Splanchnic metabolism of plasma apolipoprotein B: studies of artery-hepatic vein differences of mass and radiolabel in fasted human subjects

Hyperlipoproteinemia type 3: the forgotten phenotype

Hyperlipoproteinemia type 3 (HLP3) is caused by impaired removal of triglyceride-rich lipoproteins (TGRL) leading to accumulation of TGRL remnants with abnormal composition. High levels of these remnants, called β-VLDL, promote lipid deposition in tuberous xanthomas, atherosclerosis, premature coronary artery disease, and early myocardial infarction. Recent genetic and molecular studies suggest more genes than previously appreciated may contribute to the expression of HLP3, both through impaired hepatic TGRL processing or removal and increased TGRL production. HLP3 is often highly amenable to appropriate treatment. Nevertheless, most HLP3 probably goes undiagnosed, in part because of lack of awareness of the relatively high prevalence (about 0.2% in women and 0.4-0.5% in men older than 20 years) and largely because of infrequent use of definitive diagnostic methods.

ACAT2 is localized to hepatocytes and is the major cholesterol-esterifying enzyme in human liver

BACKGROUND

Two acyl-coenzyme A:cholesterol acyltransferase (ACAT) genes, ACAT1 and ACAT2, have been identified that encode 2 proteins responsible for intracellular cholesterol esterification.

METHODS AND RESULTS

In this study, immunohistology was used to establish their cellular localization in human liver biopsies. ACAT2 protein expression was confined to hepatocytes, whereas ACAT1 protein was found in Kupffer cells only. Studies with a highly specific ACAT2 inhibitor, pyripyropene A, in microsomal activity assays demonstrated that ACAT2 activity was highly variable among individual human liver samples, whereas ACAT1 activity was more similar in all specimens. ACAT2 provided the major cholesterol-esterifying activity in 3 of 4 human liver samples examined.

CONCLUSIONS

The data suggest that in diseases in which dysregulation of cholesterol metabolism occurs, such as hypercholesterolemia and atherosclerosis, ACAT2 should be considered a target for prevention and treatment.

The binding of very low density lipoprotein remnants to the low density lipoprotein receptor in familial defective apolipoprotein B-100

We have compared the affinity for low density lipoprotein (LDL) receptors of LDL and very low density lipoprotein (VLDL) remnants from patients with familial defective apo B-100 (FDB) with that of LDL and VLDL remnants from normal subjects. The binding affinity of FDB LDL was markedly reduced in all 14 FDB patients examined, hut the affinity of FDB remnants did not differ significantly from that of remnants prepared from normal subjects. Since the mutant form of apo B-100 present in FDB is recognized by LDL receptors with greatly reduced efficiency, we suggest that apo B plays only a minor role in the receptor-mediated uptake of VLDL remnants by the liver in man. These results are consistent with our previous suggestion that the ability of drugs that stimulate hepatic receptor activity to lower the plasma LDL level in FDB is due in part to increased hepatic uptake of lipoprotein precursors of LDL, including remnant particles with normal apo B-100 and those with mutant apo B-100.

Effect of hepatic lipase on LDL in normal men and those with coronary artery disease

Hepatic triglyceride lipase (HL) is thought to play a role in the formation of low density lipoproteins (LDLs) from small very low density lipoproteins (VLDLs) and intermediate density lipoproteins (IDLs). To analyze the possible physiological role of HL in determining LDL buoyancy, size, and chemical composition, HL activity and LDL were studied in 21 patients with coronary artery disease (CAD) and 23 normolipidemic subjects. In both groups, LDL buoyancy and size were inversely associated with HL activity levels. The effect of HL on LDL size was comparable in CAD patients and in normolipidemic subjects. HL appeared to influence LDL lipid composition primarily by affecting the surface lipid components. The free cholesterol content of LDL particles was highly correlated with HL activity in both CAD and normolipidemic individuals. The free cholesterol to phospholipid ratio in LDL particles correlated with HL in both CAD and normolipidemic subjects. When the individuals were separated according to their LDL subclass patterns, pattern B subjects had significantly higher HL than pattern A subjects in both CAD and normolipidemic groups. The analysis of the cholesterol distribution profiles across the lipoprotein density gradient confirmed that LDL buoyancy is affected by HL. These data support the hypothesis that HL modulates the physical and compositional properties of LDL and contributes to the expression of the LDL subclass phenotype, suggesting a physiological role for HL in LDL metabolism.

Coexistence of abnormalities of hepatic lipase and lipoprotein lipase in a large family

A large family is reported with familial hepatic triglyceride lipase (HTGL) deficiency and with the coexistence of reduced lipoprotein lipase (LPL) similar to the heterozygote state of LPL deficiency. The proband was initially detected because of hypertriglyceridemia and chylomicronemia. He was later demonstrated to have beta-VLDL despite an apo E3/E3 phenotype and the lack of stigmata of type III hyperlipoproteinemia. The proband had no HTGL activity in postheparin plasma. Two of his half-sisters had very low HTGL activity (39 and 31 nmol free fatty acids/min/ml; normal adult female greater than 44). His son and daughters had decreased HTGL activity (normal male and preadolescent female greater than 102), which would be expected in obligate heterozygotes for HTGL deficiency. Low HTGL activity was associated with LDL particles which were larger and more buoyant. Several family members, including the proband, had reduced LPL activity and mass less than that circumscribed by the 95% confidence-interval ellipse for normal subjects and had hyperlipidemia similar to that described in heterozygote relatives of patients with LPL deficiency. All the sibs with hyperlipidemia had a reduced LPL activity and mass, while subjects with isolated reduced HTGL (with normal LPL activity) had normal lipid phenotypes. Analysis of genomic DNA from these subjects by restriction-enzyme digestion revealed no major abnormalities in the structure of either the HTGL or the LPL gene. Compound heterozygotes for HTGL and LPL deficiency show lipoprotein physiological characteristics typical for HTGL deficiency, while their variable lipid phenotype is typical for LPL deficiency.

Large buoyant LDL-like particles in hepatic lipase deficiency

Hepatic lipase (HL) is thought to play a role in processing very low density lipoprotein to low density lipoprotein (LDL). To analyze the relationship between HL and LDL, the density, size, and chemical composition of LDL isolated from 18 normal subjects and from three subjects with reduced or absent levels of HL activity were compared. In an HL-deficient subject, the major peak of apoprotein (apo) B-containing lipoproteins ('LDL') had a density of 1.023 g/ml and a diameter of 26.4 nm compared to male control subjects (1.044 +/- 0.006 g/ml and 25.3 +/- 0.3 nm). Two half-sisters of the HL-deficient subject with half the normal levels of HL activity had LDL that also were more buoyant and slightly larger than the LDL isolated from female control subjects. The peak density and average diameter of LDL were correlated with HL activity, consistent with the hypothesis that HL influenced formation and physical characteristics of typical LDL. Apo B-100 was the major apoprotein in the 'LDL' isolated from the HL-deficient subject and contained a greater proportion of triglyceride compared to the control subjects' LDL. The absence of HL appears to prevent the production of classical LDL. Our data support the hypothesis that HL helps determine normal LDL characteristics.

Very low density lipoprotein apolipoprotein B metabolism in humans

The human plasma lipoproteins encompass a broad spectrum of particles of widely varying physical and chemical properties whose metabolism is directed by their protein components. Apolipoprotein B100 (apo B100) is the major structural protein resident in particles within the Svedberg flotation range 0-400. The largest of these, the very low density lipoprotein (VLDL), rich in triglyceride, are metabolised by sequential delipidation through a transient intermediate density lipoprotein (IDL) to cholesterol-rich low density lipoproteins (LDL). Several components contribute to the regulation of this process, including (a) the lipolytic enzymes lipoprotein lipase and hepatic lipase (b), apolipoproteins B, CII, CIII and E, and (c) the apolipoprotein B/E or LDL receptor. Lipoprotein lipase acts primarily on large VLDL of Sf 60-400. Hepatic lipase on the other hand seems to be critical for the conversion of smaller particles (Sf 12-60) to LDL (Sf 0-12). Although most apo B100 flux is directed to the production of the delipidation end product LDL, along the length of the cascade there is potential for direct removal of particles from the system, probably via the actions of cell membrane receptors. This alternative pathway is particularly evident in hypertriglyceridaemic subjects, in whom the delipidation process is retarded. VLDL metabolism shows inter subject variability even in normal individuals. In this regard, apolipoprotein E plays an important role. Normolipidaemic individuals homozygous for the apo E2 variant exhibit gross disturbances in the transit of B protein through the VLDL-IDL-LDL chain.

Development of an integrated model for analysis of the kinetics of apolipoprotein B in plasma very low density lipoproteins, intermediate density lipoproteins, and low density lipoproteins

To quantify more precisely the metabolism of apolipoprotein B (apo B) in human beings, an integrated model was developed for the analysis of the isotope kinetics of apo B in very low density lipoproteins (VLDL), intermediate density lipoproteins (IDL), and low density lipoproteins (LDL). The experimental basis for model development was a series of 30 triple-isotope studies in which patients received autologous 131I-VLDL, 125I-IDL, and [3H]glycerol as a precursor of VLDL triglycerides. The currently proposed model contains the following components: (a) a VLDL delipidation cascade that has a variable number of subcompartments, (b) a slowly catabolized pool of VLDL, (c) an IDL compartment consisting of two closely connected subcompartments, one of which is outside the immediate circulation, and (d) a two-compartment subsystem for LDL. Because mass data indicate that not all VLDL were converted to LDL, the model allows for irreversible removal of apo B from VLDL (or IDL) subsystems. It accounts for apparent "direct" input of LDL by postulating an early, rapidly metabolized compartment of VLDL that is converted directly to IDL. The model appears to be consistent with specific activity curves from the current triple-isotope studies and with present concepts of lipoprotein physiology; it also can be used to quantify pathways of lipoprotein apo B transport in normal and abnormal states.

Plasma apolipoprotein B metabolism in familial type III dysbetalipoproteinaemia

Lipoprotein metabolism was studied in eleven patients with Type III hyperlipoproteinaemia, one with normolipidaemic dysbetalipoproteinaemia and eight controls. Apolipoprotein B kinetics in very low density, intermediate density and low density lipoproteins (VLDL, IDL and LDL) was investigated. Fractional catabolic rates (FCRs) of VLDL-apo B and IDL-apo B were lower (P less than 0.005 and P less than 0.001) in the patients: 0.064 +/- 0.018 and 0.059 +/- 0.006 h-1 respectively, (mean +/- SEM), compared with 0.219 +/- 0.035 and 0.243 +/- 0.028 h-1. Synthetic rates (SRs) of IDL-apo B varied widely from 1.5 mg kg-1 day-1 in the subject with normolipidaemic dysbetalipoproteinaemia to 2.8-25.2 mg kg-1 day-1 in Type III. The mean time for conversion of IDL-apo B to LDL-apo B was prolonged, 18.7 h compared with 3.8 h in the controls (P less than 0.001). LDL-apo B pool size and SR were lower in the patients (P less than 0.05 for both). Two patients treated with gemfibrozil showed reduced hyperlipidaemia and decreased SR of VLDL-apo B and IDL-apo B. Dysbetalipoproteinaemia is associated with pronounced impairment of IDL and VLDL-remnant catabolism, lipoprotein levels reflecting an interaction between this defect and SR of these lipoproteins.

Effect of heparin-induced lipolysis on the distribution of apolipoprotein e among lipoprotein subclasses. Studies with patients deficient in hepatic triglyceride lipase and lipoprotein lipase

In normal subjects, apolipoprotein E (apo E) is present on very low density lipoproteins (VLDL) (fraction I) and on particles of a size intermediate between VLDL and low density lipoproteins (LDL) (fraction II). The major portion of apo E is, however, on particles smaller than LDL but larger than the average high density lipoproteins (HDL) (fraction III). To investigate the possible role of the vascular lipases in determining this distribution of apo E among the plasma lipoproteins, we studied subjects with primary deficiency of either hepatic lipase or of lipoprotein lipase and compared them with normal subjects. Subjects with familial hepatic triglyceride lipase deficiency (n = 2) differ markedly from normal in that fraction II is the dominant apo E-containing group of lipoproteins. When lipolysis of VLDL was enhanced in these subjects upon release of lipoprotein lipase by intravenous heparin, a shift of the apo E from VLDL into fractions II and III was observed. In contrast, apolipoproteins CII and CIII (apo CII and CIII, respectively) did not accumulate in intermediate-sized particles but were shifted markedly from triglyceride rich lipoproteins to HDL after treatment with heparin. In subjects with primary lipoprotein lipase deficiency (n = 4), apo E was confined to fractions I and III. Release of hepatic triglyceride lipase by heparin injection in these subjects produced a shift of apo E from fraction I to III with no significant increase in fraction II. This movement of apo E from large VLDL and chylomicron-sized particles occurred with little hydrolysis of triglyceride and no significant shift of apo CII or CIII into HDL from triglyceride rich lipoproteins. When both lipoprotein lipase and hepatic triglyceride lipase were released by intravenous heparin injection into normal subjects (n = 3), fraction I declined and the apo E content of fraction III increased by an equivalent amount. Either moderate or no change was noted in the intermediate sized particles (fraction II). These data strongly support the hypothesis that fraction II is the product of the action of lipoprotein lipase upon triglyceride rich lipoproteins and is highly dependent on hepatic triglyceride lipase for its further catabolism. In addition, the hydrolysis by hepatic triglyceride lipase of triglyceride rich lipoproteins in general results in a preferential loss of apo E and its transfer to a specific group of large HDL.

Inhibition of cholesterol synthesis reduces low-density-lipoprotein apoprotein B production without decreasing very-low-density-lipoprotein apoprotein B synthesis in rabbits

The kinetics of the apoprotein B (apo B) of very-low-density (VLDL; d less than 1.006) and low-density (LDL; d 1.019-1.063) lipoproteins were studied in six rabbits by using radioiodinated homologous lipoproteins, before and during oral administration of mevinolin (5 mg/kg per day), a competitive inhibitor of 3-hydroxy-3-methylglutaryl-coenzyme A reductase (EC 1.1.1.34), to explore the mechanism by which the drug reduces LDL synthesis. Before treatment LDL-apo B production greatly exceeded VLDL-apo B production in all animals, indicating that a large proportion of plasma LDL was derived from a VLDL-independent pathway. Five animals responded to mevinolin with a fall in plasma cholesterol (mean change - 53%; P less than 0.01). This was associated with a 66% decrease in LDL-apo B synthesis (P less than 0.05). In contrast, VLDL-apo B synthesis was unaffected by mevinolin. Furthermore, in all but one animal the decrement in LDL-apo B synthesis was greater than the rate of VLDL-apo B synthesis before treatment, demonstrating that mevinolin had reduced the VLDL-independent production of LDL.

Metabolism of apolipoproteins B-48 and B-100 of triglyceride-rich lipoproteins in normal and lipoprotein lipase-deficient humans

The metabolism of apolipoproteins B-48 and B-100 (apo B-48 and B-100) in large triglyceride-rich lipoproteins (300 to 1500 A in diameter) has been compared in three normal subjects and two subjects with genetically determined deficiency of lipoprotein lipase. The triglyceride-rich lipoproteins were obtained from a lipoprotein lipase-deficient donor 4 hr after a fat-rich meal in order to obtain chylomicrons (containing apo B-48) and very low density lipoproteins (VLDL) (containing apo B-100), whose properties had not been modified by the action of this enzyme. The triglyceride-rich lipoproteins were labeled with 125I and injected intravenously into recipients who had fasted overnight. In normal recipients, most of the apo B-48 was removed from the blood within 15 min, and most of the apo B-100 was removed within 30 min. In the lipoprotein lipase-deficient recipients, most of the injected apo B-100 remained in the blood for more than 8 hr; removal of apo B-48 was only slightly more rapid. In all subjects, only trace amounts of either protein were found in lipoproteins more dense than 1.006 g/ml. The results indicate that (i) the removal of the apo B of both chylomicrons and large VLDL from the blood is dependent upon the hydrolysis of their component triglycerides by lipoprotein lipase, and (ii) little or no apo B-48 of chylomicrons or apo B-100 of large VLDL is converted appreciably to low density lipoproteins (LDL). Our results suggest that the reported variability of the conversion of VLDL to LDL may be related to the size and composition of the particles secreted from the liver. The rapid production of remnant particles that are removed efficiently by the liver may minimize the opportunity for further reactions leading to the formation of LDL.

Heterogeneity of very-low-density lipoprotein metabolism in hyperlipidemic subjects

Very-low-density lipoproteins (VLDL) are triglyceride-rich lipoproteins that have been shown, by physicochemical means, to comprise more than one group of particles. Because of the potential atherogenicity of catabolized VLDL, we used the technique of heparin-affinity chromatography to separate VLDL into two classes of particles, one of which appears to contain partly catabolized VLDL. This observation is based on the higher cholesterol/triglyceride and higher apoprotein E/apoprotein C ratios in VLDL particles that are bound to heparin, resembling in this way intermediate-density lipoproteins (IDL), which are certainly derived in the main through VLDL catabolism. Further studies showed separate metabolic characteristics for the unbound and heparin-bound VLDL particles. Radiolabeled whole VLDL or unbound particles were reinjected into seven hypertriglyceridemic subjects and the kinetics studied in serial samples of plasma over the next 18-48 hours. The specific radioactivity-time curves of apoprotein B in the unbound and bound particles showed that the bound particles were derived wholly or partly from the unbound particles and in turn, were the precursors of IDL. This confirmed that heparin-bound VLDL particles represented VLDL undergoing catabolism, although in one subject about one-half of the bound particles appeared to have an origin other than through VLDL catabolism. These studies show that VLDL metabolism is heterogeneous, that the kinetics of total VLDL must be interpreted accordingly, and that the technique of heparin-affinity chromatography can be used for more detailed studies of VLDL.

Modes of action of lipid-lowering diets in man: studies of apolipoprotein B kinetics in relation to fat consumption and dietary fatty acid composition

The mechanisms by which dietary fat influences fasting plasma lipid concentrations have been investigated in hyperlipidaemic subjects. The synthetic and fractional catabolic (FCR) rates of the apoprotein B (apo B) of very-low density (VLDL) and low-density (LDL) lipoproteins were measured using radioiodinated autologous lipoproteins. Reductions of LDL concentration in eight subjects during low-fat (25% of energy) diets were largely explained by diminished synthesis (-20%, P less than 0.02), and possibly also by an increased FCR (+15%, P = 0.05) of LDL, compared with observations made during a high-fat (45% of energy) diet of similar fatty acid composition. VLDL apo B synthesis and FCR were not significantly altered. When a diet rich in polyunsaturated fatty acids was exchanged for one high in saturated fatty acids (fat providing 45% of energy on both occasions) in four subjects, the synthetic rates of both VLDL apo B (-31%, P less than 0.02) and LDL apo B (-23%, P less than 0.10) were reduced while their FCRs were unchanged.

Lipoprotein metabolism during acute inhibition of hepatic triglyceride lipase in the cynomolgus monkey

The role of the enzyme hepatic triglyceride lipase was investigated in a primate model, the cynomolgus monkey. Antisera produced against human postheparin hepatic lipase fully inhibited cynomolgus monkey posttheparin plasma hepatic triglyceride lipase activity. Lipoprotein lipase activity was not inhibited by this antisera. Hepatic triglyceride lipase activity in liver biopsies was decreased by 65-90% after intravenous infusion of this antisera into the cynomolgus monkey. After a 3-h infusion of the antisera, analytic ultracentrifugation revealed an increase in mass of very low density lipoproteins (S(f) 20-400). Very low density lipoprotein triglyceride isolated by isopycnic ultracentrifugation increased by 60-300%. Analytic ultracentrifugation revealed an increase in mass of lipoproteins with flotation greater than S(f) 9 (n = 4). The total mass of intermediate density lipoproteins (S(f) 12-20) approximately doubled during the 3 h of in vivo enzyme inhibition. While more rapidly floating low density lipoproteins (S(f) 9-12) increased, the total mass of low density lipoproteins decreased after infusion of the antibodies. The changes in high density lipoproteins did not differ from those in control experiments. In order to determine whether the increases of plasma concentrations of very low density lipoproteins were due to an increase in the rate of synthesis or a decrease in the rate of clearance of these particles, the metabolism of radiolabeled homologous very low density lipoproteins was studied during intravenous infusion of immunoglobulin G prepared from the antisera against hepatic triglyceride lipase (n = 3) or preimmune goat sera (n = 3). Studies performed in the same animals during saline infusion were used as controls for each immunoglobulin infusion. There was a twofold increase in the apparent half-life of the very low density lipoprotein apolipoprotein-B tracer in animals receiving the antibody, consistent with a decreased catabolism of very low density lipoproteins. Concomitantly, the rise in low density lipoprotein apoprotein-B specific activity was markedly delayed. None of these changes were observed during infusion of preimmune immunoglobulin G.Hepatic triglyceride lipase participates with lipoprotein lipase in the hydrolysis of the lipid in very low density lipoproteins, intermediate density lipoproteins, and the larger low density lipoproteins (S(f) 9-12). Thus, hepatic triglyceride lipase appears to function in a parallel role with lipoprotein lipase in the conversion of very low density and intermediate density lipoproteins to low density lipoproteins (S(f) 0-9).

Roles of lipoprotein lipase and hepatic triglyceride lipase in the catabolism in vivo of triglyceride-rich lipoproteins

To define the roles, in vivo, of hepatic triglyceride lipase and lipoprotein lipase in the catabolism of triglyceride-rich lipoproteins, we investigated the relationship between the activities of the above enzymes in postheparin plasma and the fractional removal rates of very low density lipoproteins (VLDL) and VLDL remnant particles. In 22 patients, the fractional removal rates of VLDL and VLDL-remnant particles were determined from analyses of the disappearance of radioiodinated Sf 60-400 and Sf 12-60 lipoprotein B apoprotein. The maximal activities of hepatic triglyceride lipase and lipoprotein lipase were determined in plasma samples drawn 2-60 minutes after heparin injection (60 U/kg). A positive correlation was observed between the fractional removal rate of VLDL and postheparin plasma lipoprotein lipase activity (r = 0.65). When all 22 patients were considered together, no relationship was demonstrable between remnant fractional removal and postheparin plasma lipoprotein lipase activity. However, humans may be subdivided with respect to the way in which they catabolize remnants. In some, all remnant may be catabolized to form LDL. In others, some of the remnant may also be directly removed from the circulation. Those subjects in whom previous studies indicate that all remnant is converted to LDL demonstrated a positive correlation between remnant fractional removal rate and postheparin plasma lipoprotein lipase activity (n = 8, r - 0.83). No correlations between postheparin plasma hepatic triglyceride lipase activity and any of the fractional removal rates were found. These data are consistent with the following: 1) lipoprotein lipase plays a key regulatory role in the catabolism of triglyceride-rich lipoproteins; 2) this role applies only to those catabolic involving the formation of particles of higher density VLDL remnants and low density lipoprotein; and 3) hepatic triglyceride lipase plays no rate-limiting role in the catabolism of VLDL or VLDL-remnant particles.

Myelomatosis with type III hyperlipoproteinemia: clinical and metabolic studies

We investigated the metabolism of intermediate-density lipoproteins (IDL [1.006 to 1.019 g per milliliter]) and low-density lipoproteins (LDL [1.019 to 1.063 g per milliliter]) in two men with Type III hyperlipoproteinemia associated with myelomatosis. In vivo kinetic studies using radiolabeled autologous lipoproteins demonstrated a greatly reduced fractional catabolic rate of IDL, relative to control values (patients vs. normal, 0.006 and 0.025 per hour vs. 0.20 +/- 0.08 per hour [mean +/- S.E.M]) and a greatly prolonged IDL-to-LDL conversion time (45 and 17 hours vs. 5.4 +/- 1.6 hours). In studies in vitro, LDL from both patients failed to bind to the LDL receptor of normal blood lymphocytes, whereas LDL from subjects with familial Type III hyperlipoproteinemia bound normally to the receptor. In one patient immunoglobulin was shown to be associated with IDL and LDL. Thus, hyperlipoproteinemia reflected an impaired metabolism of IDL, probably secondary to the binding of immunoglobulin to the lipoproteins. A similar impairment of receptor-mediated LDL catabolism did not elevate the plasma LDL concentration because of the low IDL-to-LDL conversion rate.

Cholesterol homeostasis of skin fibroblasts after incubation with postabsorptive and postprandial lipoproteins. The effect of a fatty meal

To determine if lipoproteins formed after a fatty meal deliver more cholesterol to cultured skin fibroblasts than do lipoproteins in the basal state, very low density lipoproteins and remnants (d less than 1.019), low density lipoproteins (LDL), and high density lipoproteins (HDL) were isolated from plasma obtained before, and 3 and 6 hours after, consumption of a high fat-cholesterol formula by seven normal males. Binding of 125I-LDL to cells and cell cholesterol content were determined after incubation of normal human skin fibroblasts for 48 hours with the lipoprotein fractions at 5% or 15% of plasma concentration. Activity of 3-hydroxy-3-methylglutaryl coenzyme A reductase was also measured after preincubation of cells with HLD for 48 hours. Despite a 40% increase in unesterified cholesterol in the d less than 1.019 fraction at 3 hours compared to the 0-hour fraction, the 3-hour d less than 1.019 fraction did not decrease LDL binding or increase cell cholesterol more than did the 0-hour fraction. Preincubation of cells with LDL, concentrations of which were unchanged by feeding, decreased LDL binding and increased cellular cholesterol. These effects also were not altered by the meal. HDL lipids and apo A-I were decreased at 3 hours, but not at 6 hours. Effects of HDL on LDL binding and cellular cholesterol were not altered by feeding, but the 3-hour and 6-hour fractions increased 3-hydroxy-3-methylglutaryl coenzyme A reductase activity, while the 0-hour fraction had little effect. These data indicate that consumption of a high fat-cholesterol meal as a bolus does not acutely alter the cholesterol delivery capacity of serum lipoproteins of normal male subjects.

[The apolipoproteins in man ]

Twelve different apolipoproteins have been described in human serum. Apo A-I and apo A-II are essential for the structure of the HDL particles and for the function of LCAT activity. Apo B is the main protein in LDL but does also occur in the triglyceride-rich particles. Apo B represents the binding protein for the LDL-receptor pathway. The C-apolipoproteins are located on the surface of VLDL. They are transferred to HDL throughout the catabolism of VLDL and affect lipoprotein lipase activity. This enzyme is also affected by the E-apolipoproteins which occur in the triglyceride-rich particles as well as in HDL. Apo E is the binding site for another specific cell receptor. The concentration and metabolism of apolipoproteins is affected by diet, drugs, hormones, body weight, alcohol, cigarettes, physical exercises, liver and renal diseases. There is a close relation between apolipoproteins and atherosclerosis.

Increased hepatitic cholesterol production due to liver hypertrophy in rat experimental nephrosis

Control and nephrotic rats were compared as to the liver contents of cholesterol, phospholipid and the activity of microsomal 3-hydroxy-3-methylglutaryl-coenzyme A reductase. Whole liver homogenates as well as endoplasmic reticulum membrane samples showed increased free cholesterol and phospholipid mass in the nephrotics. Correction of the values by the protein content indicated membrane expansion, i.e. liver hypertrophy. However, total hepatic cholesterol synthesis as measured by the reductase activity was increased in the nephrotic rat. These results are in accordance with previous studies showing enhanced cholesterol production in experimental nephrosis. In short, enhanced cholesterol mass in the liver coexists with increased hepatic synthesis in the experimental model used.