Role of the kidney in the metabolism of apolipoprotein A-IV: influence of the type of proteinuria

Increased plasma concentrations of apolipoprotein A-IV (apoA-IV) in chronic renal disease suggest a metabolic role of the kidney for this antiatherogenic protein. Therefore, we investigated patients with various forms of proteinuria and found increased serum concentrations of apoA-IV in 124 nephrotic patients compared with 274 controls (mean 21.9 +/- 9.6 vs. 14.4 +/- 4.0 mg/dl; P < 0.001). Decreasing creatinine clearance showed a strong association with increasing apoA-IV levels. However, serum albumin levels significantly modulated apoA-IV levels in patients with low creatinine clearance, resulting in lower levels of apoA-IV in patients with low compared with high albumin levels (21.4 +/- 8.6 vs. 29.2 +/- 8.4 mg/dl; P = 0.0007). Furthermore, we investigated urinary apoA-IV levels in an additional 66 patients with a wide variety of proteinuria and 30 controls. Especially patients with a tubular type of proteinuria had significantly higher amounts of apoA-IV in urine than those with a pure glomerular type of proteinuria and controls (median 45, 14, and 0.6 ng/mg creatinine, respectively). We confirmed these results in affected members of a family with Dent's disease, who are characterized by an inherited protein reabsorption defect of the proximal tubular system. In summary, our data demonstrate that the increase of apoA-IV caused by renal impairment is significantly modulated by low levels of serum albumin as a measure for the severity of the nephrotic syndrome. From this investigation of apoA-IV in urine as well as earlier immunohistochemical studies, we conclude that apoA-IV is filtered through the normal glomerulus and is subsequently reabsorbed mainly by proximal tubular cells.     

Radioimmunoassay of rat apolipoprotein A-IV

We have developed a specific and sensitive radioimmunoassay for rat apolipoprotein A-IV (apoA-IV). The protocol includes treatment of the samples for 1 h at 60 degrees C with 0.7% Tween 20. Under these conditions, linear logit-log plots have been obtained for apoA-IV in lymph and plasma lipoprotein fractions as well as for purified apoA-IV. The sensitivity of the assay is to 20 ng. Absolute mass values obtained with the assay were validated by comparison with values obtained with an independent method of colorimetric reading of apoA-IV separated by polyacrylamide gel electrophoresis from plasma high density lipoproteins. The concentration of apoA-IV in fasting plasma averaged 10.2 mg/dl and in the mesenteric duct lymph 15.8 and 12.6 mg/dl during the fasting and the fat absorption states, respectively.     

Distribution of apolipoprotein A-IV in human plasma

Human apoA-IV was purified from delipidated urinary chylomicrons. Monospecific antibodies were raised in rabbits and used to develop a double antibody radioimmunoassay (RIA). Displacement of 125I-labeled apoA-IV by plasma or purified chylomicron apoA-IV resulted in parallel displacement curves, indicating that apoA-IV from both sources share common antigenic determinants. The apoA-IV level in plasma from normal healthy fasting male subjects (n = 5) was 37.4 +/- 4.0 mg/dl, while fat-feeding increased the level to 49.1 +/- 7.9 mg/dl (P less than 0.05) at 4 hr. The apoA-IV level in plasma from abetalipoproteinemic fasting subjects was 13.7 +/- 3.1 mg/dl (n = 5). Plasma from a single fasting Tangier subject showed a reduced apoA-IV level of 21.1 mg/dl. The distribution of apoA-IV in fasting and postprandial plasma was determined by 6% agarose gel chromatography. Fifteen to 25% of plasma apoA-IV eluted in the region of plasma high density lipoprotein (HDL), with the remainder eluting in subsequent column fractions. In abetalipoproteinemic plasma this HDL fraction is reduced and lacks apoA-IV, suggesting that at least some of the apoA-IV on these particles is normally derived from triglyceride-rich lipoproteins. Lipemic plasma from a fat-fed subject showed a small rise (3%) in chylomicron-associated apoA-IV. Gel-filtered HDL and subsequent apoA-IV-containing fractions were subjected to 4-30% polyacrylamide gradient gel electrophoresis (4/30 GGE), and apoA-IV was identified by immunolocalization following transfer of proteins to nitrocellulose paper. In normal plasma apoA-IV was localized throughout all HDL fractions. In addition, normal plasma contained apoA-IV localized in a small particle (diameter 7.8-8.0 nm). This particle also contained apoA-I and lipid. A markedly elevated saturated to unsaturated cholesteryl ester ratio was present in gel-filtered plasma fractions containing small HDL, suggesting an intracellular origin of these particles. In abetalipoproteinemic plasma apoA-IV was absent from all HDL fractions except for the small HDL particles, suggesting that they are not derived from the surface of triglyceride-rich particles. All plasmas contained free apoA-IV. In contrast to gel-filtered plasma, lipoprotein subfractions of fasted normal plasma prepared in the ultracentrifuge primarily contained apoA-IV in the d greater than 1.26 g/ml fraction, suggesting an artifactual redistribution of the apolipoprotein during centrifugation. Overall, these data suggest that apoA-IV secretion into plasma is increased with fat feeding, and that apoA-IV normally exists as both a free apolipoprotein and in association with HDL particles.     

Apolipoprotein A-IV polymorphism and its effect on plasma lipid and apolipoprotein concentrations

Recently, we determined the apolipoprotein E (apoE) phenotype distribution in 2,000 randomly selected 35-year-old male individuals by slab gel isoelectric focusing of delipidated plasma samples, followed by immunoblotting using anti-apoE antiserum. These blots have been successfully re-used for immunovisualization of apoA-IV isoelectric focusing patterns. In a population sample of 1,393 individuals, four distinct apoA-IV isoforms were detected, encoded by the alleles A-IV*0, A-IV*1, A-IV*2, and A-IV*3 with gene frequencies of 0.002, 0.901, 0.079, and 0.018, respectively. The mean of plasma cholesterol, triglyceride, apoB and E levels did not differ significantly among the different apoA-IV phenotype groups. For these lipoprotein parameters, less than 0.1% of the total phenotypic variance could be accounted for by the APOA-IV gene locus. Our results did not show any effect of apoA-IV polymorphism on plasma apoA-I levels nor could we find any correlation between plasma levels of apoA-I and apoA-IV within the different apoA-IV phenotype groups. The plasma level of apoA-IV in subjects bearing the A-IV*3 allele is significantly lower than in subjects without the A-IV*3 allele (5 mg/dl versus 14 mg/dl). We therefore conclude that, in contrast to the apoE polymorphism, the polymorphism at the APOA-IV locus does not influence any of the levels of the lipoprotein parameters considered except apoA-IV.     

The APOA4 T347S variant is associated with reduced plasma TAOS in subjects with diabetes mellitus and cardiovascular disease

Apolipoprotein A-IV (apoA-IV) has been postulated to be antiatherogenic. Transgenic APOA4/Apoe-/- mice are protected against atherosclerosis, with plasma apoA-IV displaying antioxidant activity in vitro. In humans, there is an inverse relationship between apoA-IV levels and risk of coronary heart disease (CHD). Furthermore, the APOA4 T347S rare allele has been associated with increased risk of CHD and reduced apoA-IV levels. Reduced total antioxidant status (TAOS) due to increased oxidative stress is implicated in the process of atherogenesis. Thus, this study aimed to examine the association between the APOA4 T347S variant and TAOS in diabetic patients with (n = 196) or without (n = 509) cardiovascular disease (CVD). A higher percentage of CVD patients were present in the lowest quartile of TAOS, compared with the rest (P = 0.04). Overall, there was no association between genotype and TAOS. However, in patients with CVD, homozygotes for the S347 allele had significantly lower TAOS compared with TT and TS subjects (31.2 +/- 9.89% and 42.5 +/- 13.04% TAOS, respectively; P = 0.0024), an effect that was not seen in the patients without CVD. This study offers direct support for an antioxidant capacity of apoA-IV, thus providing some explanation for the antiatherogenic role of apoA-IV and the higher CVD risk in S347 homozygotes.     

Increased plasma apoA-IV level is a marker of abnormal postprandial lipemia: a study in normoponderal and obese subjects.

Plasma apolipoprotein A-IV (apoA-IV) levels are found elevated in hypertriglyceridemic patients. However, the relationship between plasma apoA-IV level and postprandial lipemia is not well known and remains to be elucidated. Thus, our objective was to study the relationship between plasma apoA-IV and postprandial TG after an oral fat load test (OFLT). Plasma apoA-IV was measured at fast and during an OFLT in 16 normotriglyceridemic, normoglucose-tolerant android obese subjects (BMI = 34.6 +/- 2.9 kg/m(2)) and 30 normal weight controls (BMI = 22.2 +/- 2.3 kg/m(2)). In spite of not statistically different fasting plasma TG levels in controls and obese patients, the former group showed an altered TG response after OFLT, featuring increased nonchylomicron TG area under the curve (AUC) compared with controls (516 +/- 138 vs. 426 +/- 119 mmol/l x min, P < 0.05). As compared to controls, obese patients showed increased apoA-IV levels both at fast (138.5 +/- 22.4 vs. 124.0 +/- 22.8 mg/l, P < 0.05) and during the OFLT (apoA-IV AUC: 79,833 +/- 14,281 vs. 68,176 +/- 17,463 mg/l x min, P < 0.05). Among the whole population studied, as among the control and obese subgroups, fasting plasma apoA-IV correlated significantly with AUC of plasma TG (r = 0.60, P < 0.001), AUC of chymomicron TG (r = 0.45, P < 0.01), and AUC of nonchylomicron TG (r = 0.62, P < 0.001). In the multivariate analysis, fasting apoA-IV level constituted an independent and highly significant determinant of AUC of plasma TG, AUC of chymomicron TG, AUC of nonchylomicron TG, and incremental AUC of plasma TG. In conclusion, we show a strong link between fasting apoA-IV and postprandial TG metabolism. Plasma fasting apoA-IV is shown to be a good marker of TG response after an OFLT, providing additional information on post-load TG response in conjunction with other known factors such as fasting TGs.     

Quantitation of apolipoprotein A-IV in human plasma using a competitive enzyme-linked immunosorbent assay

A method for measuring human apolipoprotein A-IV has been developed using the competitive enzyme-linked immunosorbent assay (ELISA) system. The assay described is relatively easy, rapid, and inexpensive to perform, uses convenient dilutions of plasma (1/8-1/32) but is sensitive enough to quantitate the apoA-IV content of lipoproteins following gel filtration of small (0.3-0.5 ml) volumes of plasma. The working range is 100-600 ng of apoA-IV per 50-microliters sample and the intra- and interassay coefficients of variations are 7.5 and 10.2% (means), respectively. The mean apoA-IV concentration of 100 subjects was found to be 16.4 +/- 5.4 mg/dl. The assay can be performed on untreated plasma samples which may be stored frozen (-20 degrees C) for up to 2 months.     

Correlation between apolipoprotein A-IV and triglyceride concentrations in human sera

Apolipoprotein A-IV concentration was measured by a newly developed competitive enzyme immunoassay in sera from fasted human subjects (n = 105) whose triglyceride concentrations ranged from 20 to 474 mg/dl (total cholesterol below 260 mg/dl) and in which chylomicrons could not be detected. Mean (+/- SD) apolipoprotein A-IV concentration was 13.0 +/- 2.6 mg/dl in sera with triglyceride levels ranging from 20 to 100 mg/dl, 16.9 +/- 3.7 mg/dl in sera with triglyceride levels ranging from 101 to 250 mg/dl, and 22.7 +/- 6.7 mg/dl in sera with triglyceride levels ranging from 251 to 474 mg/dl. The differences among the three groups were highly significant (P less than 0.001). Moreover, variations of apolipoprotein A-IV concentrations according to the triglyceride levels were noted within the normo-triglyceridemic population. Apolipoprotein A-IV concentration was 12.8 +/- 2.1 mg/dl for triglyceride levels ranging from 20 to 75 mg/dl and 16.4 +/- 3.8 mg/dl for triglyceride levels ranging from 76 to 150 mg/dl (P less than 0.01). In the entire population that was studied there was a significant linear correlation (r = 0.61, P less than 0.001) between the concentrations of serum apolipoprotein A-IV and triglyceride. Although the hypothesis of an unknown factor independently influencing both very low density lipoproteins and apolipoprotein A-IV cannot be ruled out, and although no apolipoprotein A-IV was found in the triglyceride-rich lipoprotein fraction after separation by gel filtration, these data suggest that, in fasting subjects, the secretion of very low density lipoproteins could contribute to the plasma apolipoprotein A-IV level.     

Association of low plasma high density lipoprotein (HDL)-cholesterol concentration with documented coronary artery disease in males with non-insulin dependent diabetes mellitus

Plasma lipid and lipoprotein concentrations were determined in 30 males without diabetes or symptomatic coronary artery disease (CAD), and compared to the values in age-matched and weight-matched males (n = 55) with non-insulin-dependent diabetes mellitus (NIDDM). Patients with NIDDM were further subdivided into those with (n = 30) and without (n =25) CAD. Mean (+/- SEM) plasma triglyceride concentrations were significantly increased (P less than 0.001) over control values (96 +/- 5 mg/dl) in patients with NIDDM, whether with (172 +/- 14 mg/dl) or without documented CAD (164 +/- 25 mg/dl). Plasma cholesterol concentrations were also higher (P less than 0.001) than normal (168 +/- 5 mg/dl) in both groups of patients with NIDDM (201 +/- 11 and 199 +/- 7 mg/dl, respectively, in patients with and without evidence of CAD). Plasma LDL-cholesterol concentrations were also greater (P less than 0.001) than normal (104 +/- 4 mg/dl) in patients with NIDDM, but were again similar in the group of diabetics (120 +/- 9 vs 128 +/- 6 mg/dl). However, plasma HDL-cholesterol concentrations were only reduced below control values in diabetes patients with CAD (30 +/- 1 mg/dl), whereas patients with NIDDM and no subjective evidence of CAD had HDL-cholesterol concentrations (37 +/- 3 mg/dl) which were similar to normal values (38 +/- 2 mg/dl). As a result, the ratio of LDL-cholesterol to HDL-cholesterol was highest in patients with NIDDM and CAD (4.2 +/- 0.3), lowest in the control population (2.8 +/- 0.2), and intermediate in those patients with NIDDM without subjective or objective evidence of CAD (3.6 +/- 0.3).(ABSTRACT TRUNCATED AT 250 WORDS)     

Altered particle size distribution of apolipoprotein A-I-containing lipoproteins in subjects with coronary artery disease

Plasma high density lipoproteins (HDL) can be separated into two subpopulations of apolipoprotein A-I-containing particles: those that also contain apoA-II [Lp(AI w AII)] and those that do not [Lp(AI w/o AII)]. These particles were isolated by immunoaffinity chromatography from 17 men (9 normolipidemic (NL), 8 hyperlipidemic (HL) with symptomatic coronary artery disease (CAD), from 17 NL men without any symptoms of CAD (healthy controls), and from 10 NL men with entirely normal coronary arteriograms (CAD-free controls). The distributions of particle size in these two subpopulations were determined by gradient gel electrophoresis and densitometric scanning. Approximately half of the Lp(AI w AII) particles in all subjects were distributed in the 8.2-9.2 nm interval. For patients with CAD, a greater fraction of the particles were small, in the 7.0-8.2 nm interval [33% in CAD vs. 26% in CAD-free controls (P less than 0.01) and 19% in healthy controls (P less than 0.0001)], and a smaller fraction of the particles were in the 9.2-11.2 nm interval (14% in CAD vs. 24% in CAD-free control (P less than 0.002) and healthy control groups (P less than 0.001). The Lp(AI w/o AII) of both control groups were primarily composed of two discrete subpopulations in the 8.2-9.2 nm and the 9.2-11.2 nm intervals. In CAD patients there were fewer particles in the 9.2-11.2 nm size interval (23% in CAD vs. 33% in CAD-free controls (P less than 0.005) and 36% in healthy controls (P less than 0.0001), and more particles in the smallest 7.0-8.2 nm size interval (32% in CAD vs. 23% in CAD-free controls (P less than 0.01) and 18% in healthy controls (P less than 0.001]. Thus, the spectrum of HDL particle sizes in patients with CAD tends to be shifted toward the smaller particle when compared with the two control groups. This was observed in both NL and HL patients with HDL cholesterol (CH) values in the normal range. As a group, CAD patients had lower HDL (42 +/- 7 mg/dl) and HDL2 (6 +/- 4 mg/dl) CH than healthy (HDL: 49 +/- 7, HDL2: 12 +/- 6 mg/dl) and CAD-free (HDL: 51 +/- 9, HDL2: 12 +/- 6 mg/dl) controls. When controls and patients were compared for their frequencies of abnormal HDL CH levels and particle sizes, abnormalities in HDL and HDL2 CH levels were not significantly more frequent (twofold) among CAD patients than among controls.(ABSTRACT TRUNCATED AT 400 WORDS)     

Distribution of apolipoprotein A-IV among lipoprotein subclasses in rat serum

The distribution of apolipoproteins (apo) A-I, A-IV, and E in sera of fed and fasted rats was studied using various methods for the isolation of lipoproteins. Serum concentrations of apoA-I and apoA-IV decreased significantly during fasting (16 and 31%, respectively), while apoE concentrations remained essentially the same. Chromatography of sera on 6% agarose columns showed that apoA-IV is present on HDL and as so-called "free" apoA-IV. The concentration of "free" apoA-IV decreased six- to seven-fold during fasting, explaining the decrease in total serum apoA-IV. Serum apoA-I and apoE are almost exclusively associated with HDL-sized particles. When sera are centrifuged at a density of 1.21 g/ml, marked quantities of apoA-I (8-9%) and apoE (11-22%) are recovered in the "lipoprotein-deficient" infranatant, suggesting that ultracentrifugation affects the integrity of serum HDL. The nature of the chromatographically separated carriers of serum apoA-IV was investigated by quantitative immunoprecipitation. From these studies, it is concluded that apoA-IV in rat serum is present in at least three fractions: 1) particles with the size and composition of HDL, containing both apoA-I and apoA-IV and possibly minor quantities of apoE; 2) HDL-sized particles containing apoA-IV, but no apoA-I or apoE; 3) "free" apoA-IV probably containing small amounts of bound cholesterol and phospholipid.     

A previously reported polymorphic plasma protein of dogs and horses, identified as apolipoprotein A-IV

By using immunoblotting with antiserum specific to human plasma apolipoprotein A-IV (apoA-IV), a previously reported polymorphic plasma protein of dogs viz postalbumin-2 (Pa2) and one of horses viz serum protein 2 (SP2), were identified as apoA-IV of these species. This along with earlier published results implied that: (1) both dog and horse show a high degree of polymorphism at the APOA4 locus with three common alleles in each of the two species; and (2) apoA-IV phenotyping in these two species can be done by analysing plasma/serum samples by a simple method of two-dimensional electrophoresis, conducted under non-denaturing conditions, followed by general-protein staining of gels.     

A previously reported polymorphic plasma protein of dogs and horses, identified as apolipoprotein A-IV

Summary. By using immunoblotting with antiserum specific to human plasma apolipoprotein A-IV (apoA-IV), a previously reported polymorphic plasma protein of dogs viz postalbumin-2 (Pa2) and one of horses viz serum protein 2 (SP2), were identified as apoA-IV of these species. This along with earlier published results implied that: (1) both dog and horse show a high degree of polymorphism at the APOA4 locus with three common alleles in each of the two species; and (2) apoA-IV phenotyping in these two species can be done by analysing plasma/serum samples by a simple method of two-dimensional electrophoresis, conducted under non-denaturing conditions, followed by general-protein staining of gels.     

In vivo metabolism of apolipoproteins A-IV and A-I associated with high density lipoprotein in normolipidemic subjects

The kinetics of apolipoprotein A-IV associated with high density lipoproteins (HDL) of plasma from fasting human subjects was followed for 15 days in five healthy normolipidemic volunteers. Purified apoA-IV and apoA-I were radioiodinated, respectively, with 125I and 131I, incubated in vitro with normal HDL, isolated at density 1.250 g/ml, and finally reinjected intravenously as HDL-125I-labeled apoA-IV and HDL-131I-labeled apoA-I. Blood samples were withdrawn at regular intervals for 15 days, and 24-h urine samples were collected. More than 93% (93.5 +/- 0.9%) of apoA-IV was recovered in apoA-I-containing lipoprotein particles after affinity chromatography on an anti-apoA-I column and 69.7 +/- 4.8% was bound to apoA-II in apoA-I:A-II particles separated on an anti-apoA-II column. 125I-labeled apoA-IV showed a much faster decay than 131I-labeled apoA-I for the first 5 days and thereafter the curves became parallel. Urinary/plasma ratios (U/P) for the 125I-labeled parallel. Urinary/plasma ratios (U/P) for the 125I-labeled apoA-IV were much higher than those for 131I-labeled apoA-I for the first days, but the U/P curves became parallel for the last 7 days, suggesting heterogeneity of apoA-IV metabolism. A heterogeneous multicompartmental model was constructed to describe the metabolism of lipoprotein particles containing apoA-IV and apoA-I and to calculate the kinetic parameters, fitting simultaneously all plasma and urine data for both tracers.(ABSTRACT TRUNCATED AT 250 WORDS)     

Human apolipoprotein A-IV reduces gastric acid secretion and diminishes ulcer formation in transgenic mice

We have investigated the involvement of human apolipoprotein A-IV (apoA-IV) in gastric acid secretion and ulcer formation in recently generated apoA-IV transgenic mice. Compared to control littermates, transgenic animals showed a gastric acid secretion decreased by 43-77% whereas only slight variations were observed in the different cell population densities within the gastric mucosa. In addition, no variation in gastrin levels was observed. Transgenics were protected against indomethacin-induced ulcer formation, with lesions diminishing by 45 to 64% compared to controls. These results indicate that endogenous apoA-IV expression can regulate gastric acid secretion and ulcer development.     

Conformational properties of human and rat apolipoprotein A-IV

Apolipoprotein A-IV has been isolated from four sources: human and rat lymph and plasma. Conformational properties of the rat and human apoA-IV in solution and denaturation changes induced by guanidine hydrochloride (Gnd X HCl) were studied using circular dichroic and fluorescence spectroscopy, and analytical sedimentation equilibrium ultracentrifugation. We have shown that both rat and human apoA-IV have similar secondary structure with negative maxima in the circular dichroic spectra at 222 nm and 207 nm. Furthermore, we have found no significant difference in the alpha-helical content of the apoA-IV from rat plasma (33%), rat lymph (37%), human plasma (35%), or human lymph (35%). Our denaturation studies with Gnd X HCl demonstrated reversibility and the fact that each apoA-IV had a tendency to self-associate in solution and the self-association could be disrupted by low concentrations of Gnd X HCl (less than or equal to 0.4 M). Unfolding of the secondary structure of each apoA-IV occurred at higher concentrations of Gnd X HCl (midpoint less than or equal to 1.0 M). The apparent free energy of denaturation of the four apoA-IV proteins calculated from changes in the circular dichroic spectra upon addition of increasing concentrations of Gnd X HCl varied in a range from 3.0 to 4.2 kcal/mol. The fluorescence experiments revealed that apoA-IV from all sources had a maximum fluorescence emission at 342.5 nm, which shifted to the red region upon addition of increasing concentrations of Gnd X HCl.(ABSTRACT TRUNCATED AT 250 WORDS)     

Studies of low density lipoprotein molecular weight in human beings with coronary artery disease

Low density lipoprotein molecular weight (LDL MW) correlates positively with coronary artery disease in cholesterol-fed nonhuman primates. To evaluate this in human beings with coronary artery disease (CAD) we measured LDL MW in 93 volunteers undergoing coronary angiography (47 controls and 46 CAD patients). LDL MW of CAD patients was less than that of controls (patients, 2.79 +/- 0.17 g/mumol; controls, 2.93 +/- 0.19 g/mumol; P less than 0.001). However, LDL MW decreased as plasma triglyceride increased and concentrations of triglyceride were greater in CAD patients than in controls. Since decreased LDL MW is likely to result, in part, from increased plasma triglyceride concentrations, we attempted to determine the effect of triglyceride on the relation of LDL MW to CAD in this study. After covariance adjustment for triglyceride, there was no LDL MW difference between CAD patients and controls. Because LDL heterogeneity has been identified in other studies and was apparent on inspection of agarose column profiles of LDL of these volunteers, we sought differences in the profiles that might distinguish coronary disease cases from controls. No differences could be found. In addition, we used density gradient ultracentrifugation to characterize LDL in more detail in a subset of volunteers who had a wide range of plasma triglyceride concentrations (50 mg/dl to 900 mg/dl). LDL mean hydrated density was inversely related to LDL MW and increased as triglyceride increased. The increase in peak density was reflected in an increase in percent of total protein in LDL found to have d greater than 1.045 g/ml and a decrease in protein in LDL of d 1.035-1.040 g/ml. These interrelationships were not apparently influenced by coronary artery status.     

Metabolism of apolipoprotein A-IV in rat

The metabolism of apolipoprotein A-IV (apo-IV) has been investigated in the rat. In this animal species, apoA-IV is a major protein constituent of plasma HDL and lymph chylomicron. The apolipoprotein is also present in the lipoprotein-deficient fraction (LDF) of plasma and lymph. In vivo studies with the radioiodinated protein showed the apoA-IV does not exchange freely between HDL and LDF and that LDF apoA-IV had a faster catabolism than HDL apoA-IV. ApoA-IV in chylomicrons is a direct precursor of apoA-IV in plasma HDL but not of that in LDF. On the other hand lymph LDF apoA-IV is an important precursor of plasma LDF apoA-IV. Transfer of apoA-IV from plasma to lymph is negligible, and since most of apoA-IV in lymph is present in LDF, we speculate that LDF apoA-IV is the major apoA-IV secretory product of the intestine. Studies aimed at identifying the site of catabolism of apoA-IV utilizing either radioiodinated or [14C]sucrose labelled apoA-IV, gave results consistent with the view that the liver plays a major role. When tested, human apoA-IV behaved in vivo in rat as the autologous protein. These findings, together with others previously published (Ghiselli, G. et al. (1987) J. Lipid Res. 27, 813-827), support the conclusion that the plasma metabolism of apoA-IV is remarkably similar in rat and human. We speculate that in mammals the rapid plasma catabolism of apoA-IV is mediated by an efficient uptake by the liver.     

GPI-specific phospholipase D associates with an apoA-I- and apoA-IV-containing complex

Glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD) is abundant in serum and associates with high density lipoproteins (HDL). We have characterized the distribution of GPI-PLD among lipoproteins in human plasma. Apolipoprotein (apo)-specific lipoproteins containing apoB (Lp[B]), apoA-I and A-II (Lp[A-I, A-II]), or apoA-I only (Lp[A-I]) were isolated using dextran sulfate and immunoaffinity chromatography. In six human plasma samples with HDL cholesterol ranging from 39 to 129 mg/dl, 79 +/- 14% (mean +/- SD) of the total plasma GPI-PLD activity was associated with Lp[A-I], 9 +/- 12% with Lp[A-I, A-II], and 1 +/- 1% with Lp[B]; and 11 +/- 10% was present in plasma devoid of these lipoproteins. Further characterization of the GPI-PLD-containing lipoproteins by gel-filtration chromatography and nondenaturing polyacrylamide and agarose gel electrophoresis revealed that these apoA-I-containing particles/complexes were small (8 nm) and migrated with pre-beta particles on agarose electrophoresis. Immunoprecipitation of GPI-PLD with a monoclonal antibody to GPI-PLD co-precipitated apoA-I and apoA-IV but little or no apoA-II, apoC-II, apoC-III, apoD, or apoE. In vitro, apoA-I but not apoA-IV or bovine serum albumin interacted directly with GPI-PLD, but did not stimulate GPI-PLD-mediated cleavage of a cell surface GPI-anchored protein. Thus, the majority of plasma GPI-PLD appears to be specifically associated with a small, discrete, and minor fraction of lipoproteins containing apoA-I and apoA-IV. -- Deeg, M. A., E. L. Bierman, and M. C. Cheung. GPI-specific phospholipase D associates with an apoA-I- and apoA-IV-containing complex. J. Lipid Res. 2001. 42: 442--451.     

Increased plasma cholestanol and 5 alpha-saturated plant sterol derivatives in subjects with sitosterolemia and xanthomatosis

We have measured plasma sterol composition in 14 subjects with sitosterolemia and xanthomatosis. In addition to elevated plasma phytosterol (campesterol 16 +/- 7 mg/dl and sitosterol 35 +/- 16 mg/dl) and normal to moderately high cholesterol levels (258 +/- 96 mg/dl), concentrations of 5 alpha-saturated stanols, cholestanol, 5 alpha-campestanol, and 5 alpha-sitostanol were at least 10 times greater than controls. Diets contained plentiful quantities of cholesterol and plant sterols, but only trace amounts of cholestanol (less than 2 mg/day) and no detectable 5 alpha-campestanol and 5 alpha-sitostanol, which indicated that the 5 alpha-saturated stanols were formed endogenously. Treatment with cholestyramine reduced plasma cholesterol and phytosterol levels by 45% and 5 alpha-saturated stanols by 55%. These results indicate that abnormally high plasma concentrations of cholestanol, 5 alpha-campestanol, and 5 alpha-sitostanol are found in subjects with sitosterolemia and xanthomatosis, and that treatment with cholestyramine effectively reduced elevated plasma sterol levels.