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PubMed1994

Transcriptional regulation of lipoprotein lipase in the heart during development in the rat.

Lipoprotein lipase (LPL) is differentially expressed in fat and heart under a variety of physiological conditions, particularly during development. LPL activity, protein and mRNA levels have been shown to rise dramatically during the first three weeks of life. The mechanism responsible for these changes in LPL in the heart during development are not fully understood. In order to address the changes in LPL in the heart during development, we measured steady state levels of LPL mRNA and the rate of LPL transcription during development. Northern blot analysis of LPL mRNA in the heart of rats from 5-120 days of age showed no changes in transcript size; however, steady state levels of LPL mRNA increased approximately 10-fold by day 20 and remained elevated thereafter. The rate of LPL transcription increased dramatically between day 5-20 and remained elevated thereafter. Thus, as opposed to other settings where posttranscriptional and posttranslational mechanisms can be important, the increased expression of LPL in rat heart that occurs during development appears to be largely due to the transcriptional activation of the gene.

Animals↗
PubMed1987

Isolation of two distinct activator proteins for lipoprotein lipase from ovine plasma.

Two distinct activator proteins for lipoprotein lipase (LPL) have been isolated in approximately equal amounts from ovine plasma. These low molecular weight proteins were readily separated from each other on the basis of size and charge. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis indicated proteins of Mr about 8000 and 5000, with pI in urea-containing gels of about 5.1 and 4.8 respectively. Each of the ovine activator proteins was as effective as human apolipoprotein C-II (apo C-II) in activating LPL, with 1 microgram/ml giving near to maximum activation, and in lowering the apparent Km of LPL for triolein substrate. As the ratio of activator to triolein increased from 0.16 to 5.2 (micrograms/mg) the apparent Km fell from about 0.5 to 0.18 mM. Whereas human apo C-II and the two ovine activators were equally effective in stimulating the hydrolysis of triolein, differences were observed between the human and ovine activators when p-nitrophenylbutyrate was used as substrate. Unlike human apo C-II, which produced significant inhibition of p-nitrophenylbutyrate hydrolysis, the ovine activators were without effect. This suggests that the interaction between the ovine activators and LPL is different from that of human apo C-II.

Animals↗
PubMed1985

Decreased cardiac lipoprotein lipase activity in rats treated chronically with adriamycin.

Rats treated chronically with the anticancer agent adriamycin (1.5 mg/kg/week X 14 weeks) exhibited cardiac and renal lesions typical of anthracycline toxicity, and had serum hyperlipidemia characterized by 4 to 10 fold elevations in all lipoprotein classes. Heparin-releasable lipoprotein lipase activity measured in perfused heart preparations was decreased 69% in adriamycin-treated rats compared to saline-treated controls. Residual (non-heparin-releasable) activity was not significantly different after adriamycin treatment. The decrease in functional cardiac lipoprotein lipase may account, at least in part, for the serum hyperlipidemia observed in adriamycin-treated rats, and might play a role in the development of heart muscle disease.

Animals↗
PubMed1998

Identification of a heparin-binding domain in the distal carboxyl-terminal region of lipoprotein lipase by site-directed mutagenesis.

The interaction of lipoprotein lipase (LPL) with heparan sulfate proteoglycans plays an important role in the metabolism and catalytic function of the enzyme. We have used site-directed mutagenesis to replace the basic residues contained in a discontinuous charge cluster (residues Lys 321, Arg 405, Arg 407, Lys 409, Lys 415, and Lys 416) of avian LPL with asparagine. The mutant proteins were expressed in Chinese hamster ovary cells and their affinity for heparin was evaluated by heparin-Sepharose chromatography. Mutation of residues Lys 321, Arg 405, Arg 407, Lys 409, and Lys 416 resulted in a decrease in affinity for heparin. The triple mutant LPL(R405N, R407N, K409N) possessed almost no high-affinity binding. The LPL mutants showed enzymatic activities ranging between 50-100% of that seen for wild-type LPL demonstrating that the overall structure of the enzyme was not significantly altered by the mutations. Mutation of previously identified heparin-binding regions of LPL results in a relatively small decrease in heparin-binding affinity, as compared with mutations in this carboxyl-terminal region, indicating that Lys 321, Arg 405, Arg 407, Lys 409, and Lys 416 constitute the major heparin-binding domain in LPL.

Amino Acid Sequence↗
PubMed2006

Preheparin serum lipoprotein lipase mass might be a biomarker of metabolic syndrome.

Lipoprotein lipase mass in preheparin serum (preheparin LPL mass) is assumed to reflect some of the LPL production in the whole body and insulin sensitivity. While metabolic syndrome is a common underlying condition for cardiovascular diseases, biological marker of this syndrome has not been fully established. To clarify the characteristics of preheparin LPL mass in metabolic syndrome, 362 Japanese subjects were studied to examine the relationship between symptoms of metabolic syndrome and preheparin LPL mass and compare with plasma adiponectin. Furthermore the relation with urinary 8-hydroxydeoxyguanosine (8-OHdG) that reflects oxidative stress to DNA was also studied. Both preheparin LPL mass and plasma adiponectin correlated positively with HDL-cholesterol and negatively with body weight and triglyceride. Only preheparin LPL mass showed a negative correlation with fasting blood glucose and HbA1c. Both mean preheparin LPL mass and plasma adiponectin decreased with an increase in severity of the metabolic syndrome with/without obesity and with/without diabetes. The correlation coefficient between preheparin LPL mass and plasma adiponectin was r=0.562. A negative correlation between preheparin LPL mass and urinary 8-OHdG was observed. These results suggest that low preheparin LPL mass may reflect systemic oxidative stress and also a biomarker of the severity of metabolic syndrome.

Adiponectin↗
PubMed2006

Catalytically inactive lipoprotein lipase overexpression increases insulin sensitivity in mice.

Abnormalities in lipoprotein lipase (LPL) function contribute to the development of hypertriglyceridemia, one of the characteristic disorders observed in the metabolic syndrome. In addition to the hydrolyzing activity of triglycerides, LPL modulates various cellular functions via its binding ability to the cell surface. Here we show the effects of catalytically inactive LPL overexpression on high-fat diet (HFD)-induced decreased systemic insulin sensitivity in mice. The binding capacity of catalytically inactive G188E-LPL to C2C12 skeletal muscle cells was not significantly different from that of wild type LPL. Insulin-stimulated IRS-1 phosphorylation and glucose uptake were increased by addition of wild type or mutant LPL in C2C12 cells. After 10 weeks' of HFD feeding, mice had significantly higher blood glucose levels than chow-fed mice in insulin tolerance tests. The blood glucose levels after insulin injection was significantly decreased in mutated LPL-overexpressing mice (G188E mice), as well as in wild type LPL-overexpressing mice (WT mice). Overexpression of catalytically inactive LPL, as well as wild type LPL, improved impaired insulin sensitivity in mice. These results show that decreased expression of LPL possibly causes the insulin resistance, in addition to hypertriglyceridemia, in metabolic syndrome.

Animals↗
PubMed1980

[Lipoprotein-lipase-activity in human milk (author's transl)].

Lipoprotein-lipase-activity has been determined in human milk between the 3rd and the 10th post-partum days in women from different ethnic origins. This activity is low in the colostrum, it increases at the 4th and 5th days and remains stable between the 6th and the 10th day. There are important variations of the LPL-activity from one woman to another. Two variation-factors have been noticed. The first is an ethnic factor. It is apparent event if the diet is exactly the same in all women. In the milk, indeed, the activity of French women is much lower. It is about three times less than the activity of Yugoslavian of African women. The second is the galactogenic powder that has been estimated by the natural duration of lactation. We can see that the activity of women, who have to stop breast-feeding because of insufficient milk, is lower than the activity of women who can decide how long they will breast-feed their child. We took as a hypothesis that the milk is a means of elimination for the mammary-LPL. This enzyme is supposed to have done first its physiological action at the surface of mammary glandular cells. Ours results show more facts in favor of this hypothesis.

Adult↗
PubMed1984

Exercise activation of myocardial lipoprotein lipase in male and estrogen-treated female rats.

Lipoprotein lipase (LPL) is the rate-limiting enzyme in the utilization of plasma triglycerides by extrahepatic tissues. The present study examined the effect of estrogen upon the circadian rhythm of rat myocardial LPL and upon the endothelial-bound and tissue components of rat myocardial LPL immediately after exhaustive exercise. Female rats were ovariectomized at 28 days. Estrogen replaced female rats (ER) received 17-beta-estradiol in sunflower oil (200 micrograms/100 g body weight/week for 4 weeks). Oil-injected female rats (OI) received sunflower oil (0.1 mL/100 g body weight/week for 4 weeks). Normal male rats (M) were studied for comparison. ER, OI, and M animals were killed every four hours starting at 8 AM. Estrogen treatment did not alter the circadian rhythm of LPL activity. A second group of animals were run to exhaustion on a motorized treadmill, starting at 9 AM. ER animals performed significantly more work (P less than 0.05) than OI or M rats. Liver glycogen concentration was reduced 96.8% to 97.8% from control in all three groups after exercise. Both the tissue and endothelial fractions of LPL activity were elevated after exercise in all three groups. Exercised ER females (116.3 +/- 9.0 units) had significantly higher cardiac tissue enzyme activity than exercised males (85.0 +/- 9.4 units). Heparin-releasable LPL activity in exercised ER rats (65.3 +/- 5.7 units) was 43% higher than in exercised M rats (45.5 +/- 5.6 units) and 53% higher than in OI female rats (42.6 +/- 8.1 units).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1996

Hormonal regulation of lipoprotein lipase activity from 5-day-old rat hepatocytes.

Lipoprotein lipase (LPL) activity is known to be synthesized, active and functional in the 1-day-old rat liver: it peaks just at birth triggered by parturition. During suckling LPL mRNA, LPL synthesis and LPL activity are still high at 5 days and then fade reaching adult values at weaning. How LPL expression is gradually extinguished is not known. Therefore we studied the effect of different doses of several hormones on LPL activity released by incubated hepatocytes from 5-day-old rats. In the presence of heparin the release of LPL activity in the medium was linear until 3 h and was always significantly increased vs. without heparin. At 3 h in the presence of heparin the main hormonal effects were: dose-dependent increase (30-60%) with dexamethasone; dose-dependent increase (20-60%) with glucagon; dose-independent decrease (50-60%) with ethinylestradiol, testosterone, progesterone and prolactin; no effect with insulin; 20-40% increase with adrenaline < 1 mM but 40-50% decrease with noradrenaline < 10 microM. Increase of LPL release by glucagon and adrenaline agrees with the increased LPL expression we previously found in an undifferentiated hepatoma cell line when the adenylate cyclase/protein kinase A pathway was activated. The effect of glucagon is concordant with our previous observations that fasting increases liver LPL activity in neonatal rats. The high estradiol levels known to be present in male and female 9-19-day-old rats might contribute to liver LPL extinction during suckling.

Animals↗
PubMed2003

Global structure and dynamics of human apolipoprotein CII in complex with micelles: evidence for increased mobility of the helix involved in the activation of lipoprotein lipase.

Apolipoprotein CII (apoCII), a surface constituent of plasma lipoproteins, is the activator for lipoprotein lipase (LPL) and is therefore central for lipid transport in blood. The three-dimensional structure of (13)C-, (15)N-enriched human full-length apoCII in complex with sodium dodecyl sulfate (SDS) micelles is reported. In addition to the structure determination, (15)N-relaxation measurements have been performed at two magnetic fields to characterize the dynamics of the backbone of apoCII in the complex. The relaxation data also provided global structural constraints, viz. the orientation of helices in the complex. In addition, global constraints were derived from the fact that apoCII helices are attached to the surface of the SDS micelle and that the hydrophobic moments of each helix faces the interior of the micelle. These three categories of global constraints, together with the local classical NMR constraints, were sufficient to define the 3D structure of the apoCII-SDS micelle complex. To our knowledge, this presents the first example in which the global structure of a protein-SDS micelle complex has been determined. The C-terminal helix of apoCII is known to be responsible for the activation of LPL. This helix is distinguished from the other helices by a higher degree of internal motion on the nanosecond time scale as shown by the relaxation data. The overall structure and the internal dynamics, combined with previous mutation data, give important clues toward a possible mechanism for the activation of LPL by apoCII.

Amino Acid Sequence↗
PubMed1990

Increasing effect of vanadate on lipoprotein lipase activity in isolated rat fat pads.

Vanadate increased lipoprotein lipase (LPL) activity in the isolated fat pads in a time- and dose-dependent manner. The increasing effect of vanadate was inhibited by amiloride, similar to that of insulin, and it also was not additive to that of insulin. Although the increasing effects of vanadate and insulin were preserved in K(+)-free medium, appreciable decreases in both effects were observed by replacement of Na+ with choline ion or omission of Ca2+ in the medium. Vanadate showed the full effect in the presence of cycloheximide at concentrations that inhibited protein synthesis of the fat pads, suggesting that the action of vanadate is not due to the increase in protein synthesis. Tetrakis (acetoxymethyl) ester of quin 2 at 50 microM concentration never inhibited the action of vanadate though it showed a little inhibition at a concentration of 300 microM. No inhibition of the action of vanadate was observed with ruthenium red. These results suggest that vanadate increases the LPL activity via a process less sensitive to the intracellular Ca2+ concentration. Adrenaline, dibutyryl cyclic AMP, and 3-isobutyl-1-methylxanthine all inhibited the action of vanadate, suggesting that the action is inhibited with increase in the intracellular concentration of cyclic AMP. Monensin and carbonyl cyanide m-chlorophenylhydrazone inhibited the action of vanadate. In contrast, the action of insulin was never inhibited by monensin. Tunicamycin and 2-deoxyglucose, at rather high concentrations, inhibited both actions. These findings suggest that vanadate increases the LPL activity through mechanisms of action involving amiloride- and monensin-sensitive pathways dependent on energy.

Adipose Tissue↗
PubMed1994

Expression of lipoprotein lipase in rat muscle: regulation by feeding and hypothyroidism.

Lipoprotein lipase (LPL) is a key enzyme in lipid metabolism and is found predominantly in adipose tissue and muscle. We examined the mechanism of regulation of LPL in muscles composed of different fiber types (soleus, extensor digitorum longus, and heart) in fed, fasted, and hypothyroid rats. In all muscles, the detergent-extractable (EXT) fraction represented approximately 95% of total LPL activity and mass. LPL activity was similar in the heparin-releasable (HR) fractions of heart and soleus (predominantly type I fibers), while in the EXT fraction LPL activity in soleus was 418 +/- 48 nEq/min per g, and in heart was 272 +/- 30 nEq/min per g (P < 0.05). However, LPL activity in extensor digitorum longus (EDL, predominantly type II fibers) was considerably lower (7.9 +/- 0.8 nEq/min per g in EXT, P < 0.0001 versus heart and soleus). LPL immunoreactive mass followed a pattern similar to LPL activity. LPL mRNA levels were quantitated by both Northern blotting and reverse transcriptase-polymerase chain reaction (RT-PCR), and were approximately equal in heart and soleus, and 5-fold lower in EDL. In response to feeding, LPL activity, mass, and mRNA levels in heart were 30% to 50% lower than in fasted rat heart, although feeding had no effect on soleus or EDL. In hypothyroid animals, muscle LPL activity was increased by 3- to 4-fold in the HR (but not EXT) fractions of heart and soleus (P < 0.05), with no change in LPL mass or mRNA. Thus, muscles with oxidative, type I fibers expressed higher levels of LPL mRNA than muscles containing glycolytic, type II fibers.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1996

Regulatory effects of lipoprotein lipase on proliferative and cytotoxic activity of NK cells.

Lipoprotein lipase (LPL) induced, in a dose-dependent fashion, a 2-fold and 11-fold increase in the proliferative response of peripheral blood lymphocytes (PBL) at 48 and 72 h, respectively; a 4- and 12-fold increase in natural killer (NK) cells, respectively; and a maximal 3-fold induction in interleukin-2 (IL-2)-treated NK cells at 72 h. T lymphocytes did not proliferate independently of the concentration of LPL used. LPL decreased the proliferative response of K562 and U937 cell lines. The effect on NK cells could be blocked by anti-LPL if it was added before LPL binding to the cell membrane. Contrary to its effects on NK proliferative response, LPL inhibited spontaneous cytotoxicity and lymphokine-activated killer activity (LAK). The effect was dose-dependent, target-dependent (U937 was more sensitive than K562 in LAK assays), but not LPL-binding time-dependent. Treatment of NK cells with heparinase overcame the inhibitory effect of LPL in spontaneous cytotoxicity. LPL binding to cell membranes, as assessed by flow cytometry, was as follows: K562 cells > monocytes > NK cells > LAK cells > U937 cells, absent in T lymphocytes and partially sensible to heparinase and IL-2 treatments. Protein kinase C translocation was observed upon treatment of NK cells with LPL. Three proteins in NK cell membrane (76, 57.2, and 27.2 kD), two in the cytosol (57.2 and 27.2 kD), and only one in ANA-1 cell membrane (76 kD) were precipitated with LPL-Sepharose. LPL receptors seem to be responsible for the proliferative and cytotoxic response observed in LPL-stimulated NK cells.

Cell Membrane↗
PubMed2005

Role of A kinase anchor proteins in the tissue-specific regulation of lipoprotein lipase.

Activation of protein kinase A by catecholamines inhibits lipoprotein lipase (LPL) activity through the elaboration of an RNA binding complex, which inhibits LPL translation by binding to the 3'-untranslated region of the LPL mRNA. To better define this process, we reconstituted the inhibitory RNA binding complex in vitro and demonstrated that the K homology (KH) domain of A kinase anchor protein (AKAP) 121/149 plays a vital role in the inhibition of LPL translation. Inhibition of LPL translation occurred in vitro only when the Calpha subunit, R subunit, and AKAP 149 were present. Using different glutathione-S-transferase fusion proteins of AKAP 149, sequences containing the KH domain were required for inhibition of LPL translation, and the inhibition of AKAP 121 expression in 3T3-F442A adipocytes with short interfering RNA resulted in loss of epinephrine-mediated translation inhibition. After epinephrine injection into mice, LPL activity was inhibited in white adipose tissue but not in brown adipose tissue (BAT) or muscle. LPL activity and synthetic rate were inhibited in vitro by the addition of epinephrine to 3T3-F442A adipocytes, but there was no effect in L6 muscle cells and cultures of brown adipocytes. Corresponding with these differences in LPL translation, AKAP 121 protein and mRNA were abundantly expressed in mouse white adipose tissue, but was either very low or undetectable in BAT and muscle. Thus, AKAP 121/149 contains a KH region that is essential to the translation inhibition of LPL in response to epinephrine. BAT and muscle do not express significant AKAP 121/149, and this likely explains some of the tissue-specific differences in LPL regulation.

3T3 Cells↗
PubMed2004

Lipoprotein lipase (LPL) mass in preheparin serum reflects insulin sensitivity.

Lipoprotein lipase (LPL) is one of the enzymes regulated by insulin and its plasma activity reflects insulin sensitivity. Although intravenous heparin injection is required to measure LPL activity, we can detect LPL mass in preheparin serum (Pr-LPL mass) by immunoassay. In this study, we examined whether Pr-LPL mass reflects insulin sensitivity. We measured Pr-LPL mass, insulin sensitivity (Si), and acute insulin release in response to a glucose bolus (AIRg) in subjects with normal glucose tolerance (NGT; n = 23), impaired glucose tolerance (IGT; n = 10), and Type II diabetes mellitus (DM; n = 48). Si and AIRg were determined by minimal model analysis. We also compared Pr-LPL mass with the homeostasis model assessment of insulin resistance (HOMA-R) and the urinary excretion of C-peptide (urine CPR). We found that Pr-LPL mass correlated significantly with Si ( r = 0.354, P < 0.01) in all the subjects. This correlation was still significant in the NGT group (P < 0.472, P < 0.05), DM group (r = 0.311, P < 0.01), and DM group with a fasting plasma glucose >150 mg/dl ( n = 20, r = 0.459. P < 0.05). Moreover, Pr-LPL mass correlated negatively with HOMA-R (r = -0.272. P < 0.05) and fasting IRI (r = -0.256, P < 0.05). By contrast, Pr-LPL mass was not correlated with either urine CPR or logAIRg that reflect the ability to secrete insulin. In conclusion, Pr-LPL mass reflects insulin sensitivity. We speculate that Pr-LPL mass might be used to assess insulin sensitivity not only in the general population but also in advanced diabetic patients.

Adult↗
PubMed2004

Effect of flax supplementation and growth promotants on lipoprotein lipase and glycogenin messenger RNA concentrations in finishing cattle.

Lipoprotein lipase (LPL) hydrolyzes triacylglycerols into monoacylglycerol and fatty acids, which are taken up by tissues and used for energy. Glycogenin is the core protein on which glycogen molecules are synthesized. There is one molecule of glycogenin per molecule of glycogen in skeletal muscle; therefore, glycogen storage is limited by the amount of glycogenin present in muscle. The objective of this study was to investigate the effect of feeding flaxseed, a source of PUFA, and administering a growth promoter on steady-state LPL and glycogenin mRNA content of muscle in finishing cattle. Sixteen crossbred steers (initial BW = 397 kg), given ad libitum access to a 92% concentrate diet for 28 d, were used in a four-treatment, 2 x 2 factorial experiment, with flaxseed supplementation (0 or 5% of dietary DM) and implanting (not implanted or implanted with Revalor-S) as the main effects. Muscle biopsies were obtained from the LM at 0, 14, and 28 d, and used to quantify LPL and glycogenin mRNA concentrations using real-time quantitative PCR. Implanting with Revalor-S did not affect LPL (P = 0.13) or glycogenin (P = 0.98) mRNA concentrations. A day x flaxseed interaction (P < 0.001) was observed for both LPL and glycogenin mRNA concentrations. No differences (P > 0.10) were observed between 0 and 5% flaxseed supplemented steers; however, at 28 d, nonflaxseed-fed steers had 4.1- and 5.7-fold increases (P < 0.001) over flaxseed steers for LPL and glycogenin mRNA concentrations, respectively. To further evaluate the effects of alpha-linolenic acid (alpha-LA) on LPL and glycogenin mRNA concentrations, muscle satellite cells were isolated from five finishing steers, and different alpha-LA concentrations were applied in culture. The RNA was isolated from the bovine satellite cells. Addition of alpha-LA numerically increased (P = 0.16) the LPL mRNA concentration 48% at 1 microM alpha-LA compared with the control. The expression of glycogenin was increased (P < 0.05) 50% at 1 microM alpha-LA compared with the control. These results suggest that flaxseed supplementation to finishing steers for 28 d decreased gene expression of both LPL and glycogenin compared with not feeding flaxseed. Alterations in local concentrations of these two proteins could affect the ability of muscle to use fatty acids and glucose for energy, and, ultimately, affect carcass quality.

Animals↗
PubMed2002

Relationship between a novel polymorphism of lipoprotein lipase gene and coronary heart disease.

OBJECTIVE: To investigate polymorphisms in the gene for lipoprotein lipase (LPL) in Chinese populations with coronary heart disease (CHD) and to inquire into the relationship between these polymorphisms in LPL gene and CHD. METHODS: Genomic DNA was extracted from patients with CHD and normal control subjects using a salting out method. The entire coding region and flanking sequences of all coding exons of the LPL gene were amplified by PCR technique and PCR products were detected by denaturing high-performance liquid chromatography (DHPLC) and sequenced with a dideoxy terminal termination method. RESULTS: A novel polymorphic site, G830A, that is within the fifth exon of the LPL gene was found. The 192 codon CGA was changed into CAA and resulted in the substitution of glutamine for arginine. Between the control and CHD groups, chi-square test showed no significant difference in the frequencies of the A/A genotype and A allele (P > 0.05). However, the frequencies of A/A genotype and A allele (0.653 and 0.786) in CHD patients with high plasma triglyceride/lowed plasma high density lipoprotein cholesterol were higher than those (0.415 and 0.642) in CHD patients without hyperlipidemia (P < 0.05). CONCLUSION: No direct association was found between the LPL Arg192-->Gln substitution polymorphism and CHD, but there is a significant positive correlation between the A/A genotype of the LPL gene and CHD associated with high triglyceride/lowed high density lipoprotein cholesterol. This study may provide new data for exploring the molecular mechanism of CHD.

Alleles↗
PubMed1973

Effects of lipoprotein lipase on the structure of chylomicrons.

Chylomicrons isolated from rat lymph were complexed with lipoprotein lipase of post-heparin plasma (chylomicrons-LPL) in order to study the effects of lipolysis on the structure of chylomicrons. Triglyceride in the chylomicron core was readily hydrolyzed to free fatty acids (FFA) and glycerol when chylomicrons-LPL were incubated at pH 8.3 in medium containing albumin. Although most of the FFA were immediately released to the medium, some were retained within chylomicrons when FFA-binding sites on albumin were not available. These observations suggest that albumin may have a specific role in the transfer of FFA across the chylomicron surface film. Chylomicrons-LPL assumed many different shapes as they were depleted of triglyceride by the lipolytic action of the enzyme, and total removal of core triglyceride resulted in empty sacks of surface film. The surface film was visualized in sections of OsO(4)-fixed chylomicrons-LPL as a thin electron-opaque line, 25-30 A wide, in areas where the underlying electron-opaque core had been replaced by zones of decreased electron opacity, and in folds of surface film extending outward from chylomicrons partially depleted of core lipid. The findings demonstrate that chylomicrons consist of a core of liquid triglyceride enveloped by a pliable and durable monolayer surface film, and that lipoprotein lipase reduces the triglyceride core without disrupting the surface film.

Animals↗