Covalent linkage of apolipoprotein e to albumin nanoparticles strongly enhances drug transport into the brain

Drug delivery to the brain is becoming more and more important but is severely restricted by the blood-brain barrier. Nanoparticles coated with polysorbates have previously been shown to enable the transport of several drugs across the blood-brain barrier, which under normal circumstances is impermeable to these compounds. Apolipoprotein E was suggested to mediate this drug transport across the blood-brain barrier. In the present study, apolipoprotein E was coupled by chemical methods to nanoparticles made of human serum albumin (HSA-NP). Loperamide, which does not cross the blood-brain barrier but exerts antinociceptive effects after direct injection into the brain, was used as model drug. Apolipoprotein E was chemically bound via linkers to loperamide-loaded HSA-NP. This preparation induced antinociceptive effects in the tail-flick test in ICR mice after i.v. injection. In contrast, nanoparticles linked to apolipoprotein E variants that do not recognize lipoprotein receptors failed to induce these effects. These results indicate that apolipoprotein E attached to the surface of nanoparticles facilitates transport of drugs across the blood-brain barrier, probably after interaction with lipoprotein receptors on the brain capillary endothelial cell membranes.     

Characterization of an abnormal species of apolipoprotein B, apolipoprotein B-37, associated with familial hypobetalipoproteinemia.

Steinberg and colleagues have previously described a unique kindred with normotriglyceridemic hypobetalipoproteinemia (1979. J. Clin. Invest. 64:292-301). In a reexamination of this kindred, we found an abnormal apolipoprotein (apo) B species, apo B-37 (203,000 mol wt), in the plasma lipoproteins of multiple members of the kindred. In affected individuals apo B-37 was found in very low density lipoproteins, along with the normal apo B species, apo B-100 and apo B-48. High density lipoproteins (HDL) also contained apo B-37, but no other apo B species. The first 13 amino-terminal amino acids of apo B-37 were identical to those of normal apo B-100. We utilized a panel of 18 different apo B-specific monoclonal antibodies and polyclonal antisera specific for apo B-37 and the thrombin cleavage products of apo B-100 to map apo B-37 in relation to apo B-100, apo B-48, and the thrombin cleavage products of apo B-100. The results of those immunochemical studies indicated that apo B-37 contains only amino-terminal domains of apo B-100. In affected individuals, the majority of apo B-37 in plasma was contained in the HDL density fraction. Within that fraction apo B-37 was found on discrete lipoprotein particles, termed Lp-B37, that had properties distinct from normal HDL particles containing apo A-I. This report documents for the first time the existence of an abnormal apo B species in humans. Further study of apo B-37 and lipoprotein particles containing apo B-37 should lead to an improved understanding of apo B structure and function.     

Sustained expression of human apolipoprotein A-I after adenoviral gene transfer in C57BL/6 mice: role of apolipoprotein A-I promoter, apolipoprotein A-I introns, and human apolipoprotein E enhancer

Elevation of HDL cholesterol, after adenoviral apolipoprotein A-I (apo A-I) gene transfer, may delay or revert ischemic cardiovascular disease, provided transgene expression is persistent. Previously, we observed transient human apo A-I expression after adenoviral gene transfer with a cytomegalovirus (CMV)-driven construct containing the human apo A-I cDNA. Therefore, the effects of promoters (CMV or 256 base pairs of the human apo A-I promoter), introns of the human apo A-I gene, and the liver-specific human apolipoprotein E (apo E) enhancer on adenovirus-mediated human apo A-I expression were evaluated in C57BL/6 mice. In the presence of the CMV promoter, human apo A-I introns prolonged expression above 20 mg/dl from 14 to 35 days. Addition of one, two, or four copies of the human apo E enhancer in these constructs resulted in a copy-dependent but transient increase in expression for 14 days. The apo A-I promoter induced 3.2-fold lower peak levels of human apo A-I than did the CMV promoter, but insertion of four apo E enhancers in the apo A-I promoter-driven construct resulted in human apo A-I levels above 20 mg/dl for 6 months. The decline between day 6 and day 35 of human apo A-I expression driven by the CMV promoter was due to (1) a 2.5-fold decline in transgene DNA levels that is not observed with apo A-I promoter-driven constructs, and (2) CMV promoter attenuation as evidenced by a 7.6-fold decline in the human apo A-I mRNA/human apo A-I DNA copy number ratio between day 6 and day 35. Hepatotoxicity, as evidenced by up to 10-fold higher serum levels of transaminases on day 6 after gene transfer with CMV promoter-driven constructs than with apo A-I promoter-driven constructs, probably caused the accelerated decline of transgene DNA. In conclusion, gene transfer with an adenovirus comprising the 256-bp apo A-I promoter, the genomic apo A-I DNA, and four apo E enhancers, all of human origin, is associated with low hepatotoxicity and with the absence of promoter shutoff resulting in human apo A-I expression above 20 mg/dl for up to 6 months.     

Quantitative analysis of apolipoprotein E secretion by human monocyte-derived macrophages in culture

Apolipoprotein E (apo E) has an impact on lipid metabolism and its production by macrophages is considered to play a protective role against atherosclerosis. Apo A-I stimulates secretion of apo E from macrophages. We developed a new method to evaluate the ability of human monocyte-derived macrophages to secrete apo E, and the effects of factors such as apo A-I were examined. Monocytes separated from peripheral venous blood were cultured. The levels of apo E in macrophage-conditioned medium were quantified by immunoblotting with an anti-human apo E antiserum conjugated with alkaline phosphatase. The basal levels of apo E secretion and the response to exogenous apo A-I in macrophages from 10 healthy volunteers were measured. Sufficient accuracy and sensitivity were confirmed and coefficient of variation of the method was 18 +/- 11% (n = 10). It was confirmed that macrophage secreted apo E in a concentration-dependent manner in response to M-CSF and apo A-I. The average apo E concentration in the conditioned medium of macrophages from 10 healthy subjects was 30.9 +/- 14.7 ng/mg cell protein. After the addition of apo A-I, the average apo E concentration increased, by about 60%, to 49.4 +/- 29.7 ng/mg cell protein (p < 0.05). There was a positive correlation between the apo A-I-induced increase and plasma LDL cholesterol levels (r = +0.54, p < 0.05).     

Measurement of very low density and low density lipoprotein apolipoprotein (Apo) B-100 and high density lipoprotein Apo A-I production in human subjects using deuterated leucine. Effect of fasting and feeding.

Six normolipidemic male subjects, after an 8-h overnight fast, were given a bolus injection and then a 15-h constant intravenous infusion of [D3]L-leucine. Subjects were studied in the fasted state and on a second occasion in the fed state (small, physiological meals were given every hour for 15 h). Apolipoproteins were isolated by preparative gradient gel electrophoresis from plasma lipoproteins separated by sequential ultracentrifugation. Incorporation of [D3]L-leucine into apolipoproteins was monitored by negative ionization, gas chromatography-mass spectrometry. Production rates were determined by multiplying plasma apolipoprotein pool sizes by fractional production rates (calculated as the rate of isotopic enrichment [IE] of each protein as a fraction of IE achieved by VLDL (d less than 1.006 g/ml) apo B-100 at plateau. VLDL apo B-100 production was greater, and LDL (1.019 less than d less than 1.063 g/ml) apo B-100 production was less in the fed compared with the fasted state (9.9 +/- 1.7 vs. 6.4 +/- 1.7 mg/kg per d, P less than 0.01, and 8.9 +/- 1.2 vs. 13.1 +/- 1.2 mg/kg per d, P less than 0.05, respectively). No mean change was observed in high density lipoprotein apo A-I production. We conclude that: (a) this stable isotope, endogenous-labeling technique, for the first time allows for the in vivo measurement of apolipoprotein production in the fasted and fed state; and (b) since LDL apo B-100 production was greater than VLDL apo B-100 production in the fasted state, this study provides in vivo evidence that LDL apo B-100 can be produced independently of VLDL apo B-100 in normolipidemic subjects.     

Atypical familial dysbetalipoproteinemia associated with apolipoprotein phenotype E3/3.

Familial dysbetalipoproteinemia has been reported to be associated uniquely with an apolipoprotein E phenotype (E2/2) that occurs in approximately 1% of all persons. We have observed the typical clinical and biochemical characteristics of this disorder in five members of a family, in all of whom the apolipoprotein E phenotype, as determined by isoelectric focusing electrophoresis, is E3/3. The disorder is present in three generations of the family: the proband, her mother, and three of the proband's five children. The proband's husband, father of all five children, also has apolipoprotein E phenotype E3/3, as do his two unaffected children. As in normal persons with phenotype E3/3, the apolipoprotein E of affected members appears to have a single residue of cysteine. When incorporated with egg lecithin into discoidal complexes, the apolipoprotein E from affected members was taken up normally into perfused livers of estradiol-treated rats, in which a high level of LDL receptors is expressed. When isoelectric focusing electrophoresis was carried out over a narrow range of pH (5-7), each of the apolipoprotein E isoforms of affected members was observed as a doublet, even after reduction of dimers of the protein with 2-mercaptoethanol and treatment with neuraminidase to minimize the content of sialylated forms of the protein. Doublets were also observed in the apolipoprotein E-2 of patients with classical dysbetalipoproteinemia, but only in the affected members of the family with atypical dysbetalipoproteinemia were the components of the doublets equally prominent. As in classical dysbetalipoproteinemia, both apolipoprotein B-100 and B-48 were present in the very low density lipoprotein fraction of plasma obtained in the postabsorptive state, indicating that remnantlike lipoproteins of both hepatic and intestinal origin accumulate. This observation, together with available evidence on the structural and functional heterogeneity of human apolipoprotein E, lead us to suggest that the disorder in this family is caused by one or two structurally abnormal forms of apolipoprotein E that contain a single residue of cysteine.     

Lipoproteins containing the truncated apolipoprotein, Apo B-89, are cleared from human plasma more rapidly than Apo B-100-containing lipoproteins in vivo.

We have reported previously on two truncations of apolipoprotein B (apo B-40 and apo B-89) in a kindred with hypobetalipoproteinemia. Premature stop codons were found to be responsible for both apo B-40 and apo B-89, but the physiologic mechanisms accounting for the reduced plasma concentrations of these proteins have not been determined in vivo. This study investigates the metabolism of apo B-89 in two subjects heterozygous for apo B-89/apo B-100 and in one apo B-40/apo B-89 compound heterozygote. In both heterozygotes total apo B concentration is approximately 30% of normal and apo B-89 is present in lower concentrations in plasma than apo B-100. After the administration of [1-13C]leucine as a primed constant infusion over 8 h, 13C enrichments of plasma leucine as well as enrichments of VLDL-, IDL-, and LDL-apo B-89 leucine and VLDL-, IDL-, and LDL-apo B-100 leucine were measured over 110 h. Enrichment values were subsequently converted to tracer/tracee ratios and a multicompartmental model was used to estimate metabolic parameters. In both apo B-89/apo B-100 heterozygotes apo B-89 and apo B-100 were produced at similar rates. Respective transport rates of apo B-89 and apo B-100 for subject 1 were 2.13 +/- 0.18 and 2.56 +/- 0.13 mg.kg-1.d-1, and for subject 2, 6.59 +/- 0.18 and 8.23 +/- 0.39 mg.kg-1.d-1. However, fractional catabolic rates of VLDL, IDL, and LDL particles containing apo B-89 were 1.4-3 times higher than the rates for corresponding apo B-100-containing particles. Metabolic parameters of apo B-89 in the apo B-40/apo B-89 compound heterozygote compared favorably with those established for apo B-89 in apo B-89/apo B-100 heterozygotes. Thus, the enhanced catabolism of VLDL, IDL, and LDL particles containing the truncated apolipoprotein is responsible for the relatively low levels of apo B-89 seen in these subjects.     

Quantitation of apolipoprotein B by polyclonal and monoclonal antibodies

A non-competitive sandwich enzyme linked immunosorbent assay for the quantitation of apolipoprotein B with polyclonal and monoclonal antibodies was developed. Polyclonal antibodies were used as 'coater'. In the assay with polyclonal antibodies, the same antibody was used as conjugate with alkaline phosphatase. For studies with monoclonal antibodies, enzyme conjugated anti-mouse immunoglobulin had to be used, since monoclonal antibodies lost their reactivity upon enzyme conjugation. Two murine monoclonal antibodies were employed: MAB B-1 with specificity for apolipoproteins (Apo) B-48 and B-100 and MAB B-5 with specificity for B-100 (Radioimmunoassay Inc.). In a reference group Apo B values of 0.82 +/- 0.20 g/l were measured with polyclonal antibodies, 0.68 +/- 0.19 g/l and 0.95 +/- 0.33 g/l with MAB B-1 and MAB B-5. In pure hypercholesterolemia, a similar increase was found with all three antibodies, while in combined hyperlipoproteinemia MAB B-5 gave greater than 40% lower values. Differences were also found with respect to the correlation between Apo B and cholesterol or triglycerides.     

Isolation and characterization of apolipoprotein B-48 and B-100 very low density lipoproteins from type III hyperlipoproteinemic subjects.

Two major species of human apolipoprotein (apo) B have been identified, apo B-48 and apo B-100, which are the predominant forms in chylomicrons and very low density lipoproteins (VLDL), respectively. Due to defective hepatic clearance, apo B-48 containing lipoproteins accumulate in the plasma of subjects with type III hyperlipoproteinemia. In the present study, we have used immunoaffinity chromatography to separate type III VLDL into a nonretained (apo B-48 VLDL) and a retained (apo B-100 VLDL) fraction. To achieve complete separation, as determined by electrophoresis and radioimmunoassay, it was necessary to employ two different insolubilized anti-apo B-100 monoclonal antibodies because of immunochemical heterogeneity within the apo B-100 VLDL fraction. The ability to separate apo B-100 VLDL from apo B-48 VLDL shows that the two apo B species are found on different particles. The apo B-48 VLDL had an electrophoretic mobility similar to chylomicrons, whereas the apo B-100 VLDL migrated similarly to total type III VLDL. Both fractions showed a concentration of particles with diameters approximately 100 nm, with apo B-48 VLDL being somewhat more heterogeneous in particle size. The two fractions were qualitatively similar in apolipoprotein composition but apo B-48 VLDL was enriched in apo E, relative to apo B-100 VLDL. Apo B-48 VLDL was enriched in cholesterol esters and deficient in triglycerides and phospholipids when compared with apo B-100 VLDL. The existence of immunochemical heterogeneity in the apo B-100 VLDL may reflect different functional subpopulations of particles within this fraction.     

Comparison of four procedures for separating apolipoprotein A- and apolipoprotein B-containing lipoproteins in plasma

Because lipoproteins containing apolipoprotein A (ApoA-I + ApoA-II) or apolipoprotein B (ApoB) seem to exert opposite effects as risk factors for coronary heart disease, we decided to determine the separability of these two major plasma lipoproteins by procedures originally designed to separate high-density from low- and very-low-density lipoproteins. The presumably ApoB-free lipoproteins isolated from normal plasma by (a) ultracentrifugation at d = 1.063; precipitation with (b) heparin-Mn2+ or (c) phosphotungstate-Mg2+; or (d) immunoprecipitation with antibodies to ApoB were characterized by quantifying cholesterol and apolipoproteins A-I, A-II, B, C-II, C-III, D, E, F, and Lp(a). ApoA- and ApoB-containing lipoproteins were completely separated only by immunoprecipitation with antibodies to ApoB. The ApoB-containing lipoproteins isolated by other procedures always contained 4% to 20% of total plasma ApoA-I and differed substantially from one another with respect to the content of some of the minor apolipoproteins. Measuring apolipoproteins was more reliable than measuring cholesterol for monitoring this separation and for expressing the concentrations of ApoA- and ApoB-containing lipoproteins.     

Relation of genetic polymorphisms of apolipoprotein E, angiotensin converting enzyme, apolipoprotein B-100, and glycoprotein IIIa and early-onset coronary heart disease.

OBJECTIVE: Apolipoprotein E (APOE) E4, apolipoprotein B-100 (APOB) Q3611 allele, the angiotensin converting enzyme (ACE) deletion (D) allele and glycoprotein IIIa (GP3A) P33 mutant allele are reported to predispose to early-onset coronary heart disease (CHD). These associations were not all confirmed in more recent studies. To determine the impact of these alleles on CHD, we examined the prevalence of these mutations in patients presenting with early-onset CHD and compared them to those manifesting CHD later in life. The delayed-onset was considered a sign of longevity and would serve as a comparative group to assess prevalence of the biochemical and genetic risk factors. METHODS: 300 patients with a history of myocardial infarction or angina pectoris and angiographically documented CHD were studied. Patients were divided into two groups: group 1 (G1 = 150 patients) presenting with these findings under the age of 50 years; while group 2 (G2 = 150 patients) were patients presenting for the first time over the age of 65 years. Prevalence of the alleles of APOE, APOB, ACE and GP3A was assessed by molecular analysis. An association of any of these genotypes with early onset CHD could lead to a higher prevalence in the younger age group. RESULTS AND CONCLUSIONS: None of the suspected alleles namely APOB Q3611 [G1: 10.7% vs. G2: 9.0%, p = 0.57], ACE D (G1: 52.0% vs. G2: 49.7%, p = 0.57), or the GP3A P33 (G1: 17.3% vs. G2: 15.7%; p = 0.58) showed any significant difference between the two groups. Subjects with APOE E4 were more frequent in the younger age group (G1: 18.3% vs. G2: 13.7%; p = 0.047), while APOE E2 was more frequent in G2 (G2: 10.0% vs. G1: 2.7%; p = 0.0002). Multivariate analysis showed an odds ratio of APOE E2 allele in G1 of 0.27 with a confidence interval of 0.10-0.73.     

The basic apolipoprotein A-I in the patients with familial lecithin:cholesterol acyltransferase deficiency

The apolipoprotein A-I (apo A-I) from the patients with familial lecithin: cholesterol acyltransferase (LCAT) deficiency has been characterized. More than 10% of total serum apo A-I was recovered in the bottom fraction (d greater than 1.21 g/ml) of the patients' sera, while in normal sera, only 4.3 +/- 2.7% (Mean +/- SD) of apo A-I was found in the bottom fraction. The lipoproteins of the sera from the patients were analysed by density gradient ultracentrifugation and by high performance liquid chromatography (HPLC). Our HPLC analysis has revealed that the association of apo A-I with a lipid-poor HDL, observed in the patients, is not due to ultracentrifugal artifacts. The serum apo A-I isoproteins and their relative concentrations in the three patients were analysed by two dimensional electrophoresis. The isoprotein distribution for each of the three patients was as follows: isoproteins 2 and 3; 12.3, 21.2 and 25.7%, suggesting that the basic isoforms (isoproteins 2 and 3) were increased compared to normal controls (3.5 + 2.3%). The profile of apo A-I isoproteins of the bottom fraction was almost the same as that of the patient's HDL, indicating the equivalent affinity of each isoprotein with lipid and/or protein of HDL. These observations are compatible with the suggestion that the reduction of HDL-cholesterol in familial LCAT deficiency may be due to rapid catabolism of HDL, resulting in the increase in the ratio of apo A-I isoprotein 2 and 3 (immature form of isoprotein A-I) to isoprotein 4 and 5 (mature form of apo A-I).     

Characterization of monoclonal antibodies against human apolipoprotein E.

From a single cell fusion, five stable hybridomas secreting antiapolipoprotein E (apo E) were obtained. The immunoglobulin (Ig)G subclasses containing the respective monoclonal antibodies were isolated and were used as the antibody component in a solid-phase radioimmunoassay. The binding of 125I-apo E to the insolubilized antibody was inhibited by unlabeled apo E but not by unlabeled apoproteins A-I, A-II, C-II, and C-III, or by low density lipoprotein immunodepleted of endogenous apo E. Competition curves were obtained with lipoprotein subfractions that had the same shape as those obtained with purified apo E. Apo E levels in normal and hyperlipidemic plasma were well correlated when measured by the five monoclonal antibodies and polyclonal anti-apo E, although differences in absolute values were observed. In normal subjects 34, 10, 20, and 36% of apo E was recovered in the very low density lipoprotein, low density lipoprotein, high density lipoprotein, and the d greater than 1.21-gl/ml fractions, respectively, whereas these values were 34, 7, 12, and 47%, respectively, in type III patients. All antibodies indicated the same subfraction distribution of apo E. The monoclonal antibodies reacted with all of the isomorphs of apo E. One of the antibodies could be clearly distinguished by its reactivity with chemically modified very low density lipoprotein.     

High-density lipoprotein apolipoproteins in urine: II. Enzyme-linked immunoassay of apolipoprotein A-I

We have developed a capture antibody, noncompetitive, enzyme-linked immunoassay for urinary apolipoprotein A-I (Apo A-I) in urine, with use of affinity-purified polyclonal antisera against Apo A-I. A 96-well microtiter plate format is used, with unconcentrated urine as sample and dilutions of serum or high-density lipoprotein (HDL) as standards. The intra- and interassay variation (CV) averaged 7.4% and 9.4%, respectively. The limit of detection is low (1.25 ng/L), and no cross-reactivity with Apo B, C, E, or A-II was detected. The mean (+/- SD) concentrations of Apo A-I in urine of patients with glomerular proteinuria were a thousandfold greater (38.4 +/- 23.1 mg/L) than in normal subjects (16.3 +/- 11.3 micrograms/L in men, 17.97 +/- 7.7 micrograms/L in women, a significant difference, P less than 0.001). Apo A-I measurements correlated very well (r = 0.92) with selectivity index assessment. The diurnal variation of the concentration of Apo A-I in urine appears to result from dilution related to fluid intake. This enzymatic method is easy to perform, can be used with large numbers of samples, and is adaptable for use in the routine clinical laboratory. The method holds promise for discriminating between normal and subclinical kidney disease populations by measuring the concentrations of urinary Apo A-I excreted on HDL particles.     

In vivo metabolism of apolipoprotein S in humans. Comparison with apolipoprotein A-I metabolism

¹²⁵I-labelled apolipoprotein (Apo) S and ¹³¹I-labelled apolipoprotein A-I were injected i.v. into healthy volunteers. Blood samples and daily urine collections were drawn periodically for 15 days. Ninety-eight percent of ¹³¹I radioactivity and > 95% of ¹²⁵I radioactivity were found in HDL after Superose gel chromatography of plasma. About 10% of each radioactivity was recovered in the d 1.250 infranate after one ultracentrifugation. Affinity chromatography on monoclonal anti-Apo A-I Sepharose column separates two lipoprotein particles containing Apo S, one retained with Apo A-I (42.5%) and the other eluting without Apo A-I (57.5%).

Kinetic parameters were calculated according to exponential curve fitting. Mean transit time was about 7.0 days for both Apo A-I and Apo S. FCR of Apo S was 50% higher than FCR of Apo A-I. Synthetic rate of Apo S was about 150 times smaller than for Apo A-I.

As heterogeneity of HDL-S was suggested by both the results of affinity chromatography and the urinary data, a compartmental model was built which fitted adequately all data. Part of the model is common to HDL-A-I and HDL-S.     

Effect of promoters and enhancers on expression, transgene DNA persistence, and hepatotoxicity after adenoviral gene transfer of human apolipoprotein A-I

Liver-directed gene transfer offers new perspectives for the treatment of inherited metabolic diseases and disorders of lipoprotein metabolism. Potent expression cassettes for transgenes in the liver may optimize gene transfer efficiency and improve the therapeutic index of gene transfer vectors. An E(1)-deleted adenovirus comprising the hepatocyte specific 256-base pair (bp) human apolipoprotein A-I (apo A-I) promoter, the genomic human apo A-I DNA, and four human apo E enhancers (AdA-I.gA-I.4xapoE) was associated with low hepatotoxicity, high transgene DNA persistence and absence of promoter shut-off, resulting in human apo A-I plasma levels above 100 mg/dl for 35 days in C57BL/6 mice. In the present investigation, the human apo A-I promoter was compared to the murine small nuclear RNA U1b, the human apolipoprotein C-II (apo C-II), and the human alpha(1) antitrypsin (hAAT) promoters and the effect of copy number and position of liver-specific human apo E enhancers in 16 adenoviral constructs was evaluated. The vector containing the 1.5-kb hAAT instead of the apo A-I promoter (AdhAAT.gA-I.4xapoE) induced 3.7-fold (p < 0.01) more human apo A-I reaching plasma levels above 300 mg/dl for 35 days. The composition of the expression cassette was a major determinant of human apo A-I transgene DNA copy number at day 35. Hepatotoxicity after adenoviral gene transfer was dependent on the promoter and the number of enhancers, and was higher with the enhancers in a 5' position. The combination of the hAAT promoter and four copies of the human apo E enhancer appears to be the expression cassette of choice for hepatocyte-specific overexpression of transgenes after gene transfer.     

Procedure for development of an enzyme-linked immunosorbent assay. Development of an assay for human apolipoprotein A-I.

A set of criteria for selection of antibodies during the development of enzyme-linked immunosorbent assay (ELISA) is described. Using these criteria, a competitive ELISA for human apo A-I using a polyclonal goat antibody was developed. The assay recognizes apo A-I from plasma and high-density lipoprotein (HDL), as well as the pure delipidated apo A-I, equally. Intra- and inter-assay variations were 5.2% and 3.5%, respectively. Recovery rate, as determined by spiking a known quantity of pure delipidated apo A-I into a reference plasma, was determined to be 101.3%. The assay was validated by comparing the concentration of apo A-I in HDL with the dye elution method. The apo A-I ELISA to apo A-I dye elution ratio was 1.01. Apo A-I concentration in Centers for Disease Control reference material determined by this method was in agreement with the reported consensus value. Repeated freezing and thawing of the samples (three freeze-thaw cycles at -20 degrees C) as well as long-term freezing (up to 1 year at -70 degrees C) did not affect the concentration of apo A-I in the samples. The assay was applicable both to normolipidemic and dyslipidemic plasmas. No immunologic difference was noted when plasma from dyslipidemic subjects was assayed. A frequent problem of long-term storage is deamidation. The values found for apo A-I in a deamidated plasma were the same as those for the corresponding fresh plasma. Plasma apo A-I values were also positively correlated with that of HDL-cholesterol.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of atorvastatin on high-density lipoprotein apolipoprotein A-I metabolism in dogs

BACKGROUND: The mechanisms involved in the decline of high-density lipoprotein (HDL) levels at a higher dose of atorvastatin have not yet been elucidated. We investigated the effects of atorvastatin on HDL-apolipoprotein (apo) A-I metabolism in dogs, a species lacking cholesteryl ester transfer protein activity. MATERIALS AND METHODS: Seven ovariectomized normolipidaemic female Beagle dogs underwent a primed constant infusion of [5,5,5-(2)H(3)] leucine to determine HDL-apo A-I kinetics before and after atorvastatin treatment (5 mg kg(-1) d(-1) for 6 weeks). Plasma lipoprotein profiles, activity of HDL-modifying enzymes involved in reverse cholesterol transport and hepatic scavenger receptor class B type I (SR-BI) expression were also studied. RESULTS: Atorvastatin treatment decreased HDL-cholesterol levels (3.56 +/- 0.24 vs. 2.64 +/- 0.15 mmol L(-1), P < 0.05). HDL-triglycerides were not affected. HDL-phospholipids levels were decreased (4.28 +/- 0.13 vs. 3.29 +/- 0.13 mmol L(-1), P < 0.05), as well as phospholipids transfer protein (PLTP) activity (0.83 +/- 0.05 vs. 0.60 +/- 0.05 pmol microL(-1) min(-1), P < 0.05). Activity of lecithin: cholesterol acyl transferase (LCAT), hepatic lipase (HL) and SR-BI expression did not change. HDL-apo A-I absolute production rate (APR) was higher after treatment (twofold, P < 0.05) as well as fractional catabolic rate (FCR) (threefold, P < 0.05). This resulted in lower HDL-apo A-I levels (2.36 +/- 0.03 vs. 1.55 +/- 0.04 g l(-1), P < 0.05). Plasma lipoprotein profiles showed a decrease in large HDL(1) levels, with lower apo A-I and higher apo E levels in this subfraction. CONCLUSIONS: Although a high dose of atorvastatin up-regulated HDL-apo A-I production, this drug also increased HDL-apo A-I FCR in dogs. This effect could be explained by a higher uptake of apo E-enriched HDL(1) by hepatic lipoprotein receptors.     

Apolipoprotein A-I variants. Naturally occurring substitutions of proline residues affect plasma concentration of apolipoprotein A-I.

Six unrelated families with genetically determined structural variants of apo A-I were found in the course of an electrophoretic screening program for apo A-I variants in dried blood samples of newborns. The following structural variations were identified by the combined use of HPLC, time-of-flight secondary ion mass spectrometry (TOF-SIMS), and automated gas phase sequencing: Pro3----Arg (1x), Pro4----Arg (1x), and Pro165----Arg (4x). All variant carriers were heterozygous for their mutant of apo A-I. Subjects heterozygous for apo A-I(Pro165----Arg) (n = 12) were found to exhibit lower mean values for apo A-I (109 +/- 16 mg/dl) and HDL cholesterol (37 +/- 9 mg/dl) than unaffected family members (n = 9): 176 +/- 41 and 64 +/- 18 mg/dl, respectively (P less than 0.001). In 9 of 12 apo A-I(Pro165----Arg) variant carriers the concentrations of apo A-I were below the fifth percentile of sex-matched controls. By two-dimensional immunoelectrophoresis as well as by densitometry the relative concentration of the variant apo A-I in heterozygous carriers of apo A-I(Pro165----Arg) was determined to account for only 30% of the total plasma apo A-I mass instead of the expected 50%. Thus, the observed apo A-I deficiency may be largely a consequence of the decreased concentration of the variant apo A-I. In the case of the apo A-I(Pro3----Arg) mutant, densitometry of HDL apolipoproteins demonstrated a distinctly increased concentration of the variant proapo A-I relative to normal proapo A-I. This phenomenon was not observed in the apo A-I(Pro4----Arg) mutant or in other mutants. This suggests that the interspecies conserved proline residue in position 3 of mature apo A-I is functionally important for the regular enzymatic conversion of proapo A-I to mature apo A-I.