Mutant Lipoprotein Lipase Research Articles

Regulation of secretion and enzymatic activity of lipoprotein lipase by C-mannosylation

Lipoprotein lipase (LPL) is a crucial enzyme in lipid metabolism and transport, and its enzymatic deficiency causes metabolic disorders, such as hypertriglyceridemia. LPL has one predicted C-mannosylation site at Trp417. In this study, we demonstrated that LPL is C-mannosylated at Trp417 by mass spectrometry. Furthermore, by using wild-type and a C-mannosylation-defective mutant of LPL-overexpressing cell lines, we revealed that both secretion efficiency and enzymatic activity of C-mannosylation-defective mutant LPL were lower than those of wild-type. These data suggest the importance of C-mannosylation for LPL functions.

GPIHBP1, the Endothelial Cell Transporter for Lipoprotein Lipase

The discovery, by Drs. Beigneux, Fong, and Young, of severe chylomicronemia in GPIHBP1-deficient mice reignited interest in the lipolytic processing of triglyceride-rich lipoproteins. GPIHBP1 is a glycophosphidylinositol-anchored protein expressed exclusively in capillary endothelial cells, and it binds lipoprotein lipase (LPL) avidly. GPIHBP1 has two noteworthy protein domains, an amino-terminal acidic domain and a cysteine-rich Lymphocyte antigen 6 (Ly6) domain, and both are important for binding LPL. The localization of GPIHBP1 in capillaries, along with its ability to bind LPL, initially prompted speculation that GPIHBP1 served as an LPL binding site within the capillary lumen (creating “a platform for lipolysis”). More recent studies have identified a second and more intriguing role for GPIHBP1—picking up LPL in the subendothelial spaces (where it is secreted by myocytes and adipocytes) and then transporting it across endothelial cells to the capillary lumen, where it acts to hydrolyze the triglycerides within the plasma lipoproteins. Over the past few years, significant progress has been made in defining the amino acids within LPL and GPIHBP1 residues that are important for GPIHBP1–LPL interactions. Also, the human genetics of chylomicronemia has come into sharper focus. Some cases of chylomicronemia in humans are caused by mutant GPIHBP1 proteins that lack the ability to bind LPL, and others are caused by mutant LPL proteins that lack the ability to bind to GPIHBP1. The discovery of GPIHBP1 as the LPL transporter is a significant development in our understanding of lipoprotein physiology, and one that should lead to a vastly improved understanding of triglyceride metabolism and hyperlipidemia.

A 60-y-old chylomicronemia patient homozygous for missense mutation (G188E) in the lipoprotein lipase gene showed no accelerated atherosclerosis

Background Familial lipoprotein lipase (LPL) deficiency is a rare autosomal recessive disorder caused by mutations in the LPL gene. Patients with LPL deficiency have chylomicronemia; however, whether they develop accelerated atherosclerosis remains unclear. Methods We investigated clinical and mutational characteristics of a 60-y-old Japanese patient with chylomicronemia. Results The patient's fasting plasma triglyceride levels were > 9.0 mmol/l. In postheparin plasma, one fifth of the normal LPL protein mass was present; however, LPL activity was undetectable. Molecular analysis of the LPL gene showed the patient to be a homozygote of missense mutation replacing glycine with glutamine at codon 188 (G188E), which had been known to produce mutant LPL protein lacking lipolytic activity. Ultrasonographic examination of the patient's carotid and femoral arteries showed no accelerated atherosclerosis. Moreover, 64-slice mechanical multidetector-row computer tomography (MDCT) angiography did not detect any accelerated atherosclerotic lesions in the patient's coronary arteries. The patient had none of the risk factors such as smoking, hypertension, and diabetes. Conclusions Our case suggests that accelerated atherosclerosis may not develop in patients with LPL deficiency, when they have no risk factors.

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.

The recovery of dysfunctional lipoprotein lipase (Asp204-Glu) activity by modification of substrate

Functional deficiency of lipoprotein lipase (LPL) was found in a patient with severe hypertriglyceridemia. The patient was 39-year-old man with a plasma triglyceride level of 2032 mg/dl, and suffered from recurrent pancreatitis. His post heparin plasma LPL mass was almost normal, but the LPL activity was remarkably decreased. Gene analysis showed that homozygote missense mutation (204 Asp (GA C)-Glu (GA G)) exists in exon 5 of LPL gene. The patient LPL purified from post heparin plasma scarcely hydrolyzed VLDL-triglyceride and also triolein emulsified with Triton X-100 or phosphatidylcholine. When phosphatidylethenolamine, phosphatidylserine and cardiolipin were used as an emulsifier for triolein, triolein-hydrolyzing activity of the patient's LPL was observed and was much higher than that of wild-type LPL. Mutant LPL gene (Asp204-Glu) was made by site-direct mutagenesis and was transfected to COS-1 cell. The expressed LPL (Asp204-Glu) also showed the same properties. These results suggested that the LPL (Asp204-Glu) is a functional deficiency, and the activity could be recovered by using acidic phospholipids as an emulsifier.

Enhanced bridging function and augmented monocyte adhesion by lipoprotein lipase N9: insights into increased risk of coronary artery disease in N9 carriers

Lipoprotein lipase (LPL) is central to triacylglycerol (TG) metabolism, having both hydrolytic and bridging functions. The common LPL gene variant D9N is associated with raised TG, reduced HDL-cholesterol concentrations and increased risk of coronary artery disease (CAD). To investigate the functional basis for the phenotype in N9 carriers, CHO K1 cells were stably transfected with wild type (D9) or mutant (N9) LPL cDNA. LPL RNA expression levels, monomer-to-dimer ratios, and dimer specific activities were similar in D9 and N9 cells. Significantly enhanced binding (4.6-fold) and internalisation (2.6-fold) of 125I-LDL by N9 compared with D9 cells was eradicated by pre-treatment with either heparin or heparinase, confirming involvement of LPL and cell surface proteoglycans. N9 cells bound and internalised 3.8- and 4.4-fold more oxidised 125I-LDL, respectively, than D9 cells (both P<0.0001). Binding of monocytes was 7-fold greater to plates coated with purified LPL-N9 dimer compared with LPL-D9 ( P≤0.005). Thus once on the cell surface, LPL-N9 enhances bridging, as assessed both by LDL binding and internalisation, and monocyte adhesion. This augmented LPL-N9 bridging provides a mechanism for the reported increased CAD risk in N9 carriers.

Successful pregnancy outcome in a patient with severe chylomicronemia due to compound heterozygosity for mutant lipoprotein lipase

Objectives: Familial chylomicronemia syndrome is characterized by massive accumulation of plasma chylomicrons, which typically results from an absolute deficiency of lipoprotein lipase (LPL). Chylomicronemia in pregnancy is a rare, but serious clinical problem and can be found in patients with underlying molecular defects in the LPL gene. We report the course and treatment of an 18 yr-old primigravida who had LPL deficiency and hypertriglyceridemia since birth. We also analyzed the molecular basis of her LPL deficiency. Design and methods: The patient’s antenatal course was complicated by extreme elevations of plasma triglycerides. Her management included a very low fat diet, pharmacotherapy with gemfibrozil in the third trimester, and intermittent hospitalization with periods of fasting supplemented by IV glucose feeding. We used DNA sequencing to determine whether mutations in LPL were present. Results: At 38 weeks of gestation, labor was induced, and the patient delivered a healthy 2.77 kilogram male. Postnatal triglycerides fell to prenatal levels. DNA sequencing showed that she was a compound heterozygote for mutant LPL: I > T194 and R > H243. Conclusions: This experience indicates that vigilance is required during pregnancy in patients with familial chylomicronemia due to mutant LPL. Gemfibrozil was used in this patient without apparent adverse effects. Compound heterozygosity for LPL mutations is an important underlying mechanism for LPL deficiency.

A compound heterozygote for a novel missense mutation (G105R) in exon 3 and a missense mutation (D204E) in exon 5 of the lipoprotein lipase gene in a Japanese infant with hyperchylomicronaemia

We systematically investigated the molecular defects resulting in primary lipoprotein lipase (LPL) deficiency in a Japanese male infant (hereafter called 'the patient') with severe fasting hypertriglyceridaemia (type I hyperlipoproteinaemia). The primary LPL deficiency was diagnosed on the basis of the findings that no LPL activity was detected in post-heparin plasma (PHP) and that the immunoreactive LPL mass in PHP was less than 2% of the control level. The patient was a compound heterozygote for a novel missense mutation (G(568)GA-->AGA/Gly(105)-->Arg; G105R) in exon 3 and a missense mutation (GAC(867)-->GAG/Asp(204)-->Glu; D204E) in exon 5 of the LPL gene. The biological significance of both missense mutations was examined by an in vitro study of the expression of the mutant G105R LPL cDNA and D204E LPL cDNA in COS-1 cells. Both mutant LPLs were catalytically inactive and were barely released by heparin from the expressing COS-1 cells. These findings explain the failure to detect LPL activity and immunoreactive LPL mass in the patient's PHP. The G105R allele could be detected by digestion with the BsmAI restriction enzyme, and the D204E allele by digestion with HincII. The patient inherited the G105R allele from his mother and the D204E allele from his father. His parents were heterozygotes for the corresponding mutant allele, but normolipidaemic. The novel G105R missense mutation could not be detected by conventional analysis of single-strand conformation polymorphism, but it was identified by extensive sequencing of the entire exons and their flanking regions in the LPL gene.

A newly identified lipoprotein lipase (LPL) gene mutation (F270L) in a Japanese patient with familial LPL deficiency

We have systematically investigated the molecular defects resulting in a primary lipoprotein lipase (LPL) deficiency in a Japanese male infant (proband SH) with fasting hyperchylomicronemia. Neither LPL activity nor immunoreactive LPL mass was detected in pre- or postheparin plasma from proband SH. DNA sequence analysis of the LPL gene of proband SH revealed homozygosity for a novel missense mutation of F270L (Phe 270→Leu/TT T 1065→TT G ) in exon 6. The function of the mutant F270L LPL was determined by both biochemical and immunocytochemical studies. In vitro expression experiments on the mutant F270L LPL cDNA in COS-1 cells demonstrated that the mutant LPL protein was synthesized as a catalytically inactive form and its total amount was almost equal to that of the normal LPL. Moreover, the synthesized mutant LPL was non-releasable by heparin because the intracellular transport of the mutant LPL to the cell surface – by which normal LPL becomes heparin-releasable – was impaired due to the abnormal structure of the mutant LPL protein. These findings explain the failure to detect LPL activities and masses in pre- and postheparin plasma of the proband. The mutant F270L allele generated an XcmI restriction enzyme site in exon 6 of the LPL gene. The carrier status of F270L in the proband’s family members was examined by digestion with XcmI. The proband was ascertained to be homozygous for the F270L mutation and his parents and sister were all heterozygous. The LPL activities and masses of the parents and the sister (carriers) were half or less than half of the control values. Regarding the phenotype of the carriers, the mother with a sign of hyperinsulinemia manifested hypertriglyceridemia (type IV hyperlipoproteinemia), whereas the healthy father and the sister were normolipidemic. Hyperinsulinemia may be a strong determinant of hypertriglyceridemia in subjects with heterozygous LPL deficiency.

Compound heterozygosity of Leu252Val and Leu252Arg causing lipoprotein lipase deficiency in a chinese patient with hypertriglyceridemia.

We investigated lipoprotein lipase (LPL) gene mutations in a Chinese male with severe hypertriglyceridemia and recurrent pancreatitis. We screened for LPL sequence mutation in the LPL gene in this patient, his relatives and 160 unrelated hypertriglyceridaemic subjects. We determined the postheparin plasma LPL activity of subjects carrying a LPL mutation and studied the in vitro expression of mutant LPL in COS-1 cells. The proband was found to be a compound heterozygote for a novel Leu252Val and a reported Leu252Arg mutation in the LPL gene. He had low plasma levels of postheparin LPL activity and mass. The two mutations segregated independently in his family. In vitro expression analysis showed that Leu252Arg abolished both the catalytic function and secretion of LPL, while Leu252Val abolished the catalytic function but only reduced secretion by about half. We have also detected heterozygous Leu252Val and Leu252Arg mutations each in one hypertriglyceridaemic individual. These results indicated that the leucine 252 is critical for the catalytic activity and secretion of LPL. Why the substitution by valine instead of arginine resulted only in a partial suppression of LPL secretion, remains to be investigated. Leu252Val and Leu252Arg are the likely cause of hypertriglyceridemia in these subjects because of their deleterious effects on LPL activity or secretion. Leu252Val/Leu252Arg is the first compound heterozygous mutation known to occur in the same codon of the LPL gene. So far they are found only in Chinese.

Identification of the epitope of a monoclonal antibody that inhibits heparin binding of lipoprotein lipase: new evidence for a carboxyl-terminal heparin-binding domain

A panel of 13 monoclonal antibodies to avian lipoprotein lipase (LPL) was screened for inhibition of LPL binding to primary avian adipocytes. One monoclonal antibody, designated xCAL (monoclonal antibody to chicken adipose lipoprotein lipase) 3-6a, was found to inhibit the binding of LPL to primary avian adipocytes. In solid phase assays, xCAL 3-6a inhibited the binding of LPL to both heparan sulfate and heparin. XCAL 3-6a did not inhibit the catalytic activity of the avian enzyme. The monoclonal antibody was not found to cross-react significantly with bovine lipoprotein lipase. In order to determine the location of the epitope of xCAL 3-6a on lipoprotein lipase, several avian lipoprotein lipase deletion mutants were constructed and produced as glutathione S-transferase (GST) fusion proteins in E. coli. These mutants were screened for their ability to react with xCAL 3-6a using Western blotting. The minimum continuous fragment of lipoprotein lipase that was required for reactivity contained the amino acids 310 to 450. Site-directed mutagenesis of basic residues 321, 405, 407, 409, 415, and 416 revealed that Arg 405 is necessary for the interaction of LPL with xCAL 3-6a. Additional deletions of either the amino- or carboxyl-terminal portion of the fragment containing residues 310–450 resulted in loss of antibody binding, suggesting that the epitope is a discontinuous one that is formed when the termini are brought together through protein folding. Heparin-Sepharose chromatography of wild-type LPL and a mutant LPL in which the well-characterized heparin-binding sequence (Arg 281–Lys 282–Arg 284) has been mutated was carried out in the presence and absence of xCAL 3-6a. These experiments indicate that lipoprotein lipase contains a heparin-binding domain, in addition to Arg 281–Arg 284, that can be blocked by xCAL 3-6a.—Sendak, R. A., K. Melford, A. Kao, and A. Bensadoun. Identification of the epitope of a monoclonal antibody that inhibits heparin binding of lipoprotein lipase: new evidence for a carboxyl-terminal heparin-binding domain.

The Mutation Gly142→ Glu in Human Lipoprotein Lipase Produces a Missorted Protein That Is Diverted to Lysosomes

While the molecular characterization of lipoprotein lipase (LPL) activation is progressing, the intracellular processing, transport, and secretion signals of LPL are still poorly known. The aim of this paper is to study are involvement of glycine 142 in LPL secretion and to elucidate the intracellular destination of the altered protein that remains inside the cell. We mutated the human LPL cDNA by site-directed mutagenesis in order to produce the G142e hLPL in which the glycine 142 was replaced by a glutamic acid. The wild type human LPL (WT hLPL) and the mutant G142E hLPL were expressed by transient transfection in COS1 cells. Using Western blot assays we identified a single band that had the same molecular weight for both proteins. However, Western blots of culture media did not reveal any specific band for the mutant protein, and ELISA experiments showed that the extracellular mass of the mutant LPL was only 25% of the WT protein, indicating defective secretion of the altered enzyme. Heparin increased LPL secretion in the case of the WT hLPL but did not have any stimulatory effect when acting on G142E hLPL-transfected cells. However, heparin-Sepharose chromatography revealed that both proteins presented the same heparin affinity. Metabolic labeling and radioimmunoprecipitation studies showed that both the WT and the mutant hLPL intracellular levels decreased upon chase time. Furthermore, leupeptin had a greater effect on the intracellular level of the mutant enzyme, thus indicating its higher intracellular degradation. Immunofluorescent studies using confocal microscopy indicated high colocalization of the LPL labeling and the Lamp1 lysosomal labeling in G142E hLPL-expressing cells. This result was confirmed using immunoelectron microscopy, which in addition showed gold labeling in Golgi stacks. This finding together with experiments performed with endoglycosidase H digestion of immunoprecipitated radiolabeled LPL, indicated that the mutant enzyme entered the Golgi compartment. The results reported in this paper show that the G142E hLPL is not efficiently secreted to the extracellular medium, but it is missorted to lysosomes for intracellular degradation. This finding suggests that lysosomal missorting might be a mechanism of cell quality control of secreted LPL.

Prevalence of alleles encoding defective lipoprotein lipase in hypertriglyceridemic patients of French Canadian descent

It has previously been estimated that due to genetic "founder effects," 97% of lipoprotein lipase (LPL) gene alleles conferring type I hyperlipoproteinemia (HLP) in French Canadians encode one of the following mutant LPL forms: Gly188-->Glu, Pro207-->Leu, or Asp250-->Asn. Although the genetic basis of type I HLP is known to be homozygosity for LPL deficiency, that for other forms of HLP, especially types IV, and V HLP, is not clear. It is also unclear whether hypertriglyceridemia due to very low density lipoprotein (VLDL) overproduction can be distinguished phenotypically from that due to defective catabolism of plasma lipoprotein triglycerides. The present study took advantage of the unique circumstances inherent in the relatively genetically isolated French Canadian population to address these questions. This study was carried out in order to determine the prevalence of these three mutant LPL alleles, and of a fourth encoding LPL Asn291-->Ser, in French Canadian patients with hypertriglyceridemia. The prevalence of heterozygosity for one of the four LPL mutant alleles in nondiabetic, nonobese hypertriglyceridemic subjects was 16 of 95 type IV HLP (17%) and 4 of 26 type V HLP cases (15%). These alleles were not found in over 150 normotriglyceridemic subjects, supporting the likelihood that the mutant alleles were at least partially responsible for HLP. In addition, heterozygosity for LPL deficiency due to one of these mutations apparently did not contribute to hypoalphalipoproteinemia, and was observed in 3 of 39 subjects with type III HLP. The results suggest that in French Canadians, 15-20% of type IV and V HLP cases are associated with these genetic defects in plasma triglyceride catabolism.

Lipoprotein Lipase:Role of Intramolecular Disulfide Bonds in Enzyme Catalysis

Lipoprotein lipase (LPL) catalyzes the hydrolysis of the triacylglycerol component of triacylglycerol-rich lipoproteins. There are 4 cysteine pairs that are completely conserved among LPLs of all species known. We examined the functional importance of each of the cysteine pairs in enzyme catalysis by examining LPLs produced in Cos cells by transfection. Immunoreactive LPL was produced by vectors encoding the wildtype LPL and each of the 4 cysteine-pair mutant LPLs. Enzyme activity was detectable in the wildtype enzyme, but not in 3 of the 4 Cys→Ser mutant enzymes (C216S/C239S, C264S/C275S, and C278S/C283S). Interestingly, LPL activity was also present in the mutant (C418S/C438S), which affects the C-terminal cysteine pair, with a specific activity ∼50% higher than that of wildtype. There is evidence that LPL contains two distinct domains consisting of the N-terminal three-quarters of the sequence connected by a flexible region to the C-terminal domain comprising the rest of the molecule. The conservation of catalytic function despite the disruption of the only disulfide bridge in the C-terminal domain of LPL indicates that the two domains can function independently of each other in enzyme catalysis.

A Naturally Occurring Mutation at the Second Base of Codon Asparagine 43 in the Proposed N-Linked Glycosylation Site of Human Lipoprotein Lipase: In Vivo Evidence That Asparagine 43 Is Essential for Catalysis and Secretion

The patient was a 20-year-old male. His fasting plasma triglyceride and cholesterol levels were 1258 mg/dl and 138 mg/dl, respectively. The lipoprotein lipase(LPL) activity and mass from postheparin plasma of the patient were 0.00 μmol/ml/h (normal range: 5.51±1.12) and 23 ng/ml (normal range: 220±42), respectively. DNA sequence analysis of the LPL gene from the patient revealed a homozymous nucleotide change: a A → G transition at nucleotide position 383, resulting in an amino acid substitution of Ser for Asn43, which is believed to be an N-linked glycosylation site of the LPL mature protein. Expression studies of this mutant LPL cDNA produced an inactive LPL protein which was not secreted into the media.

Lipoprotein lipase domain function.

Human lipoprotein lipase (LPL) monomer consists of two domains, a larger NH2-terminal domain that contains catalytic residues and a smaller COOH-terminal domain that modulates substrate specificity and is a major determinant of heparin binding. Analyses of NH2-terminal domain function were performed after site-directed mutagenesis of the putative active-site serine residue, while COOH-terminal domain function was assessed following reaction with a monoclonal antibody. The native enzyme and mutant LPL in which serine 132 was replaced with alanine, cysteine, or glycine were transiently expressed in COS-7 cells. Mutant proteins were synthesized and secreted at levels comparable to native LPL; however, none of the mutants retained enzymatic activity. The mutant with alanine replacing serine 132 was purified and shown to be inactive with both esterase and lipase substrates; however, binding to a 1,2-didodecanoyl-sn-glycero-3-phosphatidylcholine monolayer was comparable to native LPL. These results are consistent with a catalytic, and not a lipid binding, role for serine 132. To investigate the function of the smaller COOH-terminal domain, LPL lipolytic and esterolytic activities as well as heparin binding properties were determined after reaction with a monoclonal antibody specific for this domain. Lipolytic activity was inhibited by the monoclonal antibody, whereas esterolytic activity was only marginally affected, indicating that the LPL COOH-terminal domain is required for lipolysis, perhaps by promoting interaction with insoluble substrates. Also, the affinity of antibody-reacted LPL for heparin was not significantly different from that of LPL alone, suggesting that (i) the heparin-binding site is physically distinct from the COOH-terminal domain region required for lipolysis and (ii) binding of antibody did not cause dimer dissociation. A model is proposed for the two LPL domains fulfilling different roles in the lipolytic process.

Gene-environment interaction in the conversion of a mild-to-severe phenotype in a patient homozygous for a Ser172--&gt;Cys mutation in the lipoprotein lipase gene.

Normal pregnancy is associated with a two- to threefold increase in plasma triglyceride levels, particularly in the third trimester, due both to the overproduction of VLDLs and to the possible suppression of lipoprotein lipase (LPL) activity. Numerous mutations in the human LPL gene causing complete LPL deficiency have been described, but naturally occurring mutations that result in defective LPL with partial activity have not yet been reported. Here we describe a 30-yr-old woman who was first diagnosed with LPL deficiency during pregnancy after she developed pancreatitis. Her plasma triglyceride levels remained mildly elevated at approximately 300 mg/dl (3.4 mmol/liter) after the first pregnancy but rose significantly after she became pregnant again (1800 to 2000 mg/dl) (20.2 to 22.5 mmol/liter). DNA sequence analysis of the LPL gene showed that the patient is homozygous for a Ser172-->Cys missense mutation in exon 5. In vitro mutagenesis revealed that the Ser172-->Cys mutation caused a mutant LPL protein that had residual activity higher than that seen in all eight other missense mutations in patients with LPL deficiency identified in our laboratory. We propose that some mutations in the LPL gene produce a defective LPL with partial activity, which usually leads to mild hypertriglyceridemia.

Functional topology of a surface loop shielding the catalytic center in lipoprotein lipase

Lipoprotein lipase (LPL), hepatic lipase, and pancreatic lipase show high sequence homology to one another. The crystal structure of pancreatic lipase suggests that it contains a trypsin-like Asp-His-Ser catalytic triad at the active center, which is shielded by a disulfide bridge-bounded surface loop that must be repositioned before the substrate can gain access to the catalytic residues. By sequence alignment, the homologous catalytic triad in LPL corresponds to Asp156-His241-Ser132, absolutely conserved residues, and the homologous surface loop to residues 217-238, a poorly conserved region. To verify these assignments, we expressed in vitro wild-type LPL and mutant LPLs having single amino acid mutations involving residue Asp156 (to His, Ser, Asn, Ala, Glu, or Gly), His241 (to Asn, Ala, Arg, Gln, or Trp), or Ser132 (to Gly, Ala, Thu, or Asp) individually. All 15 mutant LPLs were totally devoid of enzyme activity, while wild-type LPL and other mutant LPLs containing substitutions in other positions were fully active. We further replaced the 22-residue LPL loop which shields the catalytic center either partially (replacing 6 of 22 residues) or completely with the corresponding hepatic lipase loop. The partial loop-replacement chimeric LPL was found to be fully active, and the complete loop-replacement mutant had approximately 60% activity, although the primary sequence of the hepatic lipase loop is quite different. In contrast, replacement with the pancreatic lipase loop completely inactivated the enzyme. Our results are consistent with Asp156-His241-Ser132 being the catalytic triad in lipoprotein lipase.(ABSTRACT TRUNCATED AT 250 WORDS)

A missense (Asp250—-Asn) mutation in the lipoprotein lipase gene in two unrelated families with familial lipoprotein lipase deficiency.

We have identified the molecular basis for familial lipoprotein lipase (LPL) deficiency in two unrelated families with the syndrome of familial hyperchylomicronemia. All 10 exons of the LPL gene were amplified from the two probands' genomic DNA by polymerase chain reaction. In family 1 of French descent, direct sequencing of the amplification products revealed that the patient was heterozygous for two missense mutations, Gly188----Glu (in exon 5) and Asp250----Asn (in exon 6). In family 2 of Italian descent, sequencing of multiple amplification products cloned in plasmids indicated that the patient was a compound heterozygote harboring two mutations, Arg243----His and Asp250----Asn, both in exon 6. Studies using polymerase chain reaction, restriction enzyme digestion (the Gly188----Glu mutation disrupts an Ava II site, the Arg243----His mutation, a Hha I site, and the Asp250----Asn mutation, a Taq I site), and allele-specific oligonucleotide hybridization confirmed that the patients were indeed compound heterozygous for the respective mutations. LPL constructs carrying the three mutations were expressed individually in Cos cells. All three mutant LPLs were synthesized and secreted efficiently; one (Asp250----Asn) had minimal (approximately 5%) catalytic activity and the other two were totally inactive. The three mutations occurred in highly conserved regions of the LPL gene. The fact that the newly identified Asp250----Asn mutation produced an almost totally inactive LPL and the location of this residue with respect to the three-dimensional structure of the highly homologous human pancreatic lipase suggest that Asp250 may be involved in a charge interaction with an alpha-helix in the amino terminal region of LPL. The occurrence of this mutation in two unrelated families of different ancestries (French and Italian) indicates either two independent mutational events affecting unrelated individuals or a common shared ancestral allele. Screening for the Asp250----Asn mutation should be included in future genetic epidemiology studies on LPL deficiency and familial combined hyperlipidemia.

Effects of substitutions of glycine and asparagine for serine 132 on activity and binding of human lipoprotein lipase to very low density lipoproteins

For studying the role of Ser 132 in the putative catalytic site of human lipoprotein lipase (LPL), mutant LPL cDNAs expressing LPLs with amino acid substitutions of Gly or Asn for Ser 132 were obtained by site-directed mutagenesis, and were expressed in COS-1 cells. Considerable amounts of LPL enzyme protein mass were detected in the culture medium of COS-1 cells transfected with wild-type LPL, LPL-Gly 132, or LPL-Asn 132. LPL-Gly 132 hydrolyzed Triton X-100-triolein and tributyrin as effectively as wild-type LPL, whereas LPL-Asn 132 showed no activity. LPL-Asn 132 bound to very low density lipoproteins as effectively as wild-type LPL.