglyceridaemia. This study evaluated whether oral and intravenous fat tolerance tests are appropriate for the in vivo analysis of chylomicron clearance in dogs. Plasma and chylomicron triglyceride concentrations were measured in eight beagles after the administration of a cream-based meal of 2·35 g fat kg −l bodyweight. The changes in each parameter were determined chiefly by the activity of lipoprotein lipase, which was measured in plasma collected after the intravenous injection of heparin and did not appear to be influenced by intestinal fat absorption. The inclusion of retinyl palmitate in the meal provided additional information on the metabolic fate of chylomicron remnants. After the intravenous injection of 0·1 g Intralipid kg −1 bodyweight, there was an initial linear decay in plasma triglyceride concentrations that represented the maximal elimination rate K 1. This was followed by a second exponential component so that the plasma triglyceride concentration returned to baseline by 60 minutes. Lipoprotein lipase was the major determinant of K 1 and the area under the curve of plasma triglycerides.

Trafficking of Dietary Fat in Lean Rats

Despite increasing interest in the role that fuel partitioning plays in determining body composition, the relative importance of oxidative versus storage pathways in the clearance of dietary fat remains unclear. A widely held view is that the primary destination of chylomicron triglyceride fatty acids (TGFA) is adipose tissue, and the primary source of lipid fuel for skeletal muscle is non-esterified fatty acids (NEFA). An alternate view is that muscle, not adipose tissue, is the primary site of TGFA clearance. This view is supported by estimates of the total lipoprotein lipase content of muscle and adipose tissue. To directly study the partitioning of dietary fat between oxidation and storage, 14C-labeled oleic acid was fed to Sprague Dawley rats and its metabolic rate followed over 30 days. Two hours after ingestion, more than 3.5 times as much label was found in skeletal muscle tissue (2.42 +/- 0.45 nmols) and CO2 (0.25 +/- 0.01 nmols) than was found in adipose tissue (0.71 +/- 0.14 nmols). Intramuscular triglyceride was the lipid class most extensively labeled. After skeletal muscle, liver was the next most important site of TGFA clearance. Surprisingly a substantial quantity of label remained associated with the GI tract even 24 hours after ingestion. Between 2 and 10 days following ingestion there was a net decline in the 14C content of muscle, liver and GI tract, associated with a net rise in the 14C content of adipose tissue. These findings demonstrate: 1) the importance of skeletal muscle and liver in whole organism TGFA clearance, 2) the importance of intramuscular partitioning of lipid fuels between direct oxidation and storage as TG, 3) the potentially important role of the GI tract in the delivery of dietary fat to the circulation 10-24 hours following ingestion, and 4) the stability of adipose tissue as a storage site. The complex nature of the tissue-specific clearance of TGFA over time is perhaps better described by the term "trafficking" than by the more commonly used term "partitioning." Future studies of TGFA clearance combined with sampling of relevant tissues over time will provide insight into the specific roles that abnormalities in liver, muscle and adipose tissue TGFA metabolism play in the development of hypertriglyceridemic disorders and states of increased or reduced body weight.

Pea Fiber Lowers Fasting and Postprandial Blood Triglyceride Concentrations in Humans ,

The effect of a pea cell wall fiber preparation with a high content of soluble fiber on fasting and postprandial blood lipids was investigated in young healthy subjects. Inclusion of 33 g pea fiber product/10 MJ (20 g dietary fiber) in a low fiber diet was tested in five men and six women (mean age 23 y) in a strictly controlled randomized cross-over intervention study over 2 wk separated by a 2-wk period of habitual diet consumption. No significant differences in fasting concentrations of total cholesterol, LDL cholesterol or HDL cholesterol were observed, whereas total and VLDL triglyceride concentrations were lower when subjects consumed the pea fiber diet compared with the low fiber diet (P < 0.05). Postprandial response to pea fiber was studied in eight men. Addition of 12 g pea fiber product/10 MJ to a breakfast meal and 15 g/10 MJ to the following lunch meal resulted in significantly lower total triglyceride (P = 0.01), chylomicron triglyceride (P = 0.03) and insulin (P = 0.003) concentrations after the lunch meal compared with results following the same meal without pea fiber. No differences were observed in glycemic response. In conclusion, this source of dietary fiber lowered fasting and postprandial triglyceride concentrations but did not change fasting cholesterol concentrations.

Interactions between fatty acids and lipoprotein lipase: specific binding and complex formation.

Lipoprotein lipase is the extrahepatic lipase responsible for the hydrolysis of triglycerides in chylomicrons and very low-density lipoproteins. Its enzymic activity and its location on the surface of endothelial cells are affected by the presence of fatty acids, implying that the protein possesses binding sites for that ligand. In this study, we examine the binding of fatty acids to LpL and describe factors that must be considered when the dissociation constant of the acceptor-ligand equilibrium is close to the critical micelle concentration of the fatty acid. The interaction of fatty acids with lipoprotein lipase (LpL) was studied by two methods. A new direct method, based on the LpL-induced increase in the apparent critical micelle concentration of the sodium soap of the fatty acid, indicates the presence of multiple high-affinity binding sites. In the second method, the specific binding of fatty acids to LpL was measured by quantitating the blue shift in the tryptophan fluorescence of LpL that occurs upon binding the ligand. Both methods suggest the existence of 4-6 fatty acid binding sites on LpL with a dissociation constant on the order of 10(-6)-10(-7) M. Further analysis of the blue shift indicates that at higher concentrations of fatty acid, large complexes are formed consisting of 260-310 molecules of fatty acid per LPL monomer. In contrast, no large complexes are formed with fatty acids that form crystals above their solubility limit.

Effects of moderate amounts of emulsified dietary fat on postprandial lipemia and lipoproteins in normolipidemic adults

Eight normolipidemic males ingested a meal containing either 42 g fat or 31 g fat in the form of emulsions (9.0 and 9.2 m2) and a fixed amount of retinyl palmitate. Fasting and postmeal blood samples were obtained for 7 h. Serum and chylomicron triglyceride responses were related to the amount of fat ingested and peaked after 2-3 h. The chylomicron retinyl palmitate response was lower (P < or = 0.05) with the 31-g fat supply. After the 42-g fat intake, but not after the 31-g fat intake, serum free cholesterol and phospholipids increased and esterified cholesterol decreased postprandially. Significantly different responses were observed after both meals for low-density-lipoprotein (LDL) free cholesterol, very-low-density-lipoprotein (VLDL) and LDL esterified cholesterol, and high-density-lipoprotein (HDL) phospholipids. These data show that ingesting 31 g instead of 42 g fat in a meal reduces postmeal lipoprotein variations and suggest that a threshold level of dietary fat should be overcome to promote significant postprandial changes in lipoprotein particles.

Simultaneous measurements of chylomicron lipolysis and remnant removal using a doubly labeled artificial lipid emulsion: studies in normolipidemic and hyperlipidemic subjects.

An artificial chylomicron-like lipid emulsion doubly labeled with tri[(N)3H]oleoylglycerol ([3H]TO) and cholesteryl [1-14C]oleate ([14C]CO) was infused intravenously into human subjects with the purpose of simultaneously measuring the plasma disappearance rates (residence time, RT) of [14C]CO, which represents solely the splanchnic organ uptake of the remnant chylomicron core, and of [3H]TO, which combines the remnant disappearance with the shedding off of chylomicron triglycerides by the action of lipoprotein lipase. Thus, the fraction of the particle triglyceride content that is removed before the remnant is taken up is expressed as a delipidation index (DI = 1 - RT of [3H]TO/RT of [14C]CO. The present procedure has an advantage over the use of chylomicrons labeled with retinyl ester or radioactive triglycerides alone that represent, respectively, the chylomicron remnant or the whole particle metabolism only. When normal subjects as well as primary hyperlipidemic subjects were studied, the plasma triglyceride concentration was directly related to [14C]CO RT and [3H]TO RT, but inversely related to the delipidation index. There may be different patterns of relations between these parameters of chylomicron metabolism in primary and in secondary hyperlipidemias, as well as under the action of drugs that influence the metabolism of lipoproteins.

Biphasic effects of low-molecular-weight and conventional heparins on chylomicron clearance in rats.

Chylomicrons labeled in vivo with [14C]triglycerides and [3H]retinyl esters were injected in rats at a series of times after administration of conventional unfractionated heparin (UFH), low-molecular-weight heparin (LMWH), or saline. In saline controls the clearance of both chylomicron triglycerides and retinyl esters seemed to follow exponential courses, with half-lives of about 5 and 10 minutes, respectively. Five minutes after administration of LMWH or UFH, the triglyceride clearance rates were dramatically increased and were associated with an increased appearance of the radiolabel in circulating free fatty acids (FFAs). The clearance of [3H]retinol radioactivity, ie, chylomicron particles, was also enhanced 5 minutes after heparin injection. From 75% to 90% disappeared from the circulation within the first 5 minutes. Their continued disappearance was much slower, with a slope similar to that of the saline-treated rats. Hence, it was as if a new, rapid exponent had been added to the disappearance curve that accounted for most of the particle clearance. Injection of chylomicrons 1 hour after the heparins resulted in substantially slower clearance compared with saline-treated controls of both triglyceride and retinol radioactivity in rats given a high dose of LMWH or a low dose of either heparin. Appearance of label in plasma FFAs was also decreased, suggesting that impeded lipolysis was responsible, at least in part, for the impeded chylomicron clearance. Four and 24 hours after heparin injection all studied parameters of chylomicron clearance had returned to normal.(ABSTRACT TRUNCATED AT 250 WORDS)

Chylomicrons enhance endotoxin excretion in bile

Chylomicrons prevent endotoxin toxicity and increase endotoxin uptake by hepatocytes. As a consequence, less endotoxin is available to activate macrophages, thereby reducing tumor necrosis factor secretion. To determine whether the chylomicron-mediated increase in hepatocellular uptake of endotoxin results in increased endotoxin excretion into bile, we examined bile after endotoxin administration. A sublethal dose (7 micrograms/kg) of 125I-endotoxin was incubated with either rat mesenteric lymph containing nascent chylomicrons (500 mg of chylomicron triglyceride per kg of body weight) or an equal volume of normal saline (controls) for 3 h and then infused into male Sprague-Dawley rats. Bile samples were collected via a common bile duct catheter for 24 h. Infusion of endotoxin incubated with chylomicrons increased biliary excretion of endotoxin by 67% at 3 h (P < or = 0.006) and by 20% at 24 h (P < or = 0.01) compared with infusion of endotoxin incubated in saline. Endotoxin activity, as measured by the Limulus assay, was not detected in the bile of test animals. However, endotoxin activity was detected after hot phenol-water extraction of bile, demonstrating that endotoxin is inactive in the presence of bile but retains bioactivity after hepatic processing. Since the majority of an intravenous endotoxin load has been shown to be cleared by the liver, acceleration of hepatocyte clearance and biliary excretion of endotoxin may represent a component of the mechanism by which chylomicrons protect against endotoxin-induced lethality.