The metabolism of low density lipoprotein in endogenous hypertriglyceridaemia.

The metabolism of low density lipoprotein (LDL) was studied in eighteen hypertriglyceridaemic patients by injecting autologous radioiodinated LDL. Over 95% of the label was bound to the protein moiety of LDL and therefore the metabolic data reflect the fate and distribution of LDL apoprotein (apo B). The hypertriglyceridaemic subjects included ten with Type V, five with Type IV, two with Type III and one with Type IIb hyperlipoproteinaemia. For comparison identical studies were carried out in seven normal subjects and five patients with heterozygous familial hyperbetalipoproteinaemia (Type IIa). The groups differed considerably in mean LDL-cholesterol concentration. The patients with Type V lipoprotein pattern had significantly lower LDL-cholesterol concentration (mean 0.754 g/1) than the normal group (mean 1.237 g/1). Raised LDL-cholesterol levels were observed in all patients with heterozygous familial hyperbetalipoproteinaemia. The synthetic rate of LDL-apoprotein was found to be similar in all three groups (hypertriglyceridaemic, normal and hypercholesterolaemic). The highest synthetic rate was observed in the patient with Type IIb pattern. However, the fractional catabolic rate (FCR) of LDL-apoprotein differed significantly. The highest mean FCR was found in the Type V group (0.65 +/- 0.17 day-1) compared with 0.41 +/- 0.09 day-1 in the normal group and 0.185 +/- 0.05 day-1 in the Type IIa group. A strong inverse correlation was found between FCR and LDL apoprotein concentration in the whole series (r = -0.90, p less than 0.001) as well as within the Type V group (r = -0.87, p less than 0.01). These data indicate that the low plasma levels of LDL frequently observed in patients with very high plasma triglyceride levels are due to a high removal rate of LDL in these patients rather than to abnormal LDL synthesis.     

Metabolism of Very Low Density Lipoproteins in Hyperlipidaemia: Studies of Apolipoprotein B Kinetics in Man

The metabolism of very low density lipoprotein-B (VLDL-B) peptide was studied in nineteen subjects with endogenous hypertrigylceridaemia (Types V, IV and lib), three patients with heterozygous familial hyperbetalipoproteinaemia (Type Ila) and eight healthy subjects, by reinjecting autologous radioiodinated VLDL. The kinetics of VLDL-B peptide were followed. The mean turnover rate of VLDL-B peptide was significantly higher in the hypertriglyceridaemic group than in the control group but a considerable overlap in turnover rate was found between these groups. The patients with heterozygous familial hyperbetalipoproteinaemia had a normal turnover rate of VLDL-B peptide. A significant positive correlation was found between the turnover rate of VLDL-B peptide and VLDL-triglyceride concentration in the whole series. It is concluded that the underlying defect in endogenous hypertriglyceridaemia is heterogeneous. Overproduction of VLDL is a major determining factor in seme patients whereas a reduced clearance is the determining factor in others.     

Metabolism of very low density lipoproteins in hyperlipidaemia: studies of apolipoprotein B kinetics in man.

The metabolism of very low density lipoprotein-B (VLDL-B) peptide was studied in nineteen subjects with endogenous hypertriglyceridaemia (Types V, IV and IIb), three patients with heterozygous familial hyperbetalipoproteinaemia (Type IIa) and eight healthy subjects, by reinjecting autologous radioiodinated VLDL. The kinetics of VLDL-B peptide were followed. The mean turnover rate of VLDL-B peptide was significantly higher in the hypertriglyceridaemic group than in the control group but a considerable overlap in turnover rate was found between these groups. The patients with heterozygous familial hyperbetalipoproteinaemia had a normal turnover rate of VLDL-B peptide. A significant positive correlation was found between the turnover rate of VLDL-B peptide and VLDL-triglyceride concentration in the whole series. It is concluded that the underlying defect in endogenous hypertriglyceridaemia is heterogeneous. Overproduction of VLDL is a major determining factor in some patients whereas a reduced clearance is the determining factor in others.     

Changes in Iipoprotein(a), LDL-cholesterol and apolipoprotein B in homozygous familial hypercholesterolaemic patients treated with dextran sulfate LDL-apheresis

Abstract. We evaluated the effect of periodical treatment with LDL-apheresis by adsorption to dextran sulfate (Liposorber LA-15) on several aspects related to LDL and Lipoprotein(a) metabolisms, in three homozygous familial hypercholesterolaemic patients with LDL receptor deficiency. The dextran sulfate columns retained apolipoprotein B-containing particles with high affinity and capacity, in such a way that the treatment of a volume of plasma equivalent to three times the patient plasma volume resulted in an 85% decrease of circulating LDL-cholesterol and Lipoprotein(a). The continuous treatment with LDL-apheresis was highly beneficial for these patients since an average plasma concentration lower than 200 mg dl-′ for LDL-cholesterol, and lower than 25 mg dl^-′ for Lipoprotein(a) could be achieved by treating the patients once a week. After each apheresis treatment, plasma concentrations of these metabolites progressively returned to the pretreatment, steady-state, levels. The analysis of the rates of return allowed us to estimate the fractional catabolic rates. FCRs of LDL-cholesterol were 0–052, 0.049 and 0.047 pools day^-1, and those of apolipoprotein B, 0.065, 0.045 and 0.050 pools day^-1 in the three subjects, respectively. These values are much lower than those in normolipidaemic individuals as observed by others, and are in accordance with the LDL-receptor deficiency condition of our patients. Two of them had highly elevated Lipo-protein(a) plasma concentrations, and their FCRs of Lipoprotein(a) were calculated to be 0.112 and 0.066 pools day^-1. These values were significantly higher than the respective FCR of LDL-cholesterol and apolipoprotein B, which demonstrates that Lipopro-tein(a) and LDL were not metabolically homogeneous in these patients. Values of -kt for Lipoprotein(a), LDL-cholesterol and apolipoprotein B correlated during the first days inmediately after each apheresis session, suggesting that production of Lipoprotein(a) in these individuals was associated to that of apolipoprotein B. It is proposed that elevated Lipoprotein(a) plasma levels in familial hypercholesterolaemia are mainly a consequence of a high production rate rather than decreased fractional catabolic rate.     

The metabolism of low density lipoprotein in familial type II hyperlipoproteinemia

The metabolism of low density lipoprotein (LDL, beta lipoprotein) was studied in 10 normal individuals and 10 patients with familial type II hyperlipoproteinemia using purified radioiodinated LDL. Over 97% of the label was bound to the protein moiety of LDL and therefore the turnover data reflect the fate and distribution of LDL-apoprotein. Comparison of the metabolic behavior of biologically screened and unscreened labeled LDL preparations in dogs as well as the analysis of the urinary excretion of radioiodide derived from labeled LDL degradation in humans indicated that no significant denaturation resulted from the isolation, purification, and labeling techniques.The plasma concentration of LDL-cholesterol in normals was 105+/-21 mg/100 ml (mean +/-1 SD) in contrast to 254+/-47 mg/100 mg in patients with type II hyperlipoproteinemia; these values corresponded to LDL-apoprotein concentrations of 63+/-13 mg/100 ml and 153+/-30 mg/100 ml, respectively. Despite these differences in concentration, the synthetic rate of LDL-apoprotein in both groups was not significantly different (14.43+/-1.75 mg/kg per day in normals vs. 15.01+/-1.71 mg/kg per day in type II) nor was there any difference in the fraction of the total exchangeable LDL which was in the intravascular space (68.4+/-4.3% vs. 73.3+/-5.2%). However, the fractional catabolic rate of LDL in normal individuals differed significantly from that of patients with type II hyperlipoproteinemia (0.462+/-0.077/day in normals vs. 0.237+/-0.044/day in type II) and correspondingly the biological half-life of LDL was significantly prolonged (3.08+/-0.35 days normals vs. 4.68+/-0.44 days in type II).These data indicate that the pathologic elevation of plasma LDL concentration in the individuals with type II hyperlipoproteinemia studied here is due to a decreased fractional rate of LDL degradation rather than to an abnormality of LDL synthesis. This defect of catabolism may be the primary defect in type II hyperlipoproteinemia or, alternatively, may be secondary to an underlying abnormality in lipid metabolism.     

Kinetic bases of the primar hperlipidaemias: studies of apolipoprotein B turnover in geneticall defined subjects

Abstract. Autologous ¹³¹I-labelled ver low densit lipoprotein (VLDL) and ¹²⁵I-labelled low densit lipoprotein (LDL) were injected into seven normal subjects and into fort-three hperlipidaemic patients, classified into groups on the basis of famil studies and clinical findings, to quantitate VLDL and LDL apolipoprotein B kinetics. In normal subjects, mean VLDL-B peptide snthetic rate was 151 mg kg^-1 da^-1, mean LDL-B peptide snthetic rate 7-7 mg kg^-1 da^-1 and mean LDL-B fractional catabolic rate (FCR) 0–31 da^-1. In heterozgous familial hpercholesterolaemia (n= 14) VLDL-B peptide production was normal in patients with normal triglceride levels; in those with high triglceride levels there was either VLDL overproduction or a catabolic defect. LDL-B peptide snthetic rates ranged from high normal to increased (8-5-180 mg kg^-1 da^-1) and LDL-B peptide FCR values were markedl reduced (0–14-0-28 da^-1) confirming the presence of a defect in LDL catabolism but indicating over-production as well. In familial combined hperlipidaemia (n= 11) VLDL-B peptide production ranged from normal to elevated (13-9-44-4 mg kg^-1 da^-1, mean 23-8 mg kg^-1 da^-1) correlating with the VLDL triglceride level (i.e. with the phenotpic expression of the disorder). LDL-B peptide production ranged from high normal to markedl increased (8–9-19-5 mg kg^-1 da^-1, mean 12-2 mg kg^-1 da^-1) and correlated with LDL cholesterol levels (i.e. the phenotpe), (r=+ 0–66, P < 005). Three patients with unclassified hpercholesterolaemia had increased LDL-B peptide snthetic rates. One patient with remnant hperlipoproteinaemia (tpe III) had a high normal VLDL-B peptide snthetic rate, 17-3 mg kg^-1 da^-1, and a strikingl low FCR of VLDL-B. In familial hpertriglceridaemia (three patients)     

Control of 3-Hydroxy-3-Methylglutaryl-CoA Reductase Activity in Cultured Human Fibroblasts by Very Low Density Lipoproteins of Subjects with Hypertriglyceridemia

Very low density lipoproteins (VLDL) and low density lipoproteins (LDL) from human normolipemic plasma, and the VLDL, the intermediate density lipoprotein (IDL), and LDL from patients with Type III hyperlipoproteinemic plasma were tested for their abilities to suppress the activity of 3-hydroxy-3-methylglutaryl-Coenzyme A (HMG-CoA) reductase in cultured human fibroblasts from normal subjects and a Type III patient. Regulation of cholesterol synthesis in the fibroblasts of a patient with Type III hyperlipoproteinemia appears to be normal. VLDL from normal subjects, isolated by angle head ultracentrifugation (d < 1.006) or by gel filtration on BioGel A-5m, were about 5 times less effective than LDL in suppressing HMG-CoA reductase activity, based on protein content, in agreement with previous reports with normal fibroblasts. Zonal centrifugation of normal VLDL isolated by both methods showed that the VLDL contained IDL. Normal VLDL from the angle head rotor, refractionated by the zonal method, had little, if any, ability to suppress the HMG-CoA reductase activity in either normal or Type III fibroblasts. VLDL, IDL, and LDL fractionated by zonal ultracentrifugation from Type III plasma gave half-maximum inhibition at 0.2-0.5 mug of protein/ml, indistinguishable from the suppression caused by normal LDL. Type III VLDL did not suppress HMG-CoA reductase in mutant LDL receptor-negative fibroblasts. Zonally isolated VLDL obtained from one Type IV and one Type V patient gave half-maximal suppression at 5 and 0.5 mug of protein/ml, respectively. Molecular diameters and apoprotein compositions of the zonally isolated normal and Type III VLDL were similar; the major difference in composition was that Type III VLDL contained more cholesteryl esters and less triglyceride than did normal VLDL. The compositions and diameters of the Type IV and Type V VLDL were similar to normal VLDL. These findings show that the basic defect in Type III hyperlipoproteinemia is qualitatively different from the cellular defect found in familial hypercholesterolemia, since the regulation of HMG-CoA reductase activity is normal in Type III fibroblasts. The metabolic defect in hypertriglyceridemia is related to the triglyceriderich lipoproteins which, free of other lipoproteins, have an enhanced ability to interact with cultured fibroblasts to regulate HMG-CoA reductase activity. These studies suggest that, in hypertriglyceridemia, there is a mechanism for direct cellular catabolism of VLDL which is not functional for normal VLDL.     

Plasma triglyceride and low density lipoprotein metabolism

This study examines the relationship between plasma triglyceride and low density lipoprotein (LDL) levels by measuring the turnover of the native and 1,2 cyclohexanedione-treated lipoprotein in 25 healthy adults. Plasma triglyceride showed a strong positive correlation with circulating LDL apoprotein (apo LDL) mass. In order to achieve a satisfactory fit to the kinetic data it was necessary to postulate the existence of two plasma apo LDL pools (A and B). When subjects were grouped in quintiles on the basis of circulating apo LDL mass, pool A predominated in those in the lowest quintile. The fractional catabolic rate (FCR) of apo LDL from this pool was high (FCR = 0.57 +/- 0.06 pools day-1). As plasma triglyceride and apo LDL mass rose, apoprotein accumulated in the more slowly metabolized pool B as a result of an increase in the rate of input of apo LDL into the latter. The fractional clearance rate of protein from this pool remained unchanged at 0.26 +/- 0.04 pools day-1. Synthesis of apo LDL into pool B correlated with plasma triglyceride (r = 0.553, P less than 0.01), suggesting that the protein in this pool was derived from large, triglyceride-rich very low density lipoprotein.     

The Kinetics of Plasma Free Fatty Acid and Triglyceride Transport in Patients with Idiopathic Hypertriglyceridaemia and Their Relation to Carbohydrate Metabolism

Abstract. Plasma free fatty acid and triglyceride transport kinetics were assessed in 20 patients with idiopathic hypertriglyceridaemia. None of these patients had abnormal glucose tolerance. They included 10 patients in whom the serum triglyceride elevation was due to an increase in the circulating VLDL (Fredrickson Type IV) and 10 in whom the increase in plasma VLDL was associated with hyperchylomicronaemia (Fredrickson Type V). These were compared with a control group of 27 normal subjects.–Increased plasma triglyceride turnover with normal clearance was observed in the Type IV patients suggesting that the hypertriglyceridaemia in these patients was predominantly due to enhancement of plasma triglyceride production. The plasma triglyceride concentration correlated closely with the changes in triglyceride turnover rate.–Studies performed in the Type V patients showed an increase in the plasma triglyceride turnover rate in only 3 subjects, while in the remaining patients the turnover values were similar to those of the control subjects. The increase in serum triglyceride concentration found in some of the patients was due to an increase in plasma triglyceride production. However, in the majority of patients in this group impairment of plasma triglyceride clearance was the predominant abnormality.–In both hypertriglyceridaemic groups the plasma FFA flux was markedly increased and correlated significantly with the degree of hypertriglyceridaemia. The increase in triglyceride turnover observed in Type IV patients and some of the Type V patients correlated closely with the enhancement of plasma FFA flux suggesting that the increase in triglyceride production in these patients was secondary to enhanced lipolysis.–The plasma insulin response to an oral glucose load was markedly increased in both groups of hypertriglyceridaemic patients and correlated significantly with the elevation in serum triglyceride concentration. The plasma insulin response also correlated with the plasma free fatty acid turnover.–The results suggest that the initial lesion in these patients was related to insulin unresponsiveness in adipose tissue resulting in enhanced lipolysis with secondary changes in insulin secretion and plasma triglyceride transport kinetics.     

Regulation of the production and catabolism of plasma low density lipoproteins in hypertriglyceridemic subjects. Effect of weight loss.

In subjects with hypertriglyceridemia, plasma concentrations of low density lipoprotein (LDL) cholesterol are often normal or reduced. Perturbations that alter plasma very low density lipoprotein (VLDL) concentrations are associated with opposite changes in plasma LDL levels. To determine the mechanisms regulating plasma LDL levels, we used 131I-VLDL and 125I-LDL to measure the fractional catabolic rates (FCR), production rates (PR), and rates of interconversion of apoprotein B (apo B) in VLDL, intermediate density lipoprotein, and LDL in six hypertriglyceridemic subjects pre- and post-weight reduction. [2-3H]glycerol was used to quantitate VLDL triglyceride PR. All data are presented as mean +/- SD. Percent ideal body weight fell from 132 +/- 17.9 to 119 +/- 15.9% in the group, P less than 0.05. After weight loss, plasma VLDL triglyceride (486.0 +/- 364.1 vs. 191.3 +/- 65.4 mg/dl, P less than 0.05) and VLDL apo B (32.2 +/- 12.0 vs. 14.8 +/- 6.8 mg/dl, P less than 0.05) concentrations were reduced. VLDL triglyceride PR also fell after weight reduction (56.6 +/- 39.0 vs. 28.6 +/- 23.1 mg/kg per h, P less than 0.05), as did VLDL apo B PR (47.9 +/- 41.4 vs. 19.0 +/- 14.1 mg/kg per d, P less than 0.05). Pre-weight loss, plasma LDL cholesterol and apo B levels were low-normal or reduced (64.0 +/- 12.6 and 58.4 +/- 11.9 mg/dl, respectively) despite normal or elevated LDL apo B PR (17.4 +/- 7.2 mg/kg per d). The reduced cholesterol and apo B levels were associated with increased FCRs (0.68 +/- 0.29 d-1) and reduced cholesterol/protein ratios (1.01 +/- 0.18) in LDL. The plasma levels of LDL cholesterol and apo B rose after weight reduction (84.8 +/- 24.9, P less than 0.05; and 69.5 +/- 14.3 mg/dl, P less than 0.05, respectively, vs. base line). These increased concentrations resulted from a combination of events. First, the FCR for LDL apo B fell in five of six subjects with a significant reduction for the group as a whole (0.48 +/- 0.11 d-1, P less than 0.05 vs. base line). Second, the cholesterol/protein ratio increased in all six subjects with a significantly greater mean after weight loss (1.25 +/- 0.27, P less than 0.05 vs. base line). In contrast, the LDL apo B PR fell or was essentially unchanged in the six subjects after weight loss (mean, 14.4 +/- 2.8 mg/kg per d; NS vs. pre-weight loss). The changes in LDL catabolism and composition were associated with changes in the source of LDL apo B. Pre-weight loss, 73.3% of LDL was derived from VLDL, while 26.7% was directly secreted into plasma. Post-weight reduction, VLDL-derived LDL fell to 46.8% of total, while direct secretion accounted for 53.2% of LDL production. These changes were significant; P < 0.95. Thus, all subjects had direct secretion of LDL apo B and the magnitude of this source of VLDL triglyceride secretion. These results indicate that the regulation of plasma LDL levels in hypertriglyceridemic subjects is quite complex and that the rise in LDL levels after weight loss results from reduction in the fractional catabolism of this lipoprotein. The fall in the FCR is associated with changes in the source of LDL and in its composition.     

The kinetics of plasma free fatty acid and triglyceride transport in patients with idiopathic hypertriglyceridaemia and their relation to carbohydrate metabolism

Abstract. Plasma free fatty acid and triglyceride transport kinetics were assessed in 20 patients with idiopathic hypertriglyceridaemia. None of these patients had abnormal glucose tolerance. They included 10 patients in whom the serum triglyceride elevation was due to an increase in the circulating VLDL (Fredrickson Type IV) and 10 in whom the increase in plasma VLDL was associated with hyperchylomicronaemia (Fredrickson Type V). These were compared with a control group of 27 normal subjects.—Increased plasma triglyceride turnover with normal clearance was observed in the Type IV patients suggesting that the hypertriglyceridaemia in these patients was predominantly due to enhancement of plasma triglyceride production. The plasma triglyceride concentration correlated closely with the changes in triglyceride turnover rate.—Studies performed in the Type V patients showed an increase in the plasma triglyceride turnover rate in only 3 subjects, while in the remaining patients the turnover values were similar to those of the control subjects. The increase in serum triglyceride concentration found in some of the patients was due to an increase in plasma triglyceride production. However, in the majority of patients in this group impairment of plasma triglyceride clearance was the predominant abnormality.—In both hypertriglyceridaemic groups the plasma FFA flux was markedly increased and correlated significantly with the degree of hypertriglyceridaemia. The increase in triglyceride turnover observed in Type IV patients and some of the Type V patients correlated closely with the enhancement of plasma FFA flux suggesting that the increase in triglyceride production in these patients was secondary to enhanced lipolysis.—The plasma insulin response to an oral glucose load was markedly increased in both groups of hypertriglyceridaemic patients and correlated significantly with the elevation in serum triglyceride concentration. The plasma insulin response also correlated with the plasma free fatty acid turnover.—The results suggest that the initial lesion in these patients was related to insulin unresponsiveness in adipose tissue resulting in enhanced lipolysis with secondary changes in insulin secretion and plasma triglyceride transport kinetics.     

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

Abstract. 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 <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 <0.02) and LDL apo B (-23%, P <0.10) were reduced while their FCRs were unchanged.     

Different effects of continuous oestrogen–progestin and transdermal oestrogen with cyclic progestin regimens on low-density lipoprotein subclasses

Seventy-five postmenopausal women were randomly allocated to receive either continuous oral 17β-oestradiol 2mgday^?1 and norethisterone acetate 1 mg day^?1 (E₂/NETA) or transdermal treatment consisting of 28-day cycles with patches delivering 17β-oestradiol 50μgday^?1 combined with oral cyclic medroxyprogesterone acetate 10mgday^?1, on days 17–28 (E₂/MPA). At baseline, the plasma lipid and lipoprotein concentrations, composition and concentrations of low-density lipoprotein (LDL) subclasses (LDL1, LDL2 and LDL3) isolated by density-gradient ultracentrifugation were similar in the two groups. The post-heparin plasma hepatic lipase activity (HL) correlated inversely with the percentage of total LDL found in LDL1 (buoyant LDL) and directly with the percentage of LDL found in LDL3 (dense LDL). After 12 months of hormone replacement therapy (HRT), the total and LDL-cholesterol concentration of the E₂/NETA group decreased by 14% and 17% respectively (P<0.001), while in the E₂/MPA group these parameters remained unchanged. The lowering of LDL-cholesterol in the E₂/NETA group was a consequence of a significant reduction of the large, buoyant LDL particles (LDL1) from 103mgdL^?1 to 60mgdl^?1 (P<0.001) and of a decrease of cholesterol content of LDL particles in the major LDL subclass, LDL2. In the E₂/MPA group, the concentration of LDL1 decreased, but less than in the oral group. In both groups, a significant increase in the concentration of the LDL3 subclass was observed, indicating an overall shift to denser LDL particles. After 12 months, the post-heparin plasma HL activity decreased only in the E₂/NETA group (by 12%). The inverse correlation between post-heparin plasma HL activity and LDL1 persisted in both groups, but the direct correlation between HL and LDL3 vanished in the E₂/NETA group and subsided in the E₂/MPA group. Our results indicate that HRT has multiple effects on LDL subclasses and suggest that these changes cannot be explained by changes in HL activity.     

Significance of low density lipoprotein production in the regulations of plasma cholesterol level in man.

To determine whether production or catabolism of low density lipoprotein (LDL) is the major factor controlling LDL concentrations in subjects with plasma cholesterol levels from low-normal to mildly elevated, measurements of apoprotein of LDL (apoLDL) turnover were performed in 16 patients with various plasma cholesterol concentrations. Cholesterol balance studies were done simultaneously in 13 of these patients. Plasma concentrations of apoLDL and LDL-cholesterol were positively correlated with synthetic rates of apoLDL (r = 0.74, P less than 0.001; r = 0.50, P less than 0.05, respectively). No correlation was noted between the fractional catabolic rate for apoLDL and apoLDL levels (or LDL-cholesterol). For further analysis, the patients were divided into three groups with stepwise increases in apoLDL concentrations. When apoLDL levels rose significantly, from 83 +/- 5 SEM to 122 +/- 2 to 149 +/- 5 mg/dl, synthetic rates for apoLDL also increased significantly from 11.6 +/- 12. to 17.0 +/- 0.9 to 23.8 +/- 1.8 mg/d/kg ideal weight. In contrast, the fractional catabolic rate of apoLDL was not different among the three groups (0.32 +/- 0.03 vs. 0.29 +/- 0.02 vs. 0.33 +/- 0.03/d). No relation was noted between synthesis of total body cholesterol (or bile acids) and concentrations, production rates, or removal of apoLDL. Thus, concentrations of apoLDL and LDL-cholesterol in these subjects with plasma cholesterol levels from low-normal to mildly elevated were regulated mainly by synthetic rates of apoLDL and not by LDL catabolism.     

The low density lipoprotein receptor is not required for normal catabolism of Lp(a) in humans.

Lipoprotein(a) [Lp(a)] is an atherogenic lipoprotein which is similar in structure to low density lipoproteins (LDL). The role of the LDL receptor in the catabolism of Lp(a) has been controversial. We therefore investigated the in vivo catabolism of Lp(a) and LDL in five unrelated patients with homozygous familial hypercholesterolemia (FH) who have little or no LDL receptor activity. Purified 125I-Lp(a) and 131I-LDL were simultaneously injected into the homozygous FH patients, their heterozygous FH parents when available, and control subjects. The disappearance of plasma radioactivity was followed over time. As expected, the fractional catabolic rates (FCR) of 131I-LDL were markedly decreased in the homozygous FH patients (mean LDL FCR 0.190 d-1) and somewhat decreased in the heterozygous FH parents (mean LDL FCR 0.294 d-1) compared with controls (mean LDL FCR 0.401 d-1). In contrast, the catabolism of 125I-Lp(a) was not significantly different in the homozygous FH patients (mean FCR 0.251 d-1), heterozygous FH parents (mean FCR 0.254 d-1), and control subjects (mean FCR 0.287 d-1). In summary, absence of a functional LDL receptor does not result in delayed catabolism of Lp(a), indicating that the LDL receptor is not a physiologically important route of Lp(a) catabolism in humans.     

Increased ability of LDL from normolipidemic type 2 diabetic women to generate peroxides.

BACKGROUND: We assessed the ability of LDL from 30 type 1 diabetic patients (18 men, 12 women), 65 type 2 diabetic patients (35 men, 30 women), and 35 controls (19 men, 16 women) to generate peroxides. The men and women in the diabetic groups were studied separately and matched for age, body mass index, duration of diabetes, glycohemoglobin, and conventional lipid characteristics according to the presence or absence of hyperlipidemia. METHODS: The ability of LDL to form peroxides was assessed by measuring the thiobarbituric acid-reactive substances corrected for LDL-cholesterol [ratio of malondialdehyde (MDA) to LDL-cholesterol]. LDL particle size was expressed as the ratio of LDL-cholesterol to apolipoprotein B (LDL-cholesterol/apoB). RESULTS: The MDA/LDL-cholesterol ratio was higher in type 1 and type 2 diabetic patients with hyperlipidemia than in controls. The MDA/LDL-cholesterol ratio was also higher in type 2 normolipidemic women than in controls (P <0.01). The LDL-cholesterol/apoB ratio was lower in type 2 diabetic women than in type 2 diabetic men (P <0.05). The MDA/LDL-cholesterol ratio was negatively correlated with the LDL-cholesterol/apoB ratio (r = -0.78, P <0.001) in hyperlipidemic type 1 (not type 2) diabetic patients. In normolipidemic type 2 diabetic patients, the MDA/LDL-cholesterol ratio was also negatively correlated with the LDL-cholesterol/apoB ratio (r = -0.75, P <0.001) because of the highly significant negative correlation in type 2 diabetic women (r = -0.89, P <0.01). CONCLUSIONS: LDL from well-controlled type 2 diabetic women is smaller and more prone to form peroxides. This could explain why diabetic women are at greater risk of cardiovascular disease.     

Metabolic Studies in Familial Hypercholesterolemia: EVIDENCE FOR A GENE-DOSAGE EFFECT IN VIVO

To investigate the gene-dosage effect in familial hypercholesterolemia (FH), metabolic studies were conducted in a group of well-characterized patients with either heterozygous (n = 7) or homozygous (n = 7) FH and the results were compared to those obtained in normal subjects (n = 6). The turnover of (125)I-labeled low-density lipoprotein (LDL) was measured in all of the normals, all but one of the FH heterozygotes, and in all of the homozygotes. Chemical cholesterol balance was performed simultaneously with the (125)I-LDL turnover in all seven of the homozygotes.With regard to (125)I-LDL turnover, FH homozygotes, who possess two doses of the mutant FH gene, exhibited a threefold increase in the rate of apoLDL synthesis while the fractional catabolic rate (FCR) for the apoprotein was only about one-third of normal. Heterozygotes, who have only one dose of the mutant FH gene, exhibited intermediate values for both parameters; that is, the FCR was two-thirds of normal and the apoLDL synthetic rate was 1.7-fold greater than normal.THE DATA INDICATE THAT THE SINGLE GENE DEFECT IN FH PRODUCES TWO DISTINCT ABNORMALITIES OF LDL METABOLISM: (a) an increase in the synthetic rate for apoLDL and (b) a decrease in the efficiency of apoLDL catabolism. Both defects are more severe in FH homozygotes than in heterozygotes.The FCR for apoLDL in the homozygotes appeared to be fixed at congruent with 17%/d whereas the plasma LDL level varied about twofold. These findings suggest that the twofold variation in plasma LDL levels observed in these seven patients is caused by variation in the plasma apoLDL synthetic rates. Consistent with this conclusion was the finding that the correlation between the plasma LDL level and the apoLDL synthetic rates in the seven FH homozygotes was 0.943.The rate of total body cholesterol synthesis determined by chemical cholesterol balance did not appear to clearly differ between normals and patients with either one or two mutant FH genes. Two of the youngest FH homozygotes exhibited cholesterol overproduction but the other five did not. No consistent abnormality of bile acid metabolism was observed in these patients. Because the daily plasma flux of cholesterol on LDL is about threefold greater than the amount of cholesterol produced per day, a significant amount of the cholesterol liberated from LDL degradation must be reused.     

Assay of Total Plasma Apolipoprotein B Concentration in Human Subjects

We have developed a double antibody radioimmunoassay (RIA) for human apolipoprotein B (ApoB). The assay measures not only the ApoB content of beta-lipoproteins (low density lipoproteins [LDL]) but also that contained in the other lipoproteins in plasma.Purified lymph and plasma chylomicrons and plasma very low density lipoproteins (VLDL) produced displacement curves in the assay system which paralleled those produced by pure LDL. Thus, the ApoB found in chylomicrons, VLDL, and LDL were immunologically identical. ApoB accounted for about 25 and 35%, respectively, of the total protein of chylomicrons and VLDL by RIA. VLDL and LDL preparations from normal and hyperlipoproteinemic subjects also produced parallel displacement curves, suggesting that the ApoB of normal and hyperlipoproteinemic subjects were immunologically identical. High density lipoproteins and abetalipoproteinemic plasma displaced no counts, nor did the sera of several animal species produce any useful displacement curves in this system.The fasting total plasma ApoB concentration of normal subjects was 83+/-16 mg/dl (mean+/-SD). ApoB levels were high in Type II (162+/-16), and less so in Type IV (112+/-24) and Type V (105+/-17).When plasma ApoB concentration in Type IV patients was graphed against plasma glycerides, two subpopulations, which may represent different genetic or biochemical subgroups, were apparent.ApoB concentration in individuals on constant diet and drug regimen was stable over weeks to months. Greater than 90% of ApoB of normal and Type II subjects was in the d > 1.006 plasma fraction. By contrast, only 50-80% of ApoB was in the d > 1.006 fraction in Types IV and V. Thus, hypertriglyceridemia was associated primarily with a redistribution of ApoB to the lighter density fractions; by contrast, in hypercholesterolemia absolute ApoB concentration was markedly increased.     

Non-stedy-state studies of low-density-liproprotein turnover in familial hypercholesterolaemia.

1. The non-steady-state turnover of low-density lipoprotein (LDL), labelled in its apoprotein moiety (apo-B) with 131I, was determined in four patients with familial hypercholesterolaemia, three of them homozygotes. 2. The fractional and absolute catabolic rates (FCR and ACR) of LDL-apo-B were determined by relating the excretion of radioactivity, measured in urine in vitro and by whole-body counter in vivo, to plasma radioactivity and to LDL specific radioactivity respectively. 3. The FCR remained relatively constant, even after marked reduction of LDL pool size by means of plasma exchange. This confirms the existence of an intrinsic defect in LDL catabolism in familial hypercholesterolaemia. 4. LDL-apo-B synthesis, determined by summing the ACR and the daily increment in plasma LDL, was much higher in the three homozygotes than in the one heterozygote, in whom the synthetic rate was normal. 5. These results illustrate the usefulness of combining plasma exchange and whole-body radioactivy counting as a means of examining the relationship between the turnover and pool size of a 131I-labelled protein, such as LDL.     

Hepatic HMGCoA reductase in human cholelithiasis: effects of chenodeoxycholic and ursodeoxycholic acids*

Abstract. To study further the role of hepatic cholesterol synthesis in patients with gallstones, the activity of the rate-limiting step in cholesterbgenesis, hydroxy-methyl glutaryl co-enzyme A reductase (HMGCoAR), was measured in operative wedge liver biopsies from ten patients with untreated cholesterol gallstones, six with pigment stones and ten controls. Hepatic HMGCoAR was also measured in six patients with cholesterol-rich gallstones treated for 1–24 months with 14-6-18-6 mg chenodeoxycholic acid (CDCA) kg^-1 day^-1, in two patients with radiolucent pigment stones treated with 17-3 and 17-7 mg CDCA kg^-1 day^-1 and in ten other patients with cholesterol-rich stones given 4–5-7-2 mg ursodeoxycholic acid kg^-1 day^-1 for 1–6 months. HMGCoAR activity was also related to the free and esterified cholesterol content of both hepatic homogenates and the microsomal fractions. Compared with the controls (HMGCoAR activity 14-6±1 6 (SEM) pmole mg microsomal protein^-1 min^-1), the mean activity in untreated cholesterol cholelithiasis was 32 2 ±2-0 (P < 0–001), but was near normal in patients with pigment stones (16-2 + 1 5). In cholesterol gallstone patients, chenodeoxycholic acid treatment reduced the mean enzyme activity by 51% compared with the untreated gallstone group (P < 0001) and in smaller doses, ursodeoxycholic acid therapy lowered it by 40% (P < 0001) but in the two patients with pigment stones, CDCA did not seem to affect HMGCoAR activity. Despite this four-fold variation in enzyme activity, there were no significant differences in mean free or esterified cholesterol concentrations in whole liver homogenates or in the microsomal fraction from any of the patient groups. Presented in part at the European Society for Clinical Investigation, Rotterdam, April 1978 (Maton P.N. & Dowling R.H (1978) Eur J Clin Invest 8, 329 (abstract)) and at the V International Bile Acid Meeting, Freiburg, June 1978 (Maton P.N. & Dowling R.H. (1978) in: Biological Effects of Bile Acids (ed. by G. Paumgartner, A. Stiehl and W. Gerok), pp. 91–98. MTP Press, Lancaster).