Low Density Lipoprotein Apoprotein Research Articles

Essential role of phospholipase A2 activity in endothelial cell-induced modification of low density lipoprotein.

Previous studies have established that incubation of low density lipoprotein (LDL) with cultured endothelial cells (EC) converts it to a new form (EC-modified LDL) that is now recognized by a specific receptor on macrophages (the acetyl LDL receptor) and is taken up and degraded 3-10 times more rapidly than native LDL (biological modification). The formation of EC-modified LDL depended on generation of free radicals with consequent peroxidation of LDL lipids and was accompanied by extensive hydrolysis of LDL phosphatidylcholine at the 2-position. The present studies show that p-bromophenacyl bromide, a site-specific irreversible inhibitor of phospholipase A2 activity, blocks this hydrolysis and, at the same time, the enhanced macrophage degradation. We show further that during EC modification the apoprotein B of LDL undergoes considerable modification and that this also is prevented by the phospholipase inhibitor. Finally, as reported previously, changes similar to those observed on incubation of LDL with EC can be induced by incubation in the absence of cells but in the presence of a sufficiently high concentration of Cu2+. This also is accompanied by hydrolysis of phosphatidylcholine at the 2-position and breakdown of apoprotein B. These changes are also inhibited by p-bromophenacyl bromide, suggesting the presence of a phospholipase A2 activity associated with LDL as it is isolated. A hypothesis is presented linking lipid peroxidation, phosphatidylcholine hydrolysis, and changes in the LDL apoprotein during EC modification.

Effect of postpartum lactation on lipoprotein lipids and apoproteins.

To determine if postpartum lactation alters plasma lipoprotein lipid and apoprotein concentrations and composition, we studied 56 overnight fasting lactating and 16 nonlactating women approximately 6 weeks postpartum. Postpartum results are presented as absolute concentrations and as the difference from antepartum values determined at 36 weeks gestation. Antepartum lipoprotein lipid and apoprotein concentrations were generally not different in the 2 groups, with the single exception of whole plasma and low density lipoprotein (LDL) apoprotein (apo) B (probably a chance difference). When expressed as the antepartum and postpartum difference, the lactating and nonlactating groups were indistinguishable in very low density lipoprotein (VLDL) and LDL triglyceride, cholesterol, phospholipid, and apo B concentrations. However, lactating women had higher high density lipoprotein (HDL) cholesterol, phospholipid, apo A-I, and apo A-II concentrations than nonlactating women when results were expressed as differences from antepartum values or as absolute values. HDL triglyceride concentrations were not significantly different between lactating and nonlactating women by either analysis. There was no significant effect of lactation on VLDL or LDL composition, but there was a significant increase in the percent cholesterol content in HDL. We hypothesize that the increase in HDL constituents in lactation is generated in part by increased catabolism of triglyceride-rich lipoproteins by the lactating breast.

Direct synthesis of low-density lipoprotein apoprotein B in the miniature pig

The metabolism of apoprotein B (apo B) was investigated in five miniature pigs following the injection of radiolabeled, very low-density lipoproteins (VLDL). The fractional catabolic rate (FCR) for VLDL apoprotein B was 0.71 ± 0.10 h −1 (mean ± SE), the rate of flux was 0.77 ± 0.05 mg h −1 kg −1, and the pool size of apoprotein B averaged 1.26 ± 0.20 mg kg −1. Examination of precursor-product relationships between VLDL and low-density lipoprotein (LDL) apoprotein B illustrated that a significant proportion (greater than 80%) of LDL apo B was derived from some source other than VLDL catabolism. In further experiments (n = 4), 125I-VLDL and 131I-LDL were simultaneously injected into miniature pigs. The fractional catabolic rate of LDL apo B averaged 0.055 ± 0.008 h −1 and the flux rate 0.73 ± 0.07 mg h −1 kg −1. These dual-label studies allowed us to calculate that an average of 16% of VLDL apoprotein B was converted to LDL and thus the remainder was cleared directly from the circulation. Simultaneous injection of radiolabeled homologous and human VLDL indicated that the catabolism of the two tracers was qualitatively similar. However, human VLDL apo B exhibited a slower fractional catabolic rate (0.42 v 0.71 h −1 P < 0.05) and reduced rate of conversion to LDL. Therefore, low-density lipoproteins in the pig are largely produced by direct secretion into the circulation, independent of VLDL catabolism. Apo B metabolism in miniature pigs is similar to that of cynomologous and squirrel monkeys, and rats, but differs from normal humans in whom all LDL apo B is derived from VLDL catabolism.

Effects of severe caloric restriction on the degree of sialylation of apoprotein C-III in obese women.

Alterations in the relative proportions of very low density lipoprotein (VLDL) apoprotein C-III (apoC-III) isoforms have been previously observed under various dietary and metabolic states. This study was designed to evaluate the effects of a very high caloric restriction on plasma triglycerides and on the relative proportions of VLDL apoprotein C-II (apoC-II) and apoC-III isoforms in obese subjects. VLDL were isolated by preparative ultra-centrifugation from 12 obese women. ApoC-II and apoC-III subspecies were separated by analytical ultrathin-layer isoelectrofocusing. The mean body weight decreased significantly from 96.2 +/- 8.7 to 90.6 +/- 7.6 kg (p less than 0.01). Mean total and VLDL triglycerides did not vary significantly. Apoprotein C-III2 (apoC-III2) as percentage of total apoprotein C increased (p less than 0.01) and apoprotein CIII0 (apoC-III0) decreased (p less than 0.01). Apoprotein C-III1 (apoC-III1) did not vary significantly. An inverse correlation was found between the percentage variation of apoC-III2 and that of apoC-III1 (r = -0.94; p less than 0.01). The variations of apoC-III2 correlated positively (r = 0.83; p less than 0.01), while those of apoC-III1 correlated inversely (r = -0.65, p less than 0.025) with the changes of VLDL triglycerides. The apoC-III1 to apoC-III2 ratio as well as the apoC-III0 to apoC-III2 and apo C-III1 ratios decreased after diet (p from less than 0.01 to less than 0.001). Total apoC-III as well as apoC-III2 and apoC-III1 to apoC-II ratios did not vary.(ABSTRACT TRUNCATED AT 250 WORDS)

Role of low density lipoprotein in the activation of plasma lysolecithin acyltransferase activity: Effect of chemical and enzymatic modifications of the lipoprotein on enzyme activity

The effect of various chemical and enzymatic modifications of low density lipoprotein (LDL) on its ability to activate the isolated human plasma lysolecithin acyltransferase (LAT) was studied. Removal of all lipids from LDL resulted in the complete loss of LAT activation. Removal of only neutral lipids by extraction with heptane retained up to 50% of the original activity, which was not increased further by reconstitution of the LDL with the extracted lipids. Hydrolysis of the diacylphosphoglycerides of the LDL with phospholipases resulted in complete loss of LAT activation which was partially restored by the addition of egg lecithin. Hydrolysis of more than 4% of LDL protein by trypsin led to a linear decrease in activity with complete loss of activity occurring when about 25% of the LDL protein is hydrolyzed. Modification of the arginine groups of LDL reversibly inhibited the activation of LAT. Modification of lysine residues of LDL by acetylation, acetoacetylation or succinylation also abolished its ability to activate lysolecithin acylation. Cationization of the lipoprotein destroyed its activity, but the reduction and alkylation only decreased the activity slightly. Complete removal of sialic acid groups by treatment of LDL with neuraminidase reduced the activation of LAT by about 20%. From these results it is concluded that the phospholipids of LDL and the charged groups of LDL apoprotein B are essential for its activation of the acylation of lysolecithin by the lysolecithin acyltransferase of human plasma.

Apolipoprotein A and B (Sf 100-400) metabolism during bezafibrate therapy in hypertriglyceridemic subjects.

This study describes the effects of bezafibrate, an analogue of clofibrate, on the plasma lipid and lipoprotein profiles of 11 hypertriglyceridemic subjects and on their metabolism of apolipoproteins A-I, A-II, and B. The major action of the drug was to lower plasma triglyceride (by 58%; P less than 0.01). This was accompanied by a reduction in the level of very low density lipoprotein apoprotein B (Svedberg units of flotation [Sf] 60-400), whose mean residence time in the plasma fell threefold (from 3.4 to 1.0 h). Synthesis of the B protein in this fraction was not significantly altered, so the drug acts to accelerate the transit of very low density lipoprotein particles down the delipidation cascade. The metabolism of very low density lipoprotein remnant apoprotein B (Sf 12-100) changed little in response to treatment, although we detected a 30% increment (P less than 0.05) in the plasma concentration of this fraction. The mean residence time of these remnant particles in the plasma did not correlate with that of Sf 100-400 very low density lipoprotein apoprotein B, nor was this parameter altered by the drug. The most consistent and significant perturbation seen in the Sf 0-12 fraction (low density lipoprotein) was a reduction in the fractional catabolism of its apoprotein B moiety (26%; P less than 0.05). In those subjects who were grossly hypertriglyceridemic and who responded well to treatment, the level of this protein rose substantially owing to a combined increase in its synthesis and a reduction in its catabolism. In the group as a whole, high density lipoprotein cholesterol rose 13% (P less than 0.02), and detailed examination showed that this was associated with a small but significant increment in the plasma concentration of the high density lipoprotein subfraction 2. High density lipoprotein subfraction 3 also rose on the average, but this was not a consistent feature in all patients. The plasma concentrations and turnovers of the A proteins (A-I and A-II) were not significantly altered by bezafibrate therapy.

Metabolism of apolipoprotein B in large triglyceride-rich very low density lipoproteins of normal and hypertriglyceridemic subjects.

The metabolic fate of very low density lipoprotein can be examined by following the transit of its apolipoprotein B moiety through the delipidation cascade, which leads to low density lipoprotein. In this study we have used cumulative flotation ultracentrifugation to follow the metabolism of various lipoprotein subclasses that participate in this process in normal, hypertriglyceridemic (Type IV), and dysbetalipoproteinemic (Type III) subjects. Large triglyceride-rich very low density lipoproteins of Svedberg units of flotation (Sf) 100-400 were converted virtually quantitatively in normal subjects to smaller Sf 12-100 remnant particles. Only a minor fraction appeared thereafter in low density lipoproteins (Sf 0-12), most being removed directly from the plasma. Type IV hyperlipoproteinemic individuals converted the larger Sf 100-400 very low density lipoproteins to intermediate particles at approximately 50% of the control rate but thereafter their metabolism was normal (fractional clearance of Sf 12-100 particles in controls, 1.29 +/- 0.23 pools/d; in Type IV hypertriglyceridemics, 1.38 +/- 0.23 pools/d; n = 4 in each case). Since the apolipoprotein B in large triglyceride-rich particles did not contribute significantly to the mass of the low density lipoprotein apoprotein pool, the latter must come largely from another source. This was examined by following the metabolic fate of small very low density lipoproteins of Sf 20-60 or of the total lipoprotein spectrum of d less than 1.006 kg/liter (approximate Sf 20-400). The small particles were rapidly and substantially converted to low density lipoproteins, suggesting that the major precursor of the latter was to be found in this density range. Whereas only 10% of apolipoprotein B in Sf 100-400 lipoproteins reached the low density lipoprotein flotation range, greater than 40% of Sf 20-100 B protein eventually appeared in Sf 0-12 particles; and when very low density lipoprotein of d less than 1.006 kg/liter is used as a tracer of apolipoprotein B metabolism it is primarily this population of small very low density lipoprotein particles in the Sf 12-100 flotation range that is labeled. A detailed examination was made of apolipoprotein B metabolism in three dysbetalipoproteinemic subjects. The plasma clearance curves of their Sf 100-400 lipoproteins were distinctly biphasic. The quickly decaying component converted rapidly into remnants of Sf 20-60 at a near normal rate (0.56 vs. 0.62 pools/d in normal subjects). Its subsequent processing, however, was retarded. The more slowly catabolized fraction, comprising 30% of the total apolipoprotein B radioactivity, had no counterpart in normal or Type IV hyperlipoproteinemic individuals. These data, taken together, suggest that the very low density lipoprotein consists of a complex mixture of particles with different origins and fates. Within the Sf 20-100 flotation range there are at least two subcomponents. One represents remnants of larger triglyceride-rich particles which are catabolized slowly and feeds little apolipoprotein B into low density lipoprotein. The other is apparently secreted directly into this flotation interval and transfers significant amounts of B protein rapidly into Sf 0-12 lipoproteins.

The relation of risk factors to the development of atherosclerosis in saphenous-vein bypass grafts and the progression of disease in the native circulation. A study 10 years after aortocoronary bypass surgery.

We examined 82 patients 10 years after saphenous-vein aortocoronary bypass surgery to determine their angiographic status and to relate those findings to the risk factors for coronary-artery disease. Of 132 grafts shown to be patent 1 year after surgery, only 50 were unaffected at 10 years. The remainder were narrowed (43) or occluded (39). Disease progression in coronary arteries without grafts was also frequent, both in vessels that were normal (15 of 32) and in those with minor stenosis (25 of 53). New lesions did not develop in 15 patients, whereas they did in 67--in the grafts, the native vessels, or both. There was no significant difference between the two groups in the incidence of hypertension, diabetes, or smoking, whereas plasma levels of very-low-density lipoproteins (VLDLs) and low-density lipoproteins (LDLs) were higher, and high-density lipoprotein (HDL) levels were lower in those with new disease than in those without. Univariate analysis showed that plasma cholesterol and triglyceride levels were significantly higher at the time of surgery and at the 10-year examination in those with new lesions. Multivariate analysis indicated that among the lipoprotein indexes, levels of HDL cholesterol and plasma LDL apoprotein B best distinguished the two groups. The findings indicate that atherosclerosis in these patients was a progressive disease, frequently affecting both the grafts and the native vessels, and that the course of such disease may be related to the plasma lipoprotein levels.

Influence of probucol on cholesterol and lipoprotein metabolism in man.

The mechanisms for the hypocholesterolemic action of probucol were examined in 17 patients with various levels of plasma cholesterol and triglycerides (TG). All the patients were studied on a metabolic ward. The first period of 6 weeks was for control. Thereafter, probucol was started, and after 2-6 months of drug treatment, the patients were readmitted for another 6-week period for a repeat study. During treatment with probucol, the cholesterol decreased in total plasma by an average of 12%, in low density lipoproteins (LDL) by 11%, and in high density lipoproteins (HDL) by 9%. The TG in total plasma and in very low density lipoproteins (VLDL) remained unchanged during probucol treatment. Turnover of low density lipoprotein apoprotein (apoLDL) was estimated following injection of 125I-labeled apoLDL. Probucol increased the fractional catabolic rate (FCR) for apoLDL by an average of 23%, but did not change apoLDL synthesis. The drug produced no consistent changes in fecal excretion of cholesterol (neutral steroids) and bile acids, in cholesterol absorption, in lipid composition of gallbladder bile, in biliary secretion of cholesterol and bile acids, or in the activities of lipoprotein lipase and hepatic lipase. These data show that probucol lowers LDL by increasing its catabolism. This effect appears to be independent of any changes in metabolism of cholesterol or bile acids.

Turnover of very low-density lipoprotein-apoprotein B is increased by substitution of soybean protein for meat and dairy protein in the diets of hypercholesterolemic men

The effect of dietary protein source on the kinetics of plasma very low-density lipoprotein (VLDL) (Sf 60 to 400) in hypercholesterolemic men was investigated. Using a crossover design, five subjects received sequentially either 1) a high polyunsaturated fat, low cholesterol control diet containing mixed protein from meat, dairy products, and plant sources or 2) an all-plant protein experimental diet in which the meat and dairy protein of the former diet was replaced by soybean protein and soy milk. There was no significant change in the mean values for fasting serum cholesterol and triglycerides over the 6-wk period of administration of the control versus experimental diets. The kinetics of VLDL (Sf 60 to 400) apolipoprotein B were studied at the end of each dietary period after reinjection of 125I-labeled autologous VLDL (Sf 60 to 400). The VLDL apolipoprotein B pool size was similar during the experimental and control protocols; however, the fractional catabolic rate was consistently higher during the experimental protocol (8.5 ± 1.3 versus 6.5 ± 1.2 day−1, p < 0.01) and the production rate of apoprotein B was higher than control in four of five subjects (mean values 18.6 ± 3 versus 12.6 ± 1 mg/day/kg, respectively). Administration of an all-plant protein diet significantly increased the fractional turnover rate of VLDL apolipoprotein B, even when no changes in VLDL apo B pool size or VLDL lipid concentrations were observed.

Inhibition of low density lipoprotein synthesis by dietary omega-3 fatty acids in humans.

Diets rich in omega-3 fatty acids derived from fish oils lower the plasma concentrations of low density lipoproteins (LDL) and very low density lipoproteins in humans. The present study was designed to examine the mechanism(s) by which diets enriched in omega-3 fatty acids reduce plasma LDL cholesterol levels in normal subjects. Seven healthy volunteers with normal plasma lipid levels consumed two metabolically controlled diets for a period of 4 weeks each. The control diet contained predominantly saturated and monounsaturated fatty acids, whereas the fish-oil diet contained 24 gm of omega-3 fatty acids per day. The total fat and cholesterol content of the two diets were similar for each subject. Total and LDL cholesterol levels decreased from 162 +/- 26 mg/dl and 103 +/- 27 mg/dl on the control diet to 124 +/- 26 mg/dl and 82 +/- 27 mg/dl on the omega-3-rich diet. Triglyceride levels fell from 91 +/- 34 mg/dl to 52 +/- 19 mg/dl. Kinetic studies of 125I-LDL metabolism disclosed a significantly lower rate of synthesis of LDL apoprotein B on the omega-3-rich diet (9.5 +/- 1.3 mg/kg/day) as compared to the control diet (13.6 +/- 3.7 mg/kg/day; p less than 0.05). In contrast, the fractional catabolic rate was similar on both diets. We conclude that dietary omega-3 fatty acids lower plasma LDL levels in normal human subjects by reducing the rate of synthesis of apoprotein B.

Binding of 5-methoxypsoralen to human serum low density lipoproteins

5-methoxypsoralen (5-MOP) binds to human serum low density lipoproteins (LDL) according to a two-step process. Scatchard analysis of the first step yields K = 1.4 × 10 5 M −1 and 4 binding sites. It involves the LDL apoprotein. The second step corresponds to a solubilization, in the lipidic core, of ⋍ 45 molecules of 5MOP per LDL molecule. It is accompanied by a large blue shift of the 5MOP fluorescence. The ability of LDL to bind 5MOP and to carry it into various cells may explain some biological effects sometimes encountered during PUVA therapy.

Receptor binding activity of high-density lipoproteins containing apoprotein E from abetalipoproteinemic and normal neonate plasma

The receptor binding properties of lipoproteins derived from neonates and abetalipoproteinemic patients were examined. Compared to normal adults, the neonate plasma contained reduced cholesterol levels, with only 40% of the total cholesterol transported in the low-density lipoproteins (LDL). When compared at equal cholesterol concentrations, however, the total neonate lipoproteins (d < 1.21) were as effective as adult d < 1.21 lipoproteins in stimulating cholesteryl ester formation in cultured human fibroblasts. Analysis of the neonate lipoproteins explained their enhanced ability to deliver cholesterol to the cells via LDL (apoprotein B,E) receptors: the neonate d = 1.02−1.063 fraction contained, in addition to LDL, alpha 2-migrating, apoprotein E-rich high-density lipoproteins (HDL 1), which were isolated by Geon-Pevikon electrophoresis. In binding studies performed with human fibroblasts at 4°C, the neonate HDL 1 were 14-fold more effective than either neonate or adult human LDL in displacing 125I-LDL from apo-B,E receptors. The neonate HDL (d = 1.063−1.21) contained a subfraction rich in apo-E and apo(E-A-II), which was isolated by heparin-Sepharose chromatography. This fraction was also active in displacing 125I-LDL from the receptors on cultured fibroblasts. Apoprotein E-containing HDL subclasses, similar to those described in the blood of neonates, were present in the d < 1.063 and d = 1.063−1.21 lipoprotein fractions of patients with abetalipoproteinemia. These HDL with apo-E were enriched in cholesterol and were as effective as normal LDL in competing with 125I-LDL for apo-B,E receptor-mediated binding, internalization, and degradation. When incubated with cultured human fibroblasts, the HDL with apo-E from the abetalipoproteinemic subjects increased the cholesteryl ester mass three- to fourfold. These studies suggest that neonates and abetalipoproteinemic subjects may depend (at least in part) upon lipoproteins containing apo-E to deliver cholesterol to various tissues via the LDL (apo-B,E) receptor.

冠動脈疾患におけるリポ蛋白分析特にVLDLアポ蛋白について

The aim of this study is to examine the relation between Very Low Density Lipoprotein (VLDL) apoprotein E isoforms and coronary atherosclerosis. Ninety eight subjects were allocated into two groups: 77 patients with coronary atherosclerosis (group 1) and 21 controls with normal intact coronary artery (group 2). All subjects were diagnosed by selective cornary angiography. Apoprotein E isoforms were analyzed by isoelectric focusing electrophoresis and determined semiquantitatively by densitometry.Apo E2/E3 ratio in group 1 was significantly higher than that in group 2 (group 1: 0.60±0.20, group 2: 0.51±0.09, p<0.05: Wilcoxon's rank sum test). In group 1, 27 patients (35%) had more than 0.7 of Apo E2/E3 ratio, but all controls in group 2 had less than 0.69. The incidence of Apo E4 was 24.7% in group 1 and 23.8% in group 2, there being no singificant difference between two groups.VLDL Apo E2/E3 ratio is clinically useful indicator of coronary aterosclerosis.

Turnover of low density lipoproteins during inhibition of cholesterol absorption by neomycin.

The mechanisms for the hypocholesterolemic actin of neomycin were examined in six patients with various levels of plasma total cholesterol and triglycerides. All patients were studied on a metabolic ward. The first period of 6 weeks was for control. Thereafter, neomycin (1.5 g/day) was started, and the patients were readmitted for another 6-week period after 2 to 3 months of treatment with the drug. Cholesterol balance studies showed that neomycin increased fecal excretion of neutral steroids by an average of 45%; the drug also inhibited absorption of exogenous cholesterol by an average of 44%. During treatment with neomycin, the plasma total cholesterol fell by an average of 20%, low density lipoproteins (LDL) fell by 25%, and high density lipoproteins, by 16%. Neomycin did not change plasma triglyceride levels. Turnover of the apoprotein of LDL (apoLDL) was measured following injection of 125I-apoLDL. Neomycin decreased synthesis of apoLDL by 28%. The decrease in plasma apoLDL level was correlated positively with the decrease in apoLDL synthetic rate. The effect of the drug on clearance of LDL was less constant; four of six patients had an increase in fractional clearance rates of apoLDL, but the change for the whole group was not statistically significant. These data suggest that a decrease in production of LDL is a major factor in the lowering of LDL following inhibition of cholesterol absorption; however, an increase in clearance rates may occur in some patients.

Effect of dietary eggs on liver and plasma proteins of rats. I. Synthesis of serum low-density lipoproteins.

The total lipid content in the livers of rats consuming cooked whole eggs as the only source of food for 21 days was elevated to three times that of control animals. The incorporation of 14C-labelled amino acids into serum low-density lipoprotein apoproteins was increased by 52% in these rats, without affecting the incorporation into serum albumin and liver total proteins. The turnover of low-density lipoprotein in these egg-fed rats appears to be very high.

Increased catabolism of native and cyclohexanedione-modified low density lipoprotein in subjects with myeloproliferative diseases.

We previously demonstrated reduced levels of low density lipoprotein (LDL) cholesterol in association with increased total fractional catabolic rates (FCR( of LDL apoprotein B (apo B) in individuals with myeloproliferative diseases (MPD). The removal of LDL from plasma and interstitial fluid is mediated via receptor and nonreceptor pathways. We attempted to quantitate LDL catabolism via each of these pathways in subjects with MPD and control subjects. The total FCR of LDL apo B was measured using radiolabeled native LDL. The FCR of radiolabeled cyclohexanedione-modified LDL (CHD-LDL) was used to assess the nonreceptor-mediated catabolism of LDL. Total FCR (mean +/- SD) was elevated in MPD vs controls (0.78 +/- 0.32 vs 0.45 +/- 0.11, p less than 0.01). CHD-LDL FCR was also increased in MPD vs controls (0.62 +/- 0.53 vs 0.23 +/- 0.04, p less than 0.01). Studies of the plasma decay of radiolabeled native and CHD-LDL preparations after their injection into cynomolgus monkeys indicated that CHD-LDL preparations from MPD and controls were removed at the same rates in those primates and that all CHD-LDL preparations were catabolized more slowly than the native LDL preparations. Studies in vitro indicated that CHD modification of LDL significantly reduced the rate of degradation of this lipoprotein by a specific high-affinity receptor pathway in normal human monocyte-derived macrophages and cultured human fibroblasts. We conclude that the catabolism of both native and CHD-LDL apo B is increased in subjects with MPD. If CHD-LDL is a valid tracer of nonreceptor-mediated removal of native LDL in individuals with MPD, our results indicate that the reduced LDL cholesterol concentrations demonstrated in these subjects are associated with increased nonreceptor-mediated catabolism of LDL apo B. At this time, both neoplastic cells and activated monocyte-macrophages appear to be likely sites of these abnormalities.

Apolipoproteins of high, low, and very low density lipoproteins in human bile

We tested the hypothesis that apolipoproteins, the protein constituents of plasma lipoproteins, are secreted into bile. We examined human gallbladder bile obtained at surgery (N = 54) from subjects with (N = 44) and without (N = 10) gallstones and hepatic bile collected by T-tube drainage (N = 9) after cholecystectomy. Using specific radioimmunoassays for human apolipoproteins A-I and A-II, the major apoproteins of high density lipoproteins, for apolipoproteins C-II and C-III, major apoproteins of very low density lipoproteins, and for apolipoprotein B, the major apoprotein of low density lipoproteins, we found immunoreactivity for these five apolipoproteins in every bile sample studied in concentrations up to 10% of their plasma values. Using double immunodiffusion, we observed complete lines of identity between bile samples and purified apolipoproteins A-I, A-II, or C-II. Using molecular sieve chromatography, we found identical elution profiles for biliary apolipoproteins A-I, A-II and B and these same apolipoproteins purified from human plasma. When we added high density lipoproteins purified from human plasma to lipoprotein-free solutions perfusing isolated rat livers, we detected apolipoproteins A-I and A-II in bile. Similarly, when we added low density lipoproteins purified from human plasma to lipoprotein-free solutions perfusing isolated livers of rats treated with ethinyl estradiol in order to enhance hepatic uptake of low-density lipoproteins, we found apolipoprotein B in bile. These data indicate that apolipoproteins can be transported across the hepatocyte and secreted into bile.

Metabolism of apoprotein B in cynomolgus monkey: evidence for independent production of low-density lipoprotein apoprotein B.

The catabolism of very-low-density lipoprotein apoprotein B and its conversion to low-density lipoprotein was studied in five chow-fed cynomolgus monkeys following injection of radioiodinated homologous very-low-density lipoproteins. The mean (+/- SD) fractional catabolic rate of very-low-density lipoprotein apoprotein B was 0.97 +/- 0.20 h-1 and the mean (+/- SD) production rate was 0.76 +/- 0.20 mg X kg-1 X h-1. The percent of conversion of very-low-density lipoprotein apoprotein B to low-density lipoprotein ranged from 33 to 59%. In separate studies of low-density lipoprotein apoprotein B turnover performed using homologous radiolabeled low-density lipoprotein in five additional animals, the mean (+/- SD) fractional catabolic rate for low-density lipoprotein apoprotein B was 0.050 +/- 0.017 h-1 and the mean (+/- SD) apoprotein B production rate was 0.70 +/- 0.18 mg X kg-1 X h-1. Comparison of the total low-density lipoprotein apoprotein B production with that derived from very-low-density lipoprotein apoprotein B suggested that a large fraction of plasma low-density lipoprotein apoprotein B was derived from a source exclusive of circulating very-low-density lipoprotein apoprotein B. This was confirmed in two animals by simultaneous injection of radiolabeled very-low-density and low-density lipoproteins. Thus, a significant proportion of cynomolgus monkey low-density lipoproteins are produced either by direct hepatic secretion or by rapid conversion of lower-density lipoproteins before they appear in the peripheral circulation.

Low density lipoprotein-sialic acids in patients with non-insulin dependent diabetes mellitus.

The important function of sialic acids in glycoproteins is to prolong the half life of the glycoproteins in the circulation. In this study, the sialic acid content of plasma low density lipoprotein (LDL)-apoproteins (predominantly B-100 apoprotein, a sialoglycoprotein) was analyzed together with relative electrophoretic mobility of LDL in 46 hypercholesterolemic and normo-cholesterolemic diabetics and non-diabetics. There was a significant positive correlation between the LDL-sialic acid content and LDL mobility either in the non-diabetics (r = 0.534, p less than 0.01) or in the diabetics (r = 0.482, p less than 0.02). Thus, the effect of the LDL-sialic acids upon the LDL electric charge was apparent. However, the LDL-sialic acids in the hypercholesterolemic diabetics were significantly (p less than 0.01) decreased to 9.64 +/- 0.63 micrograms/mg LDL protein (mean +/- S.E.) as compared with that in the normocholesterolemic diabetics (12.85 +/- 0.62 micrograms/mg LDL protein). Since the relatively sialic acid-poor LDL particles are rather accumulated in plasma of diabetic patients, the grade of sialylation of glycoprotein seems to have no role in the interaction with peripheral cells and therefore, the glycoprotein turnover, as far as the LDL apoproteins are concerned.