Purification and characterization of lipoprotein lipase from human postheparin plasma and its comparison with purified bovine milk lipoprotein lipase

Reproducing high mechanical load during industrial processing of UHT milk: Effect on frothing capacity

In this study, possible reasons for an increased level of free fatty acids (FFAs) in Ultra-High Temperature (UHT) treated full fat (3.5% wt/wt) milk and its effect on the frothing properties of milk were investigated. Lipolysis of raw milk from 2 different breeds of cattle (Holstein and Jersey) was induced by mechanical stress and kinetics of lipolysis were compared. Frothing capacity and foam stability of shelf stable milk with different concentrations of FFAs were determined, with a good to medium initial foam volume for up to 4 mEquiv FFA · (100 g fat)-1 fat and poor foam stability with >2 mEquiv FFA · (100 g fat)-1. A combination of mechanical stress and initial condition of fresh raw milk was found to trigger lipolysis and potential sources of mechanical stress during milk processing were identified.

Structure of dimeric lipoprotein lipase reveals a pore adjacent to the active site

Lipoprotein lipase (LPL) hydrolyzes triglycerides from circulating lipoproteins, releasing free fatty acids. Active LPL is needed to prevent hypertriglyceridemia, which is a risk factor for cardiovascular disease (CVD). Using cryogenic electron microscopy (cryoEM), we determined the structure of an active LPL dimer at 3.9 Å resolution. This structure reveals an open hydrophobic pore adjacent to the active site residues. Using modeling, we demonstrate that this pore can accommodate an acyl chain from a triglyceride. Known LPL mutations that lead to hypertriglyceridemia localize to the end of the pore and cause defective substrate hydrolysis. The pore may provide additional substrate specificity and/or allow unidirectional acyl chain release from LPL. This structure also revises previous models on how LPL dimerizes, revealing a C-terminal to C-terminal interface. We hypothesize that this active C-terminal to C-terminal conformation is adopted by LPL when associated with lipoproteins in capillaries.

Structure of Dimeric Lipoprotein Lipase Reveals a Pore for Hydrolysis of Acyl Chains

Lipoprotein lipase (LPL) hydrolyzes triglycerides from circulating lipoproteins, releasing free fatty acids. Active LPL is needed to prevent hypertriglyceridemia, which is a risk factor for cardiovascular disease (CVD). Using cryogenic electron microscopy (cryoEM), we determined the structure of an active LPL dimer at 3.9 Ã… resolution. This is the first structure of a mammalian lipase with an open, hydrophobic pore adjacent to the active site. We demonstrate that the pore can accommodate an acyl chain from a triglyceride. Previously, it was thought that an open lipase conformation was defined by a displaced lid peptide, exposing the hydrophobic pocket surrounding the active site. With these previous models after the lid opened, the substrate would enter the active site, be hydrolyzed and then released in a bidirectional manner. It was assumed that the hydrophobic pocket provided the only ligand selectivity. Based on our structure, we propose a new model for lipid hydrolysis, in which the free fatty acid product travels unidirectionally through the active site pore, entering and exiting opposite sides of the protein. By this new model, the hydrophobic pore provides additional substrate specificity and provides insight into how LPL mutations in the active site pore may negatively impact LPL activity, leading to chylomicronemia. Structural similarity of LPL to other human lipases suggests that this unidirectional mechanism could be conserved but has not been observed due to the difficulty of studying lipase structure in the presence of an activating substrate. We hypothesize that the air/water interface formed during creation of samples for cryoEM triggered interfacial activation, allowing us to capture, for the first time, a fully open state of a mammalian lipase. Our new structure also revises previous models on how LPL dimerizes, revealing an unexpected C-terminal to C-terminal interface. The elucidation of a dimeric LPL structure highlights the oligomeric diversity of LPL, as now LPL homodimer, heterodimer, and helical filament structures have been elucidated. This diversity of oligomerization may provide a form of regulation as LPL travels from secretory vesicles in the cell, to the capillary, and eventually to the liver for lipoprotein remnant uptake. We hypothesize that LPL dimerizes in this active C-terminal to C-terminal conformation when associated with mobile lipoproteins in the capillary.

Expression and one-step purification of active lipoprotein lipase contemplated by biophysical considerations

Lipoprotein lipase (LPL) is essential for intravascular lipid metabolism and is of high medical relevance. Since LPL is notoriously unstable, there is an unmet need for a robust expression system producing high quantities of active and pure recombinant human LPL. We showed previously that bovine LPL purified from milk is unstable at body temperature (T_m is 34.8 °C), but in the presence of the endothelial transporter glycosylphosphatidylinositol-anchored high-density lipoprotein-binding protein 1 (GPIHBP1) LPL is stabile (T_m increases to 57.6 °C). Building on this information, we now designed an expression system for human LPL using Drosophila S2 cells grown in suspension at high cell density and at an advantageous temperature of 25 °C. We co-transfected S2 cells with human LPL, LMF1 and soluble GPIHBP1 to provide an efficient chaperoning and stabilization of LPL in all compartments during synthesis and after secretion into the conditioned medium. For LPL purification, we used heparin-Sepharose affinity chromatography, which disrupted LPL-GPIHBP1 complexes causing GPIHBP1 to elute with the flow-through of the conditioned media. This one-step purification procedure yielded high quantities of pure and active LPL (4‒28 mg/L). Purification of several human LPL variants (furin-cleavage resistant mutant R297A, active-site mutant S132A, and lipid-binding-deficient mutant W390A-W393A-W394A) as well as murine LPL underscores the versatility and robustness of this protocol. Notably, we were able to produce and purify LPL containing the cognate furin-cleavage site. This method provides an efficient and cost-effective approach to produce large quantities of LPL for biophysical and large-scale drug discovery studies.

Identification of Key Excipients for the Solubilization and Structural Characterization of Lipoprotein Lipase, An Enzyme for Hydrolysis of Triglyceride

Lipoprotein lipase (LPL) is an essential enzyme that hydrolyzes triglycerides in chylomicrons and very low-density lipoprotein into glycerol and fatty acids. One major hurdle in using LPL as a therapeutic has been its poor solubility/stability after purification. Solutions used to preserve purified LPL commonly contain either heparin, or concentrated glycerol and sodium chloride, resulting in hypertonic solutions. These solutions are not acceptable as pharmaceutical formulations. This paper describes the identification of a key excipient, sodium laurate, which can solubilize LPL in an isotonic environment without heparin or concentrated glycerol. A follow-up multi-variant study was performed to identify the effect of sodium laurate and its interaction with sodium chloride on the solubility and processing conditions of LPL. The LPL concentration (up to 14 mg/mL) achievable in pharmaceutically relevant and salt-free conditions was identified to be closely correlated to the concentration of sodium laurate, which was co-concentrated with LPL. The result that sodium laurate increases stability of LPL characterized by differential scanning calorimetry and UV absorbance spectra suggests that the mechanism of solubilization of LPL by sodium laurate is related to LPL structural stabilization. The findings indicate that substrates and their enzymatic products can be strong stabilizers for other protein molecules.

The structure of helical lipoprotein lipase reveals an unexpected twist in lipase storage

Lipases are enzymes necessary for the proper distribution and utilization of lipids in the human body. Lipoprotein lipase (LPL) is active in capillaries, where it plays a crucial role in preventing dyslipidemia by hydrolyzing triglycerides from packaged lipoproteins. Thirty years ago, the existence of a condensed and inactive LPL oligomer was proposed. Although recent work has shed light on the structure of the LPL monomer, the inactive oligomer remained opaque. Here we present a cryo-EM reconstruction of a helical LPL oligomer at 3.8-Å resolution. Helix formation is concentration-dependent, and helices are composed of inactive dihedral LPL dimers. Heparin binding stabilizes LPL helices, and the presence of substrate triggers helix disassembly. Superresolution fluorescent microscopy of endogenous LPL revealed that LPL adopts a filament-like distribution in vesicles. Mutation of one of the helical LPL interaction interfaces causes loss of the filament-like distribution. Taken together, this suggests that LPL is condensed into its inactive helical form for storage in intracellular vesicles.

Two cases with transient lipoprotein lipase (LPL) activity impairment: evidence for the possible involvement of an LPL inhibitor

Two independent severe hypertriglyceridemic infants with transiently impaired lipoprotein lipase (LPL) activity were observed and the causes were explored. Both infants were female, born prematurely with low birth weight and developed hypertriglyceridemia (Fredrickson type V hyperlipidemia: high VLDL and low LDL/HDL) a few months after birth. While mass levels of their post-heparin plasma LPL and apoprotein C-II (apo C-II), a physiological activator of LPL, were normal, their post-heparin plasma LPL activities were remarkably impaired. Both of their mothers' post-heparin plasma LPL activities were slightly or moderately impaired as well, without a decrease in the LPL mass level. No mutations in the genes for LPL and apo C-II were detected in either patient. In an in vitro study with their serum at onset, we could not detect any distinct circulating inhibitors for LPL. There was no data supporting infection or autoimmune diseases, which might have an impact on LPL activity, during the follow-up period. Levels of their plasma triglyceride (TG) and total cholesterol (TC) were decreased quickly by a dietary intervention with medium-chain triglyceride (MCT) milk and kept normal even after stopping the intervention at around age 1 year. However, their low post-heparin LPL activity persisted and returned to normal at around age 2 years. Their low HDL cholesterol levels persisted even after recovery of the TG and TC levels, although lecithin:cholesterol acyltransferase (LCAT) and cholesterol-ester-transfer protein (CETP), two key enzymes of HDL metabolism, were normal throughout the course. The exact reasons why their post-heparin LPL activities were impaired for a certain period and why their HDL cholesterol levels have remained low are still unclear.

CONCLUSION

Transiently impaired LPL activity with no defect in LPL enzyme induced severe hypertriglyceridemia in infants. The transient occurrence of inhibitor(s) for LPL was proposed.

Development and evaluation of a direct sandwich enzyme-linked immunosorbent assay for the quantification of lipoprotein lipase mass in human plasma

OBJECTIVE

The purpose of this study was to develop and evaluate a direct sandwich enzyme-linked immunosorbent assay (ELISA) for quantification of the lipoprotein lipase (LPL) immunoreactive mass in human plasma using monoclonal antibodies (MAbs) directed against LPL purified from human postheparin plasma (PHP) [corrected].

METHODS AND RESULTS

The direct sandwich-ELISA was performed using a combination of two distinct types of MAbs that recognize different epitopes on the LPL molecule. The immunoreactive mass of human LPL was specifically measured using a horseradish peroxidase-labeled anti-human LPL MAb [1(1)D2B2] as an enzyme-linked MAb, and an anti-human LPL MAb [2(10)F8F9] coated on a polystyrene microtiter plate as a solid-phase MAb. Purified human PHP-LPL was used as a standard material. The detection range of the sandwich-ELISA was 3.6-460 ng of LPL protein per mL of plasma. The intra- and interassay coefficients of variation were less than 5.9% and 3.3%, respectively. The validity of this method was additionally assured by the recovery test, which resulted in the variation only between 97.5% and 105.1%, and also by the interference test, which resulted in noninterference of LPL assay with a high concentration of triglyceride, hemoglobin, bilirubin, uric acid, or creatinine. To assess the reliability of the LPL mass values obtained with the direct sandwich-ELISA, they were compared with LPL mass values determined by the one-step sandwich-EIA (MARKIT-F LPL EIA kit) previously established by us. This comparison showed a highly significant correlation (r = +0.990) between the two sets of values. The LPL mass concentrations in PHP from 33 healthy subjects were 267 +/- 53 and 257 +/- 59 ng/mL (mean +/- SD), respectively.

CONCLUSION

The present direct sandwich-ELISA is useful for rapidly identifying certain abnormalities of LPL in PHP samples from patients with hypertriglyceridemia [corrected].

Lipid binding of apolipoprotein CII is required for stimulation of lipoprotein lipase activity against apolipoprotein CII-deficient chylomicrons

Human apolipoprotein CII (apo CII) consists of 79 amino acid residues. The amino-terminal two thirds of the molecule binds to lipid through the formation of amphipathic helixes, while the carboxy-terminal third is engaged in activation of lipoprotein lipase (LPL). On the basis of studies in model systems, it was previously concluded that fragments of apo CII spanning residues 51-79 were sufficient for activation, although they do not bind to lipid. In the present study, we used chylomicrons from an apo CII-deficient patient to reinvestigate this possibility, with a physiologically relevant substrate. Human LPL expressed very low activity against these chylomicrons. Addition of apo CII caused an immediate > 100-fold increase in lipase activity. The apo CII fragment 50-79 caused very little stimulation, though with some synthetic lipid substrates, this fragment was fully effective. LPL bound to the chylomicrons even in the absence of apo CII but apparently in a nonproductive manner. In accord with this finding, the main effect of apo CII was on the VMAX for the reaction, with little or no change in the apparent K(M). We conclude that the lipid-binding part of apo CII is needed for activity of LPL against chylomicrons. This idea is in accord with previous studies with lipid monolayers, which showed that the lipid-binding part is necessary for activation of the enzyme at high surface pressures.

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.

A monoclonal antibody against human lipoprotein lipase inhibiting heparin binding without affecting catalytic activity

A fragment of the human lipoprotein lipase (LPL) cDNA (405 bp, 5' terminal end) was cloned in an expression vector to produce a approximately 17 kDa fusion peptide and was used as antigen to produce a high titre anti-LPL monoclonal antibody (10C3 MAb). This antibody reacts with both native and denatured forms of LPL from different tissue and animal sources. Competition studies with heparin indicate that 10C3 MAb is specific for an epitope at a heparin binding site. The antibody does not inhibit LPL enzyme activity, indicating that the antigenic epitope is not situated within or in the proximity of the LPL catalytic region. With these characteristics, 10C3 MAb should prove to be a useful immunochemical tool in clinical as well as in fundamental investigations on the metabolism of triglyceride-rich lipoproteins and in studies on the functional anatomy of LPL.

Lipoprotein lipase mass and activity in plasma and their increase after heparin are separate parameters with different relations to plasma lipoproteins

Lipoprotein lipase (LPL) activity and mass in plasma and their increase after heparin administration were measured in 61 men who had suffered myocardial infarction before the age of 45 years and in 69 population-based age- and sex-matched control subjects without coronary heart disease to study the relations between these parameters in plasma and their correlations with plasma lipoproteins in subjects with a wide range of lipoprotein and LPL levels. There was a relatively large amount of LPL protein compared with LPL activity in preheparin plasma, indicating that the majority of circulating LPL is catalytically inactive. LPL mass and activity in postheparin plasma (postheparin minus preheparin values) were highly correlated, and the calculated mean specific activity (0.35 mU/ng) was in the range expected for catalytically active LPL. Hence, heparin releases mainly active LPL. The four LPL parameters (mass and activity in plasma and their increase after heparin administration) were not related to each other, except for postheparin plasma LPL mass and activity, and they showed different correlations with plasma lipoprotein lipid concentrations. There was a strong positive correlation between LPL mass in preheparin plasma and the HDL cholesterol level as well as weak negative relations to VLDL triglyceride and cholesterol concentrations in the patients. In contrast, preheparin LPL activity showed no correlation with the HDL cholesterol level but weak positive relations to VLDL triglyceride and cholesterol concentrations in the control subjects.(ABSTRACT TRUNCATED AT 250 WORDS)

Primary structure of the bovine analogues to human apolipoproteins CII and CIII. Studies on isoforms and evidence for proteolytic processing

Two major isoforms of the bovine analogue to human apolipoprotein (apo) CII were purified from plasma. They were both as effective as human apo CII in activating lipoprotein lipase. Amino acid sequencing revealed that one form contained 79 amino acid residues, and corresponded to human pro apo CII. The other form lacked the first six residues at its N-terminus. This was apparently due to cleavage of the -Gln-Asp- linkage in the sequence H2N-Ala-His-Val-Pro-Gln-Gln-Asp-Glu-, analogous to cleavages described for human apo AI and apo CII. Previous studies with human apo CII have shown that the ability to activate lipoprotein lipase resides in the C-terminal third of the molecule. This was highly conserved in the bovine analogue: of the 30 last residues, 21 are identical. Five residues in this part of human apo CII have been reported to be essential for activation of lipoprotein lipase. Only one of these, Tyr63, is present in the bovine sequence. The bovine structure contains a threonine at position 61, instead of serine in the human, and the four last residues are -Ser-Gly-Lys-Asp instead of the allegedly necessary -Lys-Gly-Glu-Glu. Three differently sialylated isoforms of the bovine analogue to human apolipoprotein CIII were also isolated and partially sequenced. All three lacked the first three N-terminal residues as compared to sequences from other species (man, dog and rat). Sequence differences were more pronounced at the ends than in the central parts of the apo CIII molecules.

Purification and characterization of lipoprotein lipase and hepatic triglyceride lipase from human postheparin plasma: production of monospecific antibody to the individual lipase

Lipoprotein lipase (LPL) and hepatic triglyceride lipase (HTGL) were purified to homogeneity from human postheparin plasma. Molecular, catalytic and immunological properties of the purified enzymes were investigated. The native molecular weights of LPL and HTGL were 67,200 and 65,500, respectively, by gel chromatography. The subunit molecular weights of LPL and HTGL were 60,600 and 64,600, respectively, suggesting that these enzymes are catalytically active in a monomeric form. In addition, the purified LPL and HTGL each gave a single protein band when they were detected as glycoproteins with a probe of concanavalin A. The purified enzyme preparations were free of detectable antithrombin III by Western blot analysis. Catalytic properties of the purified enzymes were examined using triolein-gum arabic emulsion and triolein particles stabilized with phospholipid monolayer as substrates. LPL catalyzed the complete hydrolysis of triolein to free oleate and monooleate in the presence of apolipoprotein C-II. Apparent Km values for triolein and apolipoprotein C-II were 1.0 mM and 0.6 microM, and Vmax was 40.7 mmol/h per mg. HTGL hydrolyzed triolein substrate at a rate much slower than LPL, and produced mainly free oleate with little monooleate. Apparent Km and Vmax values were 2.5 mM and 16.1 mmol/h per mg, respectively. Polyclonal antibodies were developed against the purified LPL and HTGL. The purity and specificity of these antisera were ascertained by immunotitration, Ouchterlony double diffusion and Western blot analyses. The anti-human LPL and anti-human HTGL antiserum specifically reacted with the corresponding either native or denaturated enzyme, indicating that two enzymes were immunologically distinct. We developed an assay system for LPL and HTGL in human PHP by selective immunoprecipitation of each enzyme with the corresponding antiserum.

Inhibition of purified human postheparin lipoprotein lipase by beta-adrenergic blockers in vitro

We examined the effects of five beta-adrenergic blockers on the hydrolysis of phosphatidylcholine-stabilized triolein particles by purified human postheparin lipoprotein lipase (PHLpL) in order to evaluate the possible role of direct inhibition as a mechanism of drug-induced hypertriglyceridemia. The relative inhibitory potencies were observed in the following order: propranolol much greater than pindolol greater than metoprolol greater than atenolol greater than nadolol. There was a positive correlation between the octanol/water partition coefficients of these agents and their inhibition of lipoprotein lipase, suggesting that hydrophobicity may be one of the major determinants for PHLpL inhibition. The amount of the beta-adrenergic blockers required to produce 50% inhibition of human PHLpL was much greater than that required to inhibit purified bovine lipoprotein lipase.

Human lipoprotein lipase complementary DNA sequence

Lipoprotein lipase is a key enzyme of lipid metabolism that acts to hydrolyze triglycerides, providing free fatty acids for cells and affecting the maturation of circulating lipoproteins. It has been proposed that the enzyme plays a role in the development of obesity and atherosclerosis. The human enzyme has been difficult to purify and its protein sequence was heretofore undetermined. A complementary DNA for human lipoprotein lipase that codes for a mature protein of 448 amino acids has now been cloned and sequenced. Analysis of the sequence indicates that human lipoprotein lipase, hepatic lipase, and pancreatic lipase are members of a gene family. Two distinct species of lipoprotein lipase messenger RNA that arise from alternative sites of 3'-terminal polyadenylation were detected in several different tissues.