Radioimmunoassay of human arginine-rich apolipoprotein, apoprotein E. Concentration in blood plasma and lipoproteins as affected by apoprotein E-3 deficiency

Relationships between the responses of triglyceride-rich lipoproteins in blood plasma containing apolipoproteins B-48 and B-100 to a fat-containing meal in normolipidemic humans

The concentration of triglyceride-rich lipoproteins containing apolipoprotein (apo) B-48 (chylomicrons) and apo B-100 (very low density lipoproteins) was measured in blood plasma of healthy young men after an ordinary meal containing one-third of daily energy and fat. Plasma obtained in the postabsorptive state and at intervals up to 12 hr after the meal was subjected to immunoaffinity chromatography against a monoclonal antibody to apo B-100 that does not bind apo B-48 and a minor fraction of apo B-100 rich in apo E. Measurements of the concentrations of components of the total and unbound triglyceride-rich lipoproteins separated from plasma by ultracentrifugation showed that about 80% of the increase in lipoprotein particle number was in very low density lipoproteins containing apo B-100 and only 20% was in chylomicrons containing apo B-48 that carry dietary fat from the intestine. The maximal increments and the average concentrations of apo B-48 and B-100 during the 12 hr were highly correlated (r2 = 0.80), suggesting that preferential clearance of chylomicron triglycerides by lipoprotein lipase leads to accumulation of hepatogenous very low density lipoproteins during the alimentary period. The composition of the bulk of very low density lipoproteins that were bound to the monoclonal antibody changed little and these particles contained about 90% of the cholesterol and most of the apo E that accumulated in triglyceride-rich lipoproteins. The predominant accumulation of very low density lipoprotein rather than chylomicron particles after ingestion of ordinary meals is relevant to the potential atherogenicity of postprandial lipoproteins.

Role of apolipoprotein E on cholesteryl ester-enriched low density lipoprotein particles in coronary artery atherosclerosis of hypercholesterolemic nonhuman primates

Significant differences among individuals occur in the lipoprotein response to atherogenic diets in cynomolgus and African green monkeys. The range of concentrations of total plasma cholesterol (TPC) was 100-600 mg/dl and of apolipoprotein (apo) E (quantified by enzyme-linked immunosorbent assay) was 3-20 mg/dl in the animal groups of this study. The correlation between the concentrations of TPC and of apo E was r = 0.89 in these animals. To determine which lipoprotein classes contained the majority of apo E, agarose gel-filtration chromatography was used to subfractionate whole plasma. In hypercholesterolemic monkeys, the majority of the apo E and apo B-100 coeluted within the region of low density lipoprotein (LDL). In normocholesterolemic monkeys, the majority of apo E coeluted with apo A-I and high density lipoproteins. A strong positive correlation was seen between the concentrations of plasma apo E and LDL cholesterol (r = 0.9), but there was no significant correlation between high density lipoprotein apo E and either TPC or plasma apo E concentrations. A positive correlation of r = 0.8 was found between the LDL apo E to apo B-100 molar ratio and the average LDL particle size, suggesting an increase in the number of apo E molecules on the larger LDL particles. Within individual animals, LDL were heterogeneous and the LDL subfractions of lower density (1.02 less than d less than 1.03 g/ml) had the highest proportion of apo E, although apo E was present on LDL of all densities. A strong positive correlation between plasma apo E concentration and coronary artery atherosclerosis was identified, and in stepwise regression analysis, plasma apo B concentration and the apo E to apo B molar ratio of LDL together accounted for more than 90% of the variation in the atherosclerosis end point of coronary artery intimal area. These data strongly suggest that the enrichment of LDL with cholesteryl esters and apo E, which occurs in hypercholesterolemic primates, is an atherogenic feature of the plasma lipoproteins.

Spurious elevation of serum high-density lipoprotein cholesterol in patients with cholestatic liver diseases

Strikingly discrepant values were obtained by two commercial precipitating reagents for serum HDL cholesterol determination in three patients with cholestatic liver diseases (two patients with primary biliary cirrhosis and one patient with chronic hepatitis). An abnormal alpha 2-migrating lipoprotein (slow alpha-lipoprotein) was observed in agarose gel electrophoresis for each serum. The slow alpha-lipoprotein was partly recovered in the supernatant by precipitation with polyethylene glycol, and was completely precipitated with a polyanion-containing reagent, which well explains the discrepancy. The slow alpha-lipoprotein isolated from one of the cases is notable for (1) having an intermediate particle size between normal LDL and normal HDL, (2) containing apo E as the major apolipoprotein, and (3) being enriched with cholesterol (esterified and free) and phospholipid. Cholesterol accumulation was also found in another HDL subclass, alpha 1-migrating HDL. A severe impediment in the clearance of cholesterol-loaded HDL particles from plasma was implied. Electrophoresis of serum lipoproteins and/or the measurement of serum apo E concentrations are necessary to avoid an erroneous estimation of HDL cholesterol in patients with hepatobiliary diseases.

The very-high-density lipoprotein fraction of rabbit plasma is rich in tissue-derived cholesterol

When plasma from rabbits, which several weeks earlier had been infused with [3H]cholesterol, was subjected to equilibrium density gradient ultracentrifugation, the specific radioactivity of cholesterol in the very-high-density lipoprotein (VHDL) fraction (d 1.22-1.32 g/ml) was three to 8-fold greater (mean, 5.5-fold; P less than 0.001) than that in high-density lipoproteins (HDL; d 1.06-1.21 g/ml). On size exclusion chromatography of plasma, no increase in specific radioactivity was seen in particles smaller than HDL. These findings suggest that those apolipoprotein-lipid complexes that dissociate from HDL during ultracentrifugation to form the VHDL fraction contain proportionately more tissue-derived cholesterol than do those that are more tightly bound to HDL.

Molecular and metabolic basis for the metabolic disorder normotriglyceridemic abetalipoproteinemia

We have previously described a disorder, normotriglyceridemic abetalipoproteinemia, that is characterized by the virtual absence of plasma low density lipoproteins and complete absence of apoB-100, but with apparently normal secretion of triglyceride-rich lipoproteins containing apoB-48. The patient's plasma lipoproteins were shown on polyacrylamide gels and by antibody mapping to have a new truncated apoB variant, apoB-50, circulating along with her apoB-48. We have found this individual to be homozygous for a single C-to-T nucleotide substitution at apoB codon 2252, which produces a premature in-frame stop codon. Thus, this is a rare example of homozygous hypobetalipoproteinemia. Electron photomicrographs revealed that the diameters of particles in the d less than 1.006 g/ml lipoprotein fraction, in both the postprandial and postabsorptive state, are bimodally distributed. The molar ratio of apoE to apoB in these particles is 3.5:1, similar to normal VLDL. The plasma LDL interval contains both spherical and cuboidal particles. Autologous reinfusion of labeled d less than 1.006 g/ml lipoproteins showed exponential disappearance from plasma, with an apparent half-removal time of 50 min, somewhat slower than for normal chylomicrons but within the normal range for VLDL. The calculated production rate for apoB was within the normal range in this subject. A very small amount of label was found briefly in the IDL fraction, but none at any time in LDL or HDL. Therefore, because LDL particles that contain apoB-50 lack the putative ligand domain of the LDL receptor, we conclude that the very low level of LDL is due to the rapid removal of the abnormal VLDL particles before their conversion to LDL can take place.

Effect of a high fat/cholesterol diet with or without eicosapentaenoic acid on plasma lipids, lipoproteins and lipid transfer protein activity in the marmoset

Marmosets fed a diet supplemented with 0.2% cholesterol and 10% sheep fat (by weight) developed hypercholesterolemia with a 4-fold increase in plasma cholesterol (4.28 +/- 0.57-16.38 +/- 4.22 mmol/l, mean +/- SD, P less than 0.001). This was due mainly to a 5-fold increase in the intermediate density lipoprotein (IDL) and low density lipoprotein (LDL) fraction (d = 1.006-1.063 g/ml). The proportion of plasma cholesterol in high density lipoproteins (HDL) decreased from 56% to 25% although HDL cholesterol increased from 2.40 +/- 0.42 to 4.09 +/- 0.92 mmol/l (P less than 0.001), and HDL particle radius increased from 5.10 +/- 0.18 nm to 6.06 +/- 0.73 nm (P less than 0.05). Plasma lipid transfer protein (LTP) activity increased 2.5-fold in whole plasma and 2-fold in lipoprotein-deficient plasma. The atherogenic lipoprotein profile was attenuated by adding 0.8% eicosapentaenoic acid (EPA, 20:5 n - 3, as the ethyl ester) to the atherogenic diet. Plasma cholesterol increased only 55% to 6.64 +/- 2.55 mmol/l with only an 80% increase in lipoproteins in the d = 1.006-1.063 g/ml fraction and a more favourable proportion of plasma cholesterol in HDL (44%) than without EPA. LTP activity was reduced to 1.7-fold above control in whole plasma by addition of EPA to the atherogenic diet. There was a positive correlation between plasma cholesterol and LTP activity in whole plasma (r = 0.89, P less than 0.001) and in lipoprotein-deficient plasma (r = 0.67, P less than 0.001). EPA therefore attenuated some of the adverse effects of a 0.2% cholesterol, 10% sheep fat diet on plasma lipids and lipoproteins and induced a less atherogenic profile.

Serum apolipoprotein E in children and adolescents: the Bogalusa Heart Study

Serum apolipoprotein (apo) E levels and its relationship to lipids and lipoprotein cholesterol fractions were examined in a random subsample (n = 561) of children and adolescents (7 to 17 years of age) from a total biracial community. Mean (+/- SD) levels of apo E were higher in blacks (males 4.8 +/- 1.8 mg/dL; females 5.2 +/- 1.8 mg/dL) than in whites (males 3.9 +/- 1.2 mg/dL; females 4.3 +/- 1.0 mg/dL) irrespective of sex (P less than .001). The black-white difference in apo E persisted after controlling for the covariates: sexual maturation, age, adiposity, cigarette smoking, alcohol use, and oral contraceptive use (P less than .001). A sex differential (females greater than males, P less than .01) for apo E was seen in both racial groups. Apo E levels were inversely associated with age (P less than .01) and sexual maturation (P less than .05) only in white males. Apo E related positively and significantly to total cholesterol, low-density lipoprotein cholesterol, and high-density lipoprotein cholesterol fractions (HDL2-C and HDL3-C) in certain race-sex groups. Race, HDL2-C, triglycerides (very-low density lipoprotein cholesterol), HDL3-C, and sex were identified as predictor variables for apo E, in that order, and accounted for 21% of its variability in serum. It is conceivable that the observed race-sex differences in apo E may be related to apo E-HDL subfraction, which is thought to participate in the reverse cholesterol transport.

Apo E polymorphism and the removal of remnants of triglyceride-rich lipoproteins in normolipidemic subjects during a carbohydrate-rich diet

The role of the apo E polymorphism in the removal of remnants of very low density lipoproteins and chylomicrons was studied after a carbohydrate-rich diet in 10 healthy normolipidemic volunteers with different apo E phenotypes during 7 days. The cholesterol concentration in the heparin-sepharose bound part of the VLDL + IDL fraction (d < 1.019 g/ml) was taken as an estimate of the remnant concentration. Before and after carbohydrate-rich diet retinyl palmitate, mixed with cream, was consumed by each subject the evening before the fasting venepuncture to quantify the removal of chylomicron remnants. After the diet there was a comparable mean rise in the three groups in serum and in very low density lipoprotein triglycerides of about 30% and 50%, respectively. The concentration of remnants of very low density lipoproteins increased slightly in all subjects. The concentration of retinyl palmitate in the d < 1.019 g/ml fraction was 20% lower than before this diet in the E-2 homozygous subjects. In the other two groups, however, 25 to 80% higher retinyl palmitate levels were found. It is concluded, that after a carbohydrate-rich diet there is only a slight increase of very low density lipoprotein remnants, independent of the apo E polymorphism. The removal of chylomicron remnants, however, seems to be facilitated in E-2 homozygous subjects, in contrast to a slower removal in the groups with other apo E phenotypes.

Differential distribution of apolipoprotein E isoforms in human plasma lipoproteins

The polymorphism of apolipoprotein E (apo E) accounts for a substantial amount of the genetic variance of cholesterol levels in man. The epsilon-2 allele and the epsilon-4 allele raises plasma and low density lipoprotein cholesterol levels as compared to the epsilon-3 allele. Whereas the lower cholesterol levels in carriers of the epsilon-2 allele can, at least in part, be attributed to the grossly deficient binding of apo E-2 to the apo B,E receptor, apo E-3 and E-4 bind to the same degree. We used gel filtration and ultracentrifugation to separate lipoproteins and subsequent immunoblotting analysis to study the apo E isoform distribution. We always found lipoproteins of lower density relatively enriched in apo E-4 and high density lipoproteins relatively depleted of apo E-4 as compared to apo E-3. This was also seen in plasma of heterozygous subjects that simultaneously express two apo E isoforms. Also, the apo E-A-II complex was directly shown by immunoblotting. Furthermore, when purified iodinated apo E was incubated with plasma in vitro, apo E-4 also reassociated more with lipoproteins of lower density than apo E-3. We conclude that apo E-3 and apo E-4 have a different lipoprotein particle distribution in vivo. This differential lipoprotein distribution may account for differences in the metabolism between apo E-3 and E-4.

Concentrations of apolipoproteins B, C-I, C-II, C-III, E and lipids in serum and serum lipoproteins of normal subjects during alimentary lipaemia

The effect of alimentary lipaemia, induced by ingestion of 100 g of fat (cream), on lipids and apolipoprotein B, C-I, C-II, C-III and E levels was evaluated in 16 normotriglyceridaemic subjects. Apolipoprotein concentrations were determined by enzyme immunoassay in whole serum and, in a subsample of seven subjects, in serum lipoprotein fractions of d less than 1.006 kg/l (supernatant) containing very low density lipoproteins (VLDL) and chylomicrons and of d greater than 1.006 kg/l (infranatant) containing low density lipoproteins (LDL) and high density lipoproteins (HDL). Triglyceride concentrations in the supernatant were increased by 164% 3 h after the fat ingestion. The level of alimentary lipaemia was positively related to the fasting serum concentrations of triglycerides and apolipoproteins C-II and C-III and inversely related to the fasting concentration of apolipoprotein E in the infranatant lipoprotein fraction with d greater than 1.006 kg/l. No significant changes in the serum levels of apolipoproteins B, C-I and E were found during the postprandial lipaemia. By contrast, serum concentrations of apolipoproteins C-II and C-III were decreased (compared to the fasting levels) 6 h after the fatty meal. In the 3-h samples all apolipoproteins were increased in the supernatant (VLDL + chylomicrons) and a corresponding decrease was observed in the infranatant (LDL + HDL). In conclusion, this study shows that in normotriglyceridaemic subjects a high serum level of apo C-II is not associated with a more rapid clearance of alimentary lipaemia than a low level.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of a monoclonal antibody that binds to apolipoprotein E and to lipoprotein of human plasma containing apo E. Applications to ELISA quantification of plasma apo E

This study describes the development of an enzyme-linked immunosorbent assay for human apolipoprotein E (apo E). A mouse monoclonal IgG1 antibody named E01 against apolipoprotein E was selected from five antibodies secreted by hybridomas. This antibody had a high affinity for apo E [K = 1.2 x 10(7) L.M-1 for purified apo E and K = 1.05 x 10(7) L.M-1 for native apo E in very low density lipoproteins) in liquid phase and recognized every isoform of apo E but not other proteins in VLDL. Competition experiments with 125I apo E showed that its binding affinity for the E in every density class (VLDL, HDL, LDL) and in serum was the same. This antibody was used for the quantification of human apo E in serum by enzyme linked immunoassay. E01 was coated on microtiter plates and a polyclonal peroxidase-conjugate was used as second antibody. A good correlation was observed between the values obtained for apo E using both monoclonal and polyclonal antibodies.

Enhanced metabolism of normolipidemic human plasma very low density lipoprotein in cultured cells by exogenous apolipoprotein E-3

In this investigation in cultured human fibroblasts, an attempt was made to determine the optimal metabolism of apolipoprotein (apo) B-100 lipoproteins from normolipidemic human subjects. We supplemented culture systems containing 125I-lipoproteins with exogenous recombinant or plasmatic apo E-3. Very low density lipoprotein (VLDL) fractions I, II, and III, and low density lipoproteins (LDL) were prepared from one E 4/3 and four E 3/3 subjects. Without added apo E-3, cellular metabolism (binding, cell association, and degradation) of VLDL-I, II, and III was negligible. Exogenous apo E-3 caused a many-fold enhancement of the metabolism of the three VLDL fractions, but LDL was not affected. The effects of apo E-3 were specific, not observed with apo E-2, and not observed on receptor-negative cells. Exogenous apo E-3 also enhanced down-regulation of cellular sterol synthesis by the VLDLs, but not LDL, indicating increased particle catabolism by the cells. The optimal concentrations of exogenous apo E-3 were 4 to 6 micrograms protein/15 micrograms VLDL-protein, when most of the added apo E-3 became associated with the VLDL particles. Apo E-3 failed to associate with LDL. These results demonstrate that availability and association of adequate amounts of apo E-3 are crucial for optimal cellular metabolism of apo B-100 lipoproteins along the VLDL----LDL cascade.

Dietary fat clearance in normal subjects is regulated by genetic variation in apolipoprotein E

Apolipoprotein E (apo E) plays an important role in receptor mediated clearance of lipoprotein particles from plasma. Common genetic variation in apo E exists with three alleles coding for proteins called E2, E3, and E4. In in vitro receptor binding assays, E2 binds poorly, whereas E3 and E4 function normally. Recently, the apo E phenotype has been shown to have an effect on low density lipoprotein (LDL) cholesterol levels with levels in subjects with E2 lower and E4 higher than E3. We have examined the effect of the apo E polymorphism on dietary fat clearance using the vitamin A-fat loading test, which specifically labels intestinally derived lipoproteins with retinyl palmitate (RP). 27 normal subjects were studied, 10 with E3/3, 9 with E3/2, 7 with E4/3, and 1 with E4/4. After a vitamin A-containing fatty meal, postprandial RP concentrations were measured in chylomicron (Sf greater than 1,000) and nonchylomicron (Sf less than 1,000) fractions for 14 h. Compared with E3/3 subjects, E3/2 subjects had a significantly higher nonchylomicron RP concentration (P less than 0.05) (peak heights and areas below the curves) indicating slower clearance and the E4/3, E4/4 group had a significantly lower nonchylomicron RP concentration (P less than 0.05) indicating faster clearance. The clearance in the latter group was twice that of E3/2 subjects (P less than 0.01). Thus, heterozygosity for the defective form of apo E, E2, delays, and the surprising presence of a functionally normal allele, E4, increases clearance. This apo E effect on exogenous fat clearance may explain the recently described effect of the apo E phenotypes on LDL cholesterol levels.

Effect of low density lipoprotein receptor deficiency on the metabolism of apolipoprotein B-100 in blood plasma. Kinetic studies in normal and Watanabe heritable hyperlipidemic rabbits

The kinetics of apolipoprotein (apo) B-100 in particles containing apo E (B,E particles) or lacking apo E (B particles) were studied in Watanabe heritable hyperlipidemic (WHHL) rabbits deficient in low density lipoprotein (LDL) receptors, and compared with those of normal rabbits after injection of radioiodinated very low density lipoproteins (VLDL), intermediate density lipoproteins (IDL), and LDL. In both groups results of kinetic modeling were consistent with the hypothesis that all apo B enters the plasma in VLDL, mainly as B,E particles, followed by delipidation and partial conversion to IDL and LDL, with concomitant conversion of some B,E particles to B particles. In WHHL rabbits, production of VLDL apo B was reduced by 40%, but LDL production was increased threefold. Defective removal of B,E and B particles in all three lipoprotein classes, coupled with preserved processes of delipidation, can account for the observed increases in the concentration of apo B (threefold in VLDL, fivefold in IDL, and twenty-twofold in LDL) in WHHL rabbits.

Plasma lipoprotein distribution of apolipoprotein E in familial hypercholesterolemia

Although familial hypercholesterolemia (FH) has been well characterized in terms of the etiology of the major lipoprotein abnormality, that of low density lipoproteins (LDL), less information is available on changes in other lipoproteins which could influence the atherogenic process in this disorder. The present study has focused on such potential abnormalities by studying in detail the lipoprotein association of apolipoprotein E (apo E) in a large group of subjects homozygous for FH. Total plasma apo E levels in homozygous subjects were significantly elevated (p less than 0.001) relative to heterozygous subjects which were, in turn, significantly greater (p less than 0.001) than controls (137.6 micrograms/ml, 69.4 micrograms/ml, 46.5 micrograms/ml respectively). After separation of plasma lipoproteins by 4% agarose chromatography, an increased mass of apo E in lipoproteins of intermediate size was present; this may reflect the absence of LDL receptors that normally mediate their clearance. Homozygous FH subjects also demonstrated an increased mass of apo E-enriched high density lipoproteins (HDL) of large size, but a reduction in HDL cholesterol and apo A-I. The increase in the potentially atherogenic remnant lipoproteins and the decrease in HDL are associated with an increased risk for atherosclerosis, even in the absence of the LDL elevation, which is characteristic of FH. The increase in apo E-enriched HDL could reflect a compensatory mechanism that permits reverse cholesterol transport in the absence of LDL receptors.

The Fao cell. A tissue culture model for lipoprotein synthesis and secretion. I. Characterization of the system

In this study we found that a differentiated cultured rat hepatocyte cell line, Fao, synthesizes and secretes lipoproteins qualitatively similar to those synthesized by the rat hepatocyte in vivo, but quantitatively differing considerably in apoprotein composition and density distribution. Immunoprecipitation demonstrates that all the major apoproteins are synthesized, including both forms of apoB, apoE and apoA-I. Particles of all density classes are formed, apoB associating with the lighter particles and apoA-I with the heavier. ApoE is a major apoprotein in all but the lightest density classes. The general finding is that most particles formed have a density exceeding 1.10 g/ml, while very few of the lighter, apoB-containing particles form, probably because the normal growth medium of the cultured cells is lipid-poor as compared with rat serum. In ref. [20] we show that the composition of the lipoproteins synthesized can be effectively modulated by lipid depletion and lipid supplementation of the growth medium.

Quantification of human apolipoprotein E in plasma and lipoprotein subfractions by a non-competitive enzyme immunoassay

A non-competitive enzyme linked immunosorbent assay (ELISA) has been developed to quantitate apolipoprotein E (Apo E) concentrations in serum and in isolated lipoproteins. Microtiter plates coated with affinity-purified antibodies to Apo E were used and the Apo E bound to the plates was estimated with peroxidase-labelled antibodies to Apo E. The average concentration of Apo E in the serum from normolipidemic subjects (n = 132) was 54 +/- 19 mg/l. The within and between assay coefficients of variation were 4.65 and 7.08%, respectively. The standard curves for Apo E in serum, in VLDL and in HDL were parallel. There was a good correlation (r = 0.81) between estimation of Apo E by our assay and that by electroimmunoassay. Assay sensitivity (1 ng of Apo E) was sufficient to enable a study of the distribution of Apo E in plasma lipoproteins separated by density gradient ultracentrifugal fractionation.

A simple immunotechnique for the determination of serum concentration of apolipoprotein E

Immunoturbidimetric assay for the determination of serum apolipoprotein E was developed on the Cobas-BIO centrifugal analyzer using commercially available antisera and reference sera. Within-run imprecision was 2.9% while day-to-day imprecision was 7.9%. Addition of up to 50 g/l hemoglobin or 0.15 g/l bilirubin to sera did not interfere with the determination of this apolipoprotein. Apolipoprotein E concentration was measured in sera from 40 normolipidemic and hyperlipidemic subjects by this immunoturbidimetric method (dependent variable) and radial immunodiffusion assay (independent variable). A slope of 0.90, an intercept of -7.9, and a correlation coefficient of 0.94 were obtained. Reference range for apolipoprotein E was established using sera from 100 normolipidemic subjects. The mean + 1 SD for apolipoprotein E levels of males and females were 36 + 13 mg/l and 29 + 8.4 mg/l, respectively. The distribution of apolipoprotein E among the various lipoprotein fractions of normals was significantly different than that of hypertriglyceridemic subjects.

Single radial immunodiffusion of serum apolipoproteins C-II, C-III and E--pretreatment of samples with surfactant

An assay of apolipoproteins (Apo) C-II, C-III and E in human sera by single radial immunodiffusion (SRID) is described. The procedure involves pretreatment of the serum sample with surfactant to facilitate diffusion of very low density lipoproteins in agarose. This allowed measurement of apolipoproteins in lipidemic sera by SRID in agarose gel. The intraplate coefficient of variation was less than 2.5%, and the interplate coefficient of variation, less than 3.0%. Correlation with double-antibody radioimmunoassay produced the following coefficients of correlation: gamma = 0.9931 (n = 17) for Apo C-II, gamma = 0.9221 (n = 33) for Apo C-III, and gamma = 0.9340 (n = 17) for Apo E.

Apolipoprotein profile in healthy males and its relation to maximum aerobic capacity (MAC)

In order to document possible variations of apolipoproteins in relation to the maximum aerobic capacity, 36 healthy young males of different aerobic performance were examined and the serum concentrations of apolipoprotein A-I, A-II, B, C-II, C-III, E investigated. In contrast to all other lipoproteins, significant differences between the endurance-trained and control subjects could be found in the apo A-I concentrations only (1025 +/- 92 vs 1456 +/- 179 mg/l, p less than 0.001). In addition, the apolipoprotein A-II, B, C-II, C-III, E concentrations correlated neither with the maximum aerobic capacity values nor with the relative body weight of the subjects.

In vitro metabolism of apolipoprotein E

Apolipoprotein E plays a major role in the uptake of chylomicrons and of very-low-density lipoprotein (VLDL) remnants by the liver. It has also been clearly demonstrated that apolipoprotein E rapidly and spontaneously exchanges between lipoproteins. To assess whether all lipoprotein-bound apolipoprotein E is available to participate in spontaneous transfer and/or exchange, the present study followed the fate of radiolabeled apolipoprotein E in an in vitro system. The results show that in vitro, apolipoprotein E can be considered as having both a spontaneously exchangeable pool and a nonexchangeable pool. Based upon specific radioactivity data, only a limited amount of apolipoprotein E originating in VLDL or in high-density lipoproteins (HDL) was capable of in vitro exchange with that in other lipoprotein fractions. Lipolysis of VLDL triacylglycerol by milk lipoprotein lipase, however, resulted in complete transfer of VLDL apolipoprotein E mass and radioactivity to HDL, supporting the potential for transformation of exchangeable apolipoprotein to a transferable pool in vivo. The results of these studies indicate that during the course of lipoprotein metabolism, conformational changes occur which alter the accessibility of apolipoprotein E. Such dynamic heterogeneity may have implications for the regulation of lipoprotein metabolism.

Abnormal plasma cholesterol metabolism in cigarette smokers

Plasma cholesterol metabolism was studied in young, nonobese, normolipidemic men with a moderate level of cigarette smoking (24 +/- 5d-1) and in a comparable nonsmoking normal control group. The smokers showed a decreased cholesterol net transport from cell membranes into plasma (P less than 0.001) and a decreased ratio of cholesteryl ester transfer to low and very low density lipoprotein, relative to lecithin:cholesterol acyltransferase (P less than 0.05). Apoprotein E was increased in smokers' plasma (P less than 0.05) whereas apoprotein A-I, the major apoprotein of HDL, was decreased (P less than 0.05). This pattern of abnormalities has been previously observed in several other groups of subjects at increased risk for atherosclerotic vascular disease (diabetics, dysbetalipoproteinemics, and hyperbetalipoproteinemics). These data suggest a deleterious effect of smoking on plasma lipoprotein metabolism significant even in young smokers, which could partly explain the later incidence of atherosclerotic vascular disease in this group.

Enzyme-linked immunosorbent assay for apolipoprotein C-I

A non-competitive sandwich enzyme-linked immunosorbent assay for apolipoprotein C-I was developed. Sheep antibody to this apolipoprotein C-I, purified by affinity chromatography, was used for coating the wells of a microtiter plate and as a conjugate with alkaline phosphatase. The linear range of the assay was from 80 ng to 15 ng. It was sensitive down to 5 ng. The intra-assay variation coefficient was 2.8%, and the inter-assay variation coefficient 5.3%. The mean concentration of apolipoprotein C-I was 61 +/- 20 mg/l in healthy normal males, and 65 +/- 19 mg/l in females. Apolipoprotein C-I levels were positively correlated with the total cholesterol concentration in both sexes (p less than 0.002). A significant correlation with triacylglycerol was only observed in males (p less than 0.05). A significant increase of apolipoprotein C-I was observed in type V hyperlipoproteinaemia, and in the only studied case of type III.

A rapid apolipoprotein E radioimmunoassay using solid-phase staphylococcus protein. Use of pooled plasma as a secondary standard

A rapid apolipoprotein E (apo E) radioimmunoassay, which requires a total of 24 hour incubation as compared to the usual 3-5 days, has been developed in our laboratory. Solid phase staphylococcus protein A was used to separate bound and unbound labeled antigen. Use of a pooled plasma (quality control sample) as a secondary standard to reduce interassay variation was also described.

Quantification and immunolocalization of apolipoprotein E in experimental atherosclerosis

We developed a radioimmunoassay for rabbit apolipoprotein E (apo E) for studying their plasma apo E levels and its accumulation in the aorta of rabbits fed a cholesterol diet. Delipidation of plasma did not increase the apo E immunoreactivity and this immunoreactivity was indistinguishable from that in an apo E-phospholipid complex. The antigenic determinants of apo E in lipoprotein particles were therefore fully reacted with our goat anti-apo E antibodies. In our assay system, the non-ionic detergent Tween-20 was found to be necessary to significantly reduce the non-specific binding of 125I-labeled apo E to polystyrene tubes, and yet not interfere with the assay. In rabbits (n = 6) fed a high cholesterol (1%) diet, plasma apo E increased at least 10-fold above baseline levels and reached maximal levels within 17-20 days after the onset of cho-diet feeding. These levels were sharply reduced only 10 days after resuming a normal diet. Plasma total cholesterol levels went through a similar pattern. Thus, the plasma cholesterol concentration can simply be used to monitor the increase of apo E in cholesterol-fed rabbits. All the cholesterol-fed rabbits developed atherosclerotic fatty streak lesions and apo E located mostly in the thoracic region and was significantly correlated with the accumulation of lipids in the areas of lesion. In addition, the apo E deposition was limited to the aortic areas where lipids were present. On the other hand, apo A-I was not detectable in any lesion area. Our data suggest that apo E or apo E-containing lipoproteins, may be involved in the development of atherosclerosis in cholesterol-fed rabbits.

Metabolism of lipoproteins containing apolipoprotein B-100 in blood plasma of rabbits: heterogeneity related to the presence of apolipoprotein E

Apolipoprotein B-100 is a constant component of very low density lipoproteins (VLDL), intermediate density lipoproteins (IDL), and low density lipoproteins (LDL) in mammalian blood plasma. We have found that each of these classes of lipoproteins includes particles that contain apolipoprotein E (B,E particles) as well as particles that lack this protein (B particles). These two species can be separated by immunosorption on columns of anti-apolipoprotein E bound to Sepharose. We have injected radioiodinated VLDL, IDL, and LDL intravenously into recipient rabbits and have determined the concentration of radioiodine in apolipoprotein B-100 in B,E and B particles in whole-blood plasma obtained at intervals for 24 hr. We have developed a multicompartmental model that is consistent with this new information and with current concepts of lipoprotein metabolism. The model indicates that all apolipoprotein B-100 enters the blood as VLDL, of which about 90% is in B,E particles. Most VLDL B,E particles are removed rapidly from the blood, and only a small fraction is converted to IDL and eventually to LDL (overall conversion is approximately 2%). By contrast, a much smaller fraction of VLDL B particles is removed directly, and approximately 27% is converted to LDL. In addition, some B,E particles are converted to B particles as VLDL are converted to LDL, so that most LDL particles lack apolipoprotein E. Fractional rates of irreversible removal of B,E and B particles in IDL and LDL are similar. Our results indicate that the presence of apolipoprotein E is a major determinant of the metabolic fate of VLDL particles and support the hypothesis that polyvalent binding of particles containing several molecules of apolipoprotein E promotes receptor-dependent endocytosis of hepatogenous lipoproteins and limits their conversion to lipoproteins of higher density.

Quantification of human serum apolipoprotein A-I by zone immunoelectrophoresis assay and a procedure for the preparation of an A-I standard

A zone immunoelectrophoresis assay (ZIA) for human serum apolipoprotein A-I (Apo A-I) is described. In this immunologic technique, glass capillaries filled with agarose gel containing anti-Apo A-I are used. Apo A-I produces zones of immunoprecipitates with migration distances directly proportional to the apolipoprotein concentration. Two plasmas and their isolated high density lipoprotein (HDL) fractions, having different particle size distribution patterns have been analyzed for Apo A-I using rocket electroimmunoassay in parallel with ZIA. A good agreement between the two methods was obtained. With ZIA, Apo A-I complexed with lipids in HDL yielded concentration values that were unaffected by lipoprotein particle size and by delipidation. A standard curve, linear between 0.5 and 3.5 mg Apo A-I/1, was obtained with ZIA. A protocol for the preparation of an Apo A-I standard without prior purification of the apolipoprotein is presented. Using ZIA, 75 serum samples representing a heterogeneous study population were analyzed for Apo A-I. A mean value of 1.23 g/l with a relative standard deviation of 28% was obtained. The corresponding mean for total HDL cholesterol was 1.64 mmol/l (RSD = 61%).

A variant primary structure of apolipoprotein C-II in individuals of African descent

We have isolated an isoform of the protein activator of lipoprotein lipase, apolipoprotein C-II, from the very low density lipoproteins of four patients of African ancestry with hypertriglyceridemia and eruptive or pedunculated xanthomata. This protein, which we designate apolipoprotein C-II2, differs from the previously recognized species, which we denote apolipoprotein C-II1, by substitution of glutamine for lysine at residue 55, a mutation which would require only a single-base substitution in the structural gene for apolipoprotein C-II1. Each of the patients in whom apolipoprotein C-II2 was found had approximately equal amounts of apolipoprotein C-II1 and apolipoprotein C-II2 among the apoproteins of the very low density lipoproteins, suggesting that the structural genes for these proteins are allelic. Two additional apparent heterozygotes were found among the first-degree relatives of each of two of the patients in patterns compatible with monogenic autosomal transmission. Approximately equal amounts of apolipoproteins C-II2 and C-II1 were also found by isoelectric focusing in 6 of a casual series of 50 normolipidemic blacks, but none or only trace amounts of apolipoprotein C-II2 were found in 500 samples from Caucasian subjects with hyperlipidemia. These findings suggest that this polymorphism is distributed primarily among blacks, possibly reflecting some positive Darwinian selection pressure. Whether this polymorphism has a modifying effect upon the development of hyperlipemia remains to be determined.

ApoE deficiency: markedly decreased levels of cellular ApoE mRNA

Apolipoprotein (apo) E deficiency is a rare genetic disease characterized by palmar and tuberoeruptive xanthomas, type III hyperlipoproteinemia, and premature atherosclerotic vascular disease. The plasma level of apoE in apoE deficiency is less than 0.05 mg/dl by radioimmunoassay, and no structural variants of apoE were detected by immunoblot of plasma or VLDL separated by two-dimensional gel electrophoresis. The apoE gene is present in the apoE deficient patient, and there are no major insertions or deletions in the gene by Southern blot analysis. Blood monocyte-macrophages isolated from a patient with apoE deficiency contain 1-3% of the level of apoE mRNA present in monocyte-macrophages isolated from normal subjects. The apoE mRNA in the monocyte-macrophages of the apoE deficient patient is similar in size to normal apoE mRNA. The deficiency of plasma apoE in the patient with apoE deficiency is due to a markedly decreased level of apoE mRNA and decreased production of the E apolipoprotein. The decreased apoE mRNA may be due to a defect in transcription or processing of the primary transcript of the apoE gene or to instability of the apoE mRNA. The decreased plasma level of apoE results in delayed clearance of remnants of triglyceride rich lipoproteins, hyperlipidemia, and a type III phenotype.

Discrimination between subclasses of human high-density lipoproteins by the HDL binding sites of bovine liver

The binding of human 125I-labeled HDL3 (high-density lipoproteins, rho 1.125-1.210 g/cm3) to a crude membrane fraction prepared from bovine liver closely fit the paradigm expected of a ligand binding to a single class of identical and independent sites, as demonstrated by computer-assisted binding analysis. The dissociation constant (Kd), at both 37 and 4 degrees C, was 2.9 micrograms protein/ml (approx. 2.9 X 10(-8) M); the capacity of the binding sites was 490 ng HDL3 (approx. 4.9 pmol) per mg membrane protein at 37 degrees C and 115 at 4 degrees C. Human low-density lipoproteins (LDL) and very-low-density lipoproteins (VLDL) also bound to these sites (Kd = 41 micrograms protein/ml, approx. 6.7 X 10(-8) M for LDL, and Kd = 5.7 micrograms protein/ml, approx. 7.0 X 10(-9) M for VLDL), but this observation must be considered in light of the fact that the normal circulating concentrations of these lipoproteins are much lower than those of HDL. The binding of 125I-labeled HDL3 to these sites was inhibited only slightly by 1 M NaCl, suggesting the presence of primarily hydrophobic interactions at the recognition site. The binding was not dependent on divalent cations and was not displaceable by heparin; the binding sites were sensitive to both trypsin and pronase. Of exceptional note was the finding that various subclasses of human HDL (including subclasses of immunoaffinity-isolated HDL) displaced 125I-labeled HDL3 from the hepatic HDL binding sites with different apparent affinities, indicating that these sites are capable of recognizing highly specific structural features of ligands. In particular, apolipoprotein A-I-containing lipoproteins with prebeta electrophoretic mobility bound to these sites with a strikingly lower affinity (Kd = 130 micrograms protein/ml) than did the other subclasses of HDL.