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H B Brewer

Publications and source records attributed to H B Brewer.

At least 19 recordsLinked to original sources
PubMed1992

Human lipoprotein lipase: the loop covering the catalytic site is essential for interaction with lipid substrates.

Lipoprotein lipase (LPL), a key enzyme which initiates the hydrolysis of triglycerides present in chylomicrons and very low density lipoproteins, consists of multiple functional domains which are necessary for normal activity. The catalytic domain of LPL mediates the esterase function of the enzyme but separate lipid binding sites have been proposed to be involved in the interaction of LPL with emulsified lipid substrates at the water-lipid interface. Like pancreatic lipase (PL), LPL contains a surface loop covering the catalytic pocket that may modulate access of the substrate to the active site of the enzyme. Secondary structural analysis of this loop reveals a helix-turn-helix motif with two short amphipathic helices that have hydrophobic moments of 0.64 and 0.68. In order to investigate the role of the loop in the initial interaction of LPL with its substrate, we utilized site-directed mutagenesis to generate eight constructs in which the amphipathic properties of the loop were altered and expressed them in human embryonal kidney-293 cells. Reducing the amphiphilicity without changing the predicted secondary structure of the loop abolished the ability of the lipase to hydrolyze emulsified, long chain fatty acid triglycerides (triolein) but not the water soluble substrate tributyrin. Replacing the loop of LPL with the loop of hepatic lipase, which differs in 15 of 22 amino acids but is also amphiphilic, led to the expression of an enzyme that retained both triolein and tributyrin hydrolyzing activity. Substitution of the LPL loop by a short four amino acid peptide, which may allow more direct access to the active site than the 22 amino acid loop, enhanced hydrolysis of short chain fatty acid triglycerides by more than 2-fold, while the ability to hydrolyze emulsified substrates was abolished. Thus, disruption of the amphipathic structure of the LPL loop selectively decreases the hydrolysis of emulsified lipid substrate without affecting the esterase or catalytic function of the enzyme. These studies establish that the loop with its two amphipathic helices is essential for hydrolysis of long chain fatty acid substrate by LPL providing new insight into the role of the LPL loop in lipid-substrate interactions. We propose that the interaction between the lipoprotein substrates and the amphipathic helices within this loop may in part determine lipase substrate specificity.

Amino Acid Sequence↗
PubMed1992

Heterozygosity for apolipoprotein E-4Philadelphia(Glu13----Lys, Arg145----Cys) is associated with incomplete dominance of type III hyperlipoproteinemia.

Apolipoprotein (apo) E-4Philadelphia is a double mutant of apoE in which residue 13 of the mature protein, glutamic acid (GAG), is replaced by lysine (AAG) and amino acid 145, arginine (CGT), is converted to cysteine (TGT). These mutations result in two restriction fragment length polymorphisms for the enzymes AvaI and BbvI, a smaller apparent molecular weight of apoE-4Philadelphia on sodium dodecyl polyacrylamide gels, and severe type III hyperlipoproteinemia (HLP) in a 24-year-old homozygous female (Lohse, P., Mann, W. A., Stein, E. A., and Brewer, H. B., Jr. (1991) J. Biol. Chem. 266, 10479-10484). In the current study, we have extended our analysis to include nine additional family members of the Philadelphia kindred spanning four generations. DNA and protein analysis demonstrated that the originally described propositus is a true homozygote for the epsilon-4Philadelphia allele and that six of the nine family members are heterozygous for the mutated allele and the normal epsilon-3 allele or, in one case, the epsilon-4 allele. Heterozygosity for apoE-4Philadelphia leads to the expression of a moderate form of type III HLP without clinical manifestations. These results are consistent with a dominant mode of inheritance of this dyslipoproteinemia. The simultaneous presence of unaffected individuals, heterozygotes, and a homozygote in the Philadelphia kindred makes it possible for the first time to demonstrate that the mutant apoE exhibits an incomplete or partial dominance of type III HLP. Heterozygosity for the normal epsilon-3 allele appears to have an influence on the expression of type III HLP, resulting in a phenotype intermediate between that of the two homozygous states.

Adolescent↗
PubMed1992

Counter-current chromatography of lipoproteins with a polymer phase system using the cross-axis synchronous coil planet centrifuge.

Lipoproteins were separated by counter-current chromatography using the type-XLL coil planet centrifuge. The separation was performed with a polymer phase system composed of 16% (w/w) polyethylene glycol 1000 and 12.5% (w/w) dibasic potassium phosphate by eluting the lower phase at a flow-rate of 0.5 ml/min. About 5 ml of the sample solution containing approximately 150 mg of a lipoprotein mixture were loaded. High- and low-density lipoproteins were resolved within 12 h. Each component was detected by gel electrophoresis with oil red staining.

Centrifugation↗
PubMed1992

Human lipoprotein lipase. Analysis of the catalytic triad by site-directed mutagenesis of Ser-132, Asp-156, and His-241.

Lipoprotein lipase (LPL) plays a central role in normal lipid metabolism as the key enzyme involved in the hydrolysis of triglycerides present in chylomicrons and very low density lipoproteins. LPL is a member of a family of hydrolytic enzymes that include hepatic lipase and pancreatic lipase. Based on primary sequence homology of LPL to pancreatic lipase, Ser-132, Asp-156, and His-241 have been proposed to be part of a domain required for normal enzymic activity. We have analyzed the role of these potential catalytic residues by site-directed mutagenesis and expression of the mutant LPL in human embryonic kidney-293 cells. Substitution of Ser-132, Asp-156, and His-241 by several different residues resulted in the expression of an enzyme that lacked both triolein and tributyrin esterase activities. Mutation of other conserved residues, including Ser-97, Ser-307, Asp-78, Asp-371, Asp-440, His-93, and His-439 resulted in the expression of active enzymes. Despite their effect on LPL activity, substitutions of Ser-132, Asp-156, and His-241 did not change either the heparin affinity or lipid binding properties of the mutant LPL. In summary, mutation of Ser-132, Asp-156, and His-241 specifically abolishes total hydrolytic activity without disrupting other important functional domains of LPL. These combined results strongly support the conclusion that Ser-132, Asp-156, and His-241 form the catalytic triad of LPL and are essential for LPL hydrolytic activity.

Amino Acid Sequence↗
PubMed1992

The molecular defects in lipoprotein lipase deficient patients.

The underlying molecular defects that lead to a deficiency of lipoprotein lipase in two patients from different kindreds presenting with the familial hyperchylomicronemia syndrome have been identified. Sequence analysis of amplified LPL cDNA of the patient from the Bethesda kindred revealed a single point mutation (G to A) at position 781 of the normal gene that resulted in the substitution of an alanine for a threonine at residue 176 and the loss of an SfaN1 site present in the normal LPL gene. Amplification of patient cDNA by the PCR followed by restriction enzyme digestion with SfaN1 established that the patient is a true homozygote for the defect. The proband from the second kindred was found to be a compound heterozygote for two separate allelic mutations, including a T to C transition at nucleotide 836 and a G to A mutation at base 983 that led to the substitution of Ile194 by Thr and Arg243 by His, respectively. Transient expression of the mutant LPL cDNAs from both kindreds in human embryonal kidney-293 cells resulted in the synthesis of enzymatically inactive proteins, establishing the functional significance of the mutations.

Alleles↗
PubMed1992

Correlation of apolipoprotein(a) isoproteins with Lp(a) density and distribution in fasting plasma.

Lp(a) is an LDL-like lipoprotein which contains an additional apolipoprotein called apo(a). Apo(a) exhibits a significant size polymorphism and its size is inversely correlated with plasma Lp(a) levels. We investigated the distribution of different apo(a) isoproteins in lipoprotein density fractions. Fasting plasma samples were subjected to non-equilibrium density gradient ultracentrifugation. After SDS-PAGE and anti-apo(a) immunoblotting, apo(a) concentrations in individual density fractions were evaluated by densitometry. In series I, analysis of selected density fractions from 35 coronary heart disease (CHD) patients demonstrated that although most of the apo(a) was present in the Lp(a) density range, apo(a) was consistently found in both the VLDL and IDL fractions as well. In series II, density fractions from 9 normolipidemic subjects with 6 different apo(a) isoproteins were evaluated. A strong association between the size of the apo(a) isoprotein and the density of the associated Lp(a) particle was established (r = 0.976, P less than 0.001). Lp(a) densities ranged from 1.057 g/ml for the B isoprotein to 1.09 g/ml for the S5 isoprotein. Overall, 75% of the total apo(a) was detected in the Lp(a) density range (d = 1.05-1.12 g/ml), with 9% and 10% in the LDL (d = 1.019-1.05 g/ml) and HDL (d = 1.12-1.21 g/ml) fractions, respectively. VLDL contained an average of 4% of the total apo(a) in fasting normolipidemic plasma. Two hypertriglyceridemic subjects had substantially greater amounts of apo(a) in the fasting triglyceride-rich fraction. The results of this study indicate that the size of the apo(a) isoprotein strongly influences the density of its associated Lp(a) particle and that apo(a) is consistently found in the triglyceride-rich lipoproteins of fasting plasma.

Adult↗
PubMed1992

Regulation of LDL receptor, apoB, and apoE protein and mRNA in Hep G2 cells.

The regulation of low-density lipoprotein (LDL) receptor activity, protein synthesis, and cellular mRNA content was evaluated in the human hepatoma cell line Hep G2. Incubation of the cells with LDL led to a complete downregulation of LDL receptor mRNA and LDL receptor protein synthesis. This LDL regulation of the LDL receptor and its mRNA was both time- and concentration-dependent. In contrast to protein synthesis and cellular mRNA concentrations of the LDL receptor, which were reduced to undetectable levels by prolonged incubation in the presence of LDL, LDL receptor activity was reduced to only 44% of preincubation levels. These findings support the presence of a second metabolic pathway for LDL uptake in human hepatocytic cells. The effect of LDL on cellular LDL receptor expression was specific for LDL because incubation in the presence of HDL did not affect any of these study end points. The potential coordinate regulation of the expression of the LDL receptor with its principal ligands, apolipoproteins (apo) B and E, was also investigated. In contrast to the LDL receptor mRNA downregulation with LDL incubation, cellular apoB and apoE mRNA concentrations were not affected by either LDL or HDL. Secretion of apoB, however, was significantly increased by incubating Hep G2 cells with LDL. These findings indicate that, in contrast to LDL receptor which is regulated at the mRNA level, the ligands for the LDL receptor are regulated either co- or post-translationally.

Apolipoproteins B↗
PubMed1992

The heart in Tangier disease. Severe coronary atherosclerosis with near absence of high-density lipoprotein cholesterol.

Cardiac necropsy findings are described in a 72-year-old man with Tangier disease whose plasma total cholesterol levels averaged 70 mg/dL, low-density lipoprotein cholesterol level was 45 mg/dL, and high-density lipoprotein cholesterol level was 1.4 mg/dL, and who had coronary artery bypass grafting for severe atherosclerotic coronary artery disease. At necropsy, 24 of the 72 (33%) 5-mm segments of the 4 major (right, left main, left anterior descending, and left circumflex) native coronary arteries and 4 of the 27 (15%) 5-mm segments of the saphenous vein aortocoronary bypass conduits were narrowed by more than 75% in cross-sectional area by atherosclerotic plaques. The plaques were composed primarily (91% to 97%) of fibrous tissue. Oil red O staining, polarized light microscopy, and electron microscopy revealed cholesterol deposits in the plaques and in the walls of coronary arteries, saphenous vein grafts, and aorta. Such deposits also were found in foam cells of histiocytic origin, fibroblasts in all four cardiac valves, and in Schwann cells of cardiac nerves.

Aged↗
PubMed1992

Apolipoprotein C-II deficiency syndrome due to apo C-IIHamburg: clinical and biochemical features and HphI restriction enzyme polymorphism.

We have characterized the clinical and biochemical features of three siblings of a kindred with severe hypertriglyceridaemia due to apolipoprotein C-II (apo C-II) deficiency caused by the mutation described as apo C-IIHamburg. The clinical syndrome is characterized by recurrent pancreatitis in two of three affected individuals, with discrete hepatosplenomegaly in all three patients and cholelithiasis in one. Eruptive xanthomas and lipemia retinalis were absent. Plasma lipoproteins were characterized by fasting chylomicronaemia, reduced low density lipoproteins (LDL) and low high density lipoproteins (HDL). The marked hypertriglyceridaemia could be corrected promptly by infusion of normal plasma. Apolipoprotein C-II (apo C-II) levels in homozygotes were very low (0.01 mg dl-1), and mean apo C-II levels in heterozygotes were lower (2.08 +/- 0.11 mg dl-1) than in normal family members (3.38 +/- 0.75 mg dl-1). Lipoprotein lipase and hepatic triglyceride lipase activities in post-heparin plasma were normal. Zonal ultracentrifugation revealed a marked increase in triglyceride-rich lipoproteins and reduced LDL and HDL. LDL consisted of two fractions with higher hydrated density of the main fraction compared with normals with a trend to normalization on a fat-free diet. The molecular defect in the apo C-II Hamburg gene has been previously identified as a donor splice site mutation in the second intron. This leads to abnormal splicing of the apo C-II Hamburg mRNA and apo C-II deficiency in plasma. The mutation causes the loss of an HphI restriction enzyme site present in the normal apo C-II gene.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1992

Hypertriglyceridaemia due to genetic defects in lipoprotein lipase and apolipoprotein C-II.

Hypertriglyceridaemia, as defined by fasting triglyceride levels of greater than 2.8 mmol l-1, is a prevalent dyslipoproteinaemia in our population. The underlying pathophysiological mechanisms that result in elevations of plasma triglycerides are heterogeneous and, in most cases, incompletely understood. However, in a subset of patients presenting with this lipid disorder, the biochemical and genetic defects that lead to hypertriglyceridaemia have been well characterized. These individuals present with the familial chylomicronaemia syndrome, a rare genetic disorder that is inherited as an autosomal recessive trait, and is characterized by severe fasting hypertriglyceridaemia, massive accumulations of chylomicrons in plasma, and recurrent bouts of pancreatitis. The two major causes of the familial chylomicronaemia syndrome are a deficiency of the enzyme, lipoprotein lipase (LPL), or its cofactor, apolipoprotein (apo) C-II. Together, these two proteins initiate the hydrolysis of triglycerides present in chylomicrons and very low density lipoproteins. In the past decade our understanding of the underlying molecular defects that lead to familial chylomicronaemia has been greatly enhanced by the identification of mutations in the genes for LPL and apoC-II. Characterization of these defects has provided new insights into the structure and function of apoC-II and LPL and established the important role that these two proteins play in normal triglyceride metabolism.

Apolipoprotein C-II↗
PubMed1992

Ontogenetic regulation of apolipoprotein B mRNA editing during human and rat development in vivo.

The solubilization and delivery of lipids in plasma rely on both forms of apolipoprotein B (apo B): apo B-100 and apo B-48. Apo B-48 is the translational product of apo B-100 mRNA that undergoes peritranscriptional conversion of C----U, replacing codon CAA (glutamine 2,153) with the inframe stop codon (UAA). We examined mRNA editing activity in the human and the rat by reverse transcription-polymerase chain reaction primer-extension analysis of intestine and liver total RNA. In rat intestine the percentage of apo B transcripts that undergo editing increases dramatically the day before birth (from approximately 1% to 80%), whereas the rat liver acquires an adult level of editing activity during the third postnatal week (rising from approximately 8% to 30%), when weaning is completed, bile acid composition matures, and plasma thyroid hormone levels peak. In contrast to the rat, the human intestine acquires adult levels of apo B mRNA editing relatively early in fetal development, rising from 10% at 10 weeks to approximately 80% by the end of the second trimester. Our results establish that apo B mRNA editing is 1) developmentally regulated in a tissue- and species-specific manner; 2) fully developed prenatally in both human and rat intestine, suggesting a crucial role of apo B-48 in mammalian fetal adaptation to extrauterine life; and 3) acquired early in human fetal intestine, implying a potential role for apo B-48 in prenatal lipid metabolism.

Animals↗
PubMed1992

In vivo metabolism of apolipoprotein A-I in a patient with homozygous familial hypercholesterolemia.

Familial hypercholesterolemia (FH), caused by a defect in the low density lipoprotein (LDL) receptor, results in high plasma concentrations of LDL cholesterol due to both overproduction and delayed catabolism of LDL. FH is also associated with significantly lower levels of plasma high density lipoprotein cholesterol and apolipoprotein (apo) A-I in both heterozygous and homozygous patients. However, the metabolic basis of the hypoalphalipoproteinemia in FH has not been elucidated. We investigated the kinetics of apo A-I in a homozygous FH patient and two normal control subjects by using endogenous labeling with a stable isotopically labeled amino acid. Study subjects were administered a primed constant infusion of 13C6-phenylalanine for 12 hours. Apolipoproteins were isolated from plasma drawn at selected time points and analyzed for their isotopic enrichment by gas chromatography-mass spectrometry. The fractional catabolic rate of apo A-I in the FH subject was found to be substantially increased (0.38 day-1) compared with that of the normal subjects (mean, 0.26 day-1). In addition, the apo A-I production rate was decreased in the FH subject (6.5 mg/kg.day-1) compared with the normal subjects (mean, 11.1 mg/kg.day-1). In conclusion, the low levels of high density lipoprotein cholesterol and apo A-I in this homozygous FH patient are due to the combined metabolic defects of increased apo A-I catabolism and decreased apo A-I production.

Apolipoprotein A-I↗
PubMed1992

Two different allelic mutations in the lecithin-cholesterol acyltransferase gene associated with the fish eye syndrome. Lecithin-cholesterol acyltransferase (Thr123----Ile) and lecithin-cholesterol acyltransferase (Thr347----Met).

We have elucidated the genetic defect in a 66-yr-old patient with fish eye syndrome (FES) presenting with severe corneal opacities and hypoalphalipoproteinemia. The patient's plasma concentration of high density lipoprotein (HDL) cholesterol was reduced at 7.7 mg/dl (35.1-65.3 mg/dl in controls) and the HDL cholesteryl ester content was 31% (60-80% in controls); however, total plasma cholesteryl esters were similar to normal (60% of total cholesterol vs. a mean of 66% in controls). The patient's plasma cholesterol esterification rate was slightly reduced at 51 nmol/ml per h (control subjects: 61-106 nmol/ml per h), whereas lecithin-cholesterol acyltransferase (LCAT) activity, assayed using a HDL-like exogenous proteoliposome substrate, was virtually absent (0.9 nmol/ml per h vs. 25.1-27.9 nmol/ml per h in control subjects). DNA sequence analysis of the proband's LCAT gene revealed two separate C to T transitions resulting in the substitution of Thr123 with Ile and Thr347 with Met. The mutation at codon 347 created a new restriction site for the enzyme Nla III. Analysis of the patient's polymerase chain reaction-amplified DNA containing the region of the Thr347 mutation by digestion with Nla III confirmed that the proband is a compound heterozygote for both defects. The patient's daughter, who is asymptomatic despite a 50% reduction of LCAT activity, is heterozygous for the Thr123----Ile mutation. Our data indicate that the regions adjacent to Thr123 and Thr347 of LCAT may play an important role in HDL cholesterol esterification, suggesting that these regions may contain a portion of the LCAT binding domain(s) for HDL.

Aged↗
PubMed1992

Familial apolipoprotein E deficiency and type III hyperlipoproteinemia due to a premature stop codon in the apolipoprotein E gene.

A kindred with apolipoprotein E deficiency and a truncated lower molecular weight apoE mutant, designated apoE-3Washington, has been identified. Gel electrophoresis demonstrated complete absence of the normal apoE isoproteins and the presence of a small quantity of a lower molecular weight apoE. Plasma apoE levels in the proband were approximately 4% of normal. This marked deficiency of apoE resulted in delayed uptake of chylomicron and very low density lipoprotein (VLDL) remnants by the liver, elevated plasma cholesterol levels, mild hypertriglyceridemia, and the development of type III hyperlipoproteinemia. Sequence analysis of the patient's apoE gene revealed a single nucleotide substitution of an A for a G, which converted amino acid 210 of the mature protein, tryptophan (TGG), to a premature chain termination codon (TAG), thus leading to the synthesis of a truncated E apolipoprotein of 209 amino acids with a molecular mass of 23.88 kDa. Northern blot analysis of differentiated monocyte-derived macrophages demonstrated a mutant mRNA indistinguishable in size from normal apoE mRNA. The nucleotide substitution also resulted in the formation of a new restriction site for Mae I. Using this enzyme we were able to establish that the proband is a homozygote and that her two offsprings are heterozygous for the epsilon-3Washington allele. These data demonstrate that the striking deficiency of apoE-3Washington results in a moderate form of type III hyperlipoproteinemia. The clinical presentation also suggests a dispensable role of apoE in the nervous system and in immunoregulation.

Apolipoproteins E↗
PubMed1992

In vivo metabolism of a mutant apolipoprotein, apoA-IIowa, associated with hypoalphalipoproteinemia and hereditary systemic amyloidosis.

Apolipoprotein (apo) A-I is the major protein constituent of plasma high density lipoproteins (HDL). A kindred has been identified in which a glycine to arginine mutation at residue 26 in apoA-I is associated with hypoalphalipoproteinemia and hereditary systemic amyloidosis. We isolated the mutant protein, termed apoA-IIowa, from the plasma of an affected subject and studied its in vivo metabolism compared to that of normal apoA-I in two heterozygous apoA-IIowa subjects and two normal controls. Normal and mutant apoA-I were radioiodinated with 131I and 125I, respectively, reassociated with autologous plasma lipoproteins, and simultaneously injected into all subjects. Kinetic analysis of the plasma radioactivity curves demonstrated that the mutant apoA-IIowa was rapidly cleared from plasma (mean fractional catabolic rate [FCR] 0.559 day-1) compared with normal apoA-I (mean FCR 0.244 day-1) in all four subjects. The FCR of normal apoA-I was also substantially faster in the heterozygous apoA-IIowa subjects (mean FCR 0.281 days-1) than in the normal controls (mean FCR 0.203 days-1). Despite the rapid removal from plasma of apoA-IIowa, the cumulative urinary excretion of its associated radioactivity after 2 weeks (44%) of the injected dose) was substantially less than that associated with normal apoA-I (78% of injected dose), indicating extravascular sequestration of radiolabeled apoA-IIowa.(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1991

Transcriptional regulation of the human lipoprotein lipase gene in 3T3-L1 adipocytes.

Lipoprotein lipase (LPL), a key enzyme in normal lipoprotein metabolism, has a complex pattern of regulation and tissue-specific expression. Several potential binding sites for transcription factors, including the recognition sequences for CCAAT/enhancer-binding protein and octamer-binding proteins (Oct) have been described in the 5'-flanking region of the human LPL gene. To identify elements which regulate the expression of LPL in adipocytes, plasmids containing deletion mutants of the 5'-LPL promoter region and the luciferase reporter gene were transfected in 3T3-L1 adipocytes. Deletions at -724, -565, -461, -368, -232, -167, -92, -35, and -17 relative to the transcriptional start site modified transcription from 100 to 162, 194, 185, 128, 63, 53, 29, and 0%, respectively, indicating the presence of negative (-724 to -565) and positive (-368 to -35) cis-acting regulatory elements. Transfection of HepG2 cells, which do not synthesize LPL, with the same constructs resulted in a similar pattern of expression for the majority of the deletions. However, deletions between -724 and -368 base pairs resulted in a 75-100% increase in transcription in 3T3 adipocytes but not in HepG2 cells, indicating the presence of tissue-specific regulatory element(s) in this region. An important regulatory element affecting LPL transcription in adipocytes was identified by gel mobility shift assays and DNase I footprint analysis. Using these techniques, a nuclear protein(s) in 3T3-L1 adipocytes was shown to bind specifically to a fragment which included the proximal octamer recognition site (from -46 to -39) present in the LPL promoter. The DNA-protein complex comigrates with an electrophoretic band containing the Oct-1-DNA complex in BJA-B nuclear extracts and the DNA-protein complex was selectively competed only by DNA fragments containing the octamer sequence. Preincubation of 3T3-L1 nuclear extracts with an antibody directed against the POU domain of Oct-1 inhibited the formation of the DNA-protein complex. Deletion of the proximal octanucleotide motif from the plasmid containing the -461 fragment of the LPL promoter, resulted in a 79 and 76% decrease in the level of expression in transfected 3T3-L1 adipocytes and HepG2 hepatocytes, respectively. These combined results have established that the expression of LPL in adipocytes is modulated by multiple positive and negative regulatory elements within the 5'-flanking region of the LPL gene. A proximal octamer binding sequence which specifically interacts with a nuclear protein(s) that exhibits the characteristics of Oct-1 has been identified.(ABSTRACT TRUNCATED AT 400 WORDS)

3T3 Cells↗
PubMed1991

Three genetic variants of human plasma apolipoprotein A-IV. apoA-IV-1(Thr347----Ser), apoA-IV-0(Lys167----Glu,Gln360----His), and apoA-IV-3(Glu165----Lys).

Human apolipoprotein (apo) A-IV is a genetically polymorphic glyco-protein of 376 residues. We have recently reported the molecular basis for the two most common isoproteins, apoA-IV-1 and apoA-IV-2(Gln360----His), and for two rare variants, apoA-IV-0 (4-amino acid insertion) and apoA-IV-3(Glu230----Lys). In this report, we present the genetic basis for three additional isoproteins of apoA-IV. Sequence analysis of the apoA-IV-1 allele revealed a common nucleotide substitution which converts threonine (ACT), residue 347 of the mature protein, into serine (TCT). In one out of the five subjects with the apoA-IV-1/0 phenotype we identified two point mutations: 1) replacing the positively charged lysine (AAG), amino acid 167, with a negatively charged glutamic acid (GAG), and 2) converting the neutral residue 360, glutamine (CAG), to a positively charged histidine (CAT). We have also characterized four additional heterozygotes for the apoA-IV-3 allele. One individual was found to have the previously described substitution of lysine for glutamic acid at amino acid 230. The three other subjects had the identical mutation but at a different position in the polypeptide chain, residue 165. These results indicate that one predominant allele codes for the isoproteins apoA-IV-2 and apoA-IV-0 and that at least two major allelic variants for the isoproteins apoA-IV-1 and apoA-IV-3 are present in the population analyzed.

Alleles↗