Human apolipoprotein B: analysis of internal repeats and homology with other apolipoproteins

Alizarin as a potential protector of proteins against damage caused by hydroperoxyl radical

Alizarin is a natural anthraquinone molecule with moderate antioxidative capacity. Some earlier investigations indicated that it can inhibit osteosarcoma and breast carcinoma cell proliferation by inhibiting of phosphorylation process of ERK protein (extracellular signal-regulated kinases). Several mechanisms of deactivation of one of the most reactive oxygen species, hydroperoxyl radical, by alizarin are estimated: hydrogen atom abstraction (HAA), radical adduct formation (RAF), and single electron transfer (SET). The plausibility of those mechanisms is estimated using density functional theory. The obtained results indicated HAA as the only thermodynamically plausible mechanism. For that purpose, two possible mechanistic pathways for hydrogen atom abstraction are studied in detail: hydrogen atom transfer (HAT) and proton-coupled electron transfer (PCET). Water and benzene are used as models of solvents with opposite polarity. To examine the difference between HAT and PCET is used kinetical approach based on the Transition state theory (TST) and determined rate constants (k). Important data used for a distinction between HAT and PCET mechanisms are obtained by applying the Quantum Theory of Atoms in Molecules (QTAIM), and by the analysis of single occupied molecular orbitals (SOMOs) in transition states for two examined mechanisms. The molecular docking analysis and molecular dynamic are used to predict the most probable positions of binding of alizarin to the sequence of ApoB-100 protein, a protein component of plasma low-density lipoproteins (LDL). It is found that alizarin links the nitrated polypeptide forming the π-π interactions with the amino acids Phenylalanine and Nitrotyrosine. The ability of alizarin to scavenge hydroperoxyl radical when it is in a sandwich structure between the polypeptide and radical species, as the operative reaction mechanism, is not significantly changed concerning its antioxidant capacity in the absence of polypeptide. Therefore, alizarin can protect the polypeptide from harmful hydroperoxyl radical attack, positioning itself between the polypeptide chain and the reactive oxygen species.

A correlation between secondary structure and rheological properties of low-density lipoproteins at air/water interfaces

The secondary structure of apolipoprotein B-100 is studied within the bulk phase and at the air/water interface. In these "in viro" experiments, infrared reflection absorption spectroscopy (IRRAS) study was performed at the air/water interface while circular dichroism (CD) was conducted in the bulk phase. In the bulk phase, the conformational structure containing a significant amount of β-structure, whereas varying amount of α-helix, unordered structures, and β-sheet were observed at the air/water interface depending on the low-density lipoprotein (LDL) film interfacial pressure. The present IRRAS results demonstrate the importance of interfacial pressure-induced structural conformations on the apoB-100. A correlation between the secondary structure of the apoB-100 protein and the monomolecular film elasticity at the air/water interface was also established. The orientation of apoB-100 with respect to the LDL film-normal was found to depend on the interfacial pressure exhibited by the monomolecular film. These results may shed light on LDL's pivotal role in the progression of atherosclerotic coronary artery disease as demonstrated previously by clinical trials.

Surface plasmon resonance study on functional significance of clustered organization of lectin-like oxidized LDL receptor (LOX-1)

Lectin-like oxidized low-density lipoprotein (OxLDL) receptor 1 (LOX-1) is the major OxLDL receptor of vascular endothelial cells and is involved in an early step of atherogenesis. LOX-1 exists as a disulfide-linked homodimer on the cell surface, which contains a pair of the ligand-binding domains (CTLD; C-type lectin-like domain). Recent research using living cells has suggested that the clustered state of LOX-1 dimer on the cell is functionally required. These results questioned how LOX-1 exists on the cell to achieve OxLDL binding. In this study, we revealed the functional significance of the clustered organization of the ligand-binding domain of LOX-1 with surface plasmon resonance. Biotinylated CTLD was immobilized on a streptavidin sensor chip to make CTLD clusters on the surface. In this state, the CTLD had high affinity for OxLDL with a dissociation constant (K(D)) in the nanomolar range. This value is comparable to the K(D) measured for LOX-1 on the cell. In contrast, a single homodimeric LOX-1 extracellular domain had lower affinity for OxLDL in the supra-micromolar range of K(D). Monomeric CTLD showed marginal binding to OxLDL. In combination with the analyses on the loss-of-binding mutant W150A, we concluded that the clustered organization of the properly formed homodimeric CTLD is essential for the strong binding of LOX-1 to OxLDL.

Fenofibrate, simvastatin and their combination in the management of dyslipidaemia in type 2 diabetic patients

OBJECTIVE

To evaluate the efficacy of fenofibrate, simvastatin or their combination in type 2 diabetic patients with combined dyslipidaemia.

RESEARCH DESIGN AND METHODS

241 patients, who had never previously taken lipid-lowering medications, received fenofibrate 145 mg/day, or simvastatin 40 mg/day, or fenofibrate 145 mg/day + simvastatin 40 mg/day combination for 12 months. We evaluated lipids, glycaemic, haemostatic, and inflammatory variables at baseline, and after 6 and 12 months.

RESULTS

After 12 months total cholesterol (TC), LDL cholesterol (LDL-C) and triglycerides (Tg) decreased while HDL cholesterol (HDL-C) increased in all groups, even if the values obtained with fenofibrate + simvastatin were the best. At the end of the study apolipoprotein A-1 (Apo A-1) increased with fenofibrate + simvastatin, while apolipoprotein B (Apo B) decreased in all groups compared to baseline. Plasminogen activator inhibitor-1 (PAI-1) and high-sensitivity C reactive protein (hs-CRP) decreased after 12 months compared to baseline with simvastatin, and with fenofibrate + simvastatin even if the value obtained with fenofibrate-simvastatin was the lowest. After 12 months, fibrinogen (Fg) decreased compared to baseline with fenofibrate + simvastatin.

LIMITATIONS

This study has some limitations. The first one is the relatively small sample of studied patients. The second one is the lack of an advanced lipid proteins evaluation, such as lipoprotein subfraction changes in the different treatment regimen. Finally, we have not selected patients that could show the best response to fibrate (i.e.: hypertriglyceridemics) or statins (i.e.: hypercholesterolemics) monotherapy, so the effect of these drugs administered alone may have been partly attenuated.

CONCLUSIONS

Fenofibrate + simvastatin association improved lipid parameters, prothrombotic and inflammatory factors, and appeared to have a good tolerability profile over 12 months of therapy.

Monogenic Hypocholesterolaemic Lipid Disorders and Apolipoprotein B Metabolism

The study of apolipoprotein (apo) B metabolism is central to our understanding of human lipoprotein metabolism. Moreover, the assembly and secretion of apoB-containing lipoproteins is a complex process. Increased plasma concentrations of apoB-containing lipoproteins are an important risk factor for the development of atherosclerotic coronary heart disease. In contrast, decreased levels of, but not the absence of, these apoB-containing lipoproteins is associated with resistance to atherosclerosis and potential long life. The study of inherited monogenic dyslipidaemias has been an effective means to elucidate key metabolic steps and biologically relevant mechanisms. Naturally occurring gene mutations in affected families have been useful in identifying important domains of apoB and microsomal triglyceride transfer protein (MTP) governing the metabolism of apoB-containing lipoproteins. Truncation-causing mutations in the APOB gene cause familial hypobetalipoproteinaemia, whereas mutations in MTP result in abetalipoproteinaemia; both rare conditions are characterised by marked hypocholesterolaemia. The purpose of this review is to examine the role of apoB in lipoprotein metabolism and to explore the key biochemical, clinical, metabolic and genetic features of the monogenic hypocholesterolaemic lipid disorders affecting apoB metabolism.

Charged amino acid residues 997-1000 of human apolipoprotein B100 are critical for the initiation of lipoprotein assembly and the formation of a stable lipidated primordial particle in McA-RH7777 cells

We previously demonstrated that a portion, or perhaps all, of the residues between 931 and 1000 of apolipoprotein (apo) B100 are required for the initiation of apoB-containing particle assembly. Based on our structural model of the first 1000 residues of apoB (designated as apoB:1000), we hypothesized that this domain folds into a three-sided lipovitellin-like "lipid pocket" via a hairpin-bridge mechanism. We proposed that salt bridges are formed between four tandem charged residues 717-720 in the turn of the hairpin bridge and four tandem complementary residues 997-1000 located at the C-terminal end of the model. To identify the specific motif within residues 931 and 1000 that is critical for apoB particle assembly, apoB:956 and apoB:986 were produced. To test the hairpin-bridge hypothesis, the following mutations were made: 1) residues 997-1000 deletion (apoB:996), 2) residues 717-720 deletion (apoB:1000Delta717-720), and 3) substitution of charged residues 997-1000 with alanines (apoB:996 + 4Ala). Characterization of particles secreted by stable transformants of McA-RH7777 cells demonstrated the following. 1) ApoB:956 did not form stable particles and was secreted as large lipid-rich aggregates. 2) ApoB:986 formed both a lipidated particle that was denser than HDL(3) and large lipid-rich aggregates. 3) Compared with wild-type apoB:1000, apoB:1000Delta717-720 displayed the following: (i) significantly diminished capacity to form intact lipidated particles and (ii) increased propensity to form large lipid-rich aggregates. 4) In striking contrast to wild-type apoB:1000, (i) apoB:996 and apoB:996 + 4Ala were highly susceptible to intracellular degradation, (ii) only a small proportion of the secreted proteins formed stable HDL(3)-like lipoproteins, and (iii) a majority of the secreted proteins formed large lipid-rich aggregates. We conclude that the first 1000 amino acid residues of human apoB100 are required for the initiation of nascent apoB-containing lipoprotein assembly, and residues 717-720 and 997-1000 play key roles in this process, perhaps via a hairpin-bridge mechanism.

Effects of nateglinide and glibenclamide on prothrombotic factors in naïve type 2 diabetic patients treated with metformin: a 1-year, double-blind, randomized clinical trial

OBJECTIVE

To evaluate the effect on coagulation and fibrinolysis parameters and on non-conventional cardiovascular risk factors of metformin plus nateglinide or glibenclamide in naïve type 2 diabetes patients.

PATIENTS AND METHODS

A total of 248 type 2 diabetic patients were enrolled and randomly assigned to receive nateglinide or glibenclamide, and metformin for 12 months. We assessed body mass index (BMI), glycated hemoglobin (HbA1c), fasting plasma glucose (FPG), postprandial plasma glucose (PPG), fasting plasma insulin (FPI), postprandial plasma insulin (PPI), homeostasis model assessment index (HOMA index), lipid profile with lipoprotein (a) [Lp(a)], fibrinogen (Fg), plasminogen activator inhibitor-1 (PAI-1), tissue plasminogen activator (t-PA), homocysteine (Hcy), systolic blood pressure (SBP), diastolic blood pressure (DBP).

RESULTS

After 9 months of treatment, both tested drug combinations were similarly associated with a significant reduction in FPG (nateglinide, -17.2%; glibenclamide, -16.9%, both p<0.05) compared to the baseline, while HbA1c (-17.3%, p<0.05) and PPG (-15.2%, p<0.05) significantly decreased only in the nateglinide group. After one year of treatment, compared to the baseline the nateglinide group showed a significant reduction in HbA1c (-21%, p<0.01), FPG (-20.7%), p<0.01, PPG (-21.5%, p<0.05), HOMA index (-25.4%, p<0.05); the glibenclamide group, showed a significant reduction in HbA1c (-11%, p<0.05), FPG (-23.2%, p<0.05), PPG (-11.2%, p<0.05), and HOMA index (-23.9%, p<0.05) but to a minor extent. Moreover, the HbA1c difference value from baseline observed in the nateglinide-treated group was significantly higher than that observed in the glibenclamide group. Therefore the nateglinide-treated patients showed a significant reduction in some prothrombotic parameters (PAI-1=-19%, Lp(a)=-31%, and Hcy=-32.3%, all p<0.05), whereas the glibenclamide-treated patients did not.

CONCLUSION

Nateglinide appears to improve glycemic control as well as the levels of some prothrombotic parameters compared to glibenclamide when administered in combination with metformin.

Differential effect of glimepiride and rosiglitazone on metabolic control of type 2 diabetic patients treated with metformin: a randomized, double-blind, clinical trial

AIM

Accumulating evidence suggests that combination therapy using oral antidiabetic agents with different mechanisms of action may be highly effective in achieving and maintaining target blood glucose levels. The aim of our study is to evaluate the differential effect on glucose and lipid parameters of the association between glimepiride plus metformin and rosiglitazone plus metformin in patients affected by type 2 diabetes and metabolic syndrome.

METHODS

Patients were enroled, evaluated and followed at two Italian centres. We evaluated 99 type 2 diabetic patients with metabolic syndrome (48 males and 47 females; 23 males and 24 females, aged 52 +/- 5 with glimepiride; 25 males and 23 females, aged 54 +/- 4 with cglitazone). All were required to have been diagnosed as being diabetic for at least 6 months and did not have glycaemic control with diet and oral hypoglycaemic agents such as sulphonylureas or metformin, both to the maximum tolerated dose. All patients took a fixed dose of metformin, 1500 mg/day. We administered glimepiride (2 mg/day) or rosiglitazone (4 mg/day) in a randomized, controlled, double-blind clinical study. We evaluated body mass index (BMI), glycaemic control, lipid profile [total cholesterol (TC), low-density lipoprotein-cholesterol (LDL-C), high-density lipoprotein-cholesterol and triglycerides] and lipoprotein parameters [apolipoprotein A-I and apolipoprotein B (Apo B)] during 12 months of this treatment.

RESULTS

A total of 95 patients completed the study. Significant BMI decrease was observed at 12 months in glimepiride and rosiglitazone group (p < 0.05 and p < 0.01 respectively) as well as of glycated haemoglobin decrease (p < 0.05 and p < 0.01 respectively), mean fasting plasma glucose and postprandial plasma glucose levels (p < 0.05 and p < 0.01 respectively). A decrease in fasting plasma insulin and postprandial plasma insulin at 12 months (p < 0.05 and p < 0.01 respectively) compared with the baseline value in rosiglitazone group was observed. Furthermore, homeostasis model assessment index improvement was obtained only at 9 and 12 months (p < 0.05 and p < 0.01 respectively) compared with the baseline value in rosiglitazone group. Significant TC, LDL-C and Apo B improvement (p < 0.05 respectively) was present in glimepiride group after 12 months compared with the baseline values, and these variations were significant (p < 0.05) between groups. Of the 95 patients who completed the study, 8.5% of patients in glimepiride group and 12.5% of patients in rosiglitazone group had side-effects (p = not significant). Four patients had transient side-effects in glimepiride group and six patients in rosiglitazone group. Altogether, we did not have statistically significant changes in transaminases.

CONCLUSIONS

The rosiglitazone-metformin association significantly improve the long-term control of all insulin-resistance-related parameters in comparison with the glimepiride-metformin-treated group. On the other side, glimepiride treatment is associated to a slight improvement in cholesterolaemia, not observed in the rosiglitazone-treated patients.

Crystal Structure of Human Lectin-like, Oxidized Low-Density Lipoprotein Receptor 1 Ligand Binding Domain and Its Ligand Recognition Mode to OxLDL

Lectin-like, oxidized low-density lipoprotein (LDL) receptor 1, LOX-1, is the major receptor for oxidized LDL (OxLDL) in endothelial cells. We have determined the crystal structure of the ligand binding domain of LOX-1, with a short stalk region connecting the domain to the membrane-spanning region, as a homodimer linked by an interchain disulfide bond. In vivo assays with LOX-1 mutants revealed that the "basic spine," consisting of linearly aligned arginine residues spanning over the dimer surface, is responsible for ligand binding. Single amino acid substitution in the dimer interface caused a severe reduction in LOX-1 binding activity, suggesting that the correct dimer arrangement is crucial for binding to OxLDL. Based on the LDL model structure, possible binding modes of LOX-1 to OxLDL are proposed.

Metabolic effects of pioglitazone and rosiglitazone in patients with diabetes and metabolic syndrome treated with glimepiride: A twelve-month, multicenter, double-blind, randomized, controlled, parallel-group trial

BACKGROUND

Glimepiride is approved as monotherapy and in combination with metformin or with insulin, whereas the combination of glimepiride with other antihyperglycemic drugs is under investigation.

OBJECTIVE

The aim of this study was to assess the differential effect on glucose and lipid variables and tolerability of the combination of glimepiride plus pioglitazone or rosiglitazone in patients with type 2 diabetes mellitus (DM) and metabolic syndrome.

METHODS

This 12-month, multicenter, double-blind, randomized, controlled, parallel-group trial was conducted at 3 study sites in Italy. We assessed patients with type 2 DM (duration, > or =6 months) and with metabolic syndrome. All patients were required to have poor glycemic control with, or to have experienced > or =1 adverse effect (AE) with, diet and oral hypoglycemic agents such as sulfonylureas or metformin, both given up to the maximum tolerated dose. All patients received a fixed oral dose of glimepiride, 4 mg/d divided into 2 doses, self-administered for 12 months. Patients also were randomized to receive oral pioglitazone (15 mg once daily) (G + P group) or oral rosiglitazone (4 mg once daily) (G + R group), self-administered for 12 months. We assessed body mass index (BMI), glycemic control (glycosylated hemoglobin [HbA(1c)], fasting and postprandial plasma glucose and insulin levels [FPG, PPG, FPI, and PPI, respectively], and homeostasis model assessment index), lipid profile (total cholesterol [TC], low-density lipoprotein cholesterol [LDL-C], high-density lipoprotein cholesterol [HDL-C], and triglycerides [TG]), and lipoprotein variables (apolipoprotein [apo] A-I and apo B) at baseline and at 3, 6, 9, and 12 months of treatment. Treatment tolerability was assessed at each study visit using a thorough interview of patients, and comparisons of clinical and laboratory values to baseline levels.

RESULTS

A total of 91 patients were enrolled in the study; 87 patients completed it (G + P group: 24 women, 21 men; mean [SD] age, 53 [6] years; G + R group: 20 women, 22 men; mean [SD] age, 54 [5] years). Patients in the G + P and G + R groups experienced significant increases in mean BMI at 12 months compared with baseline (4.92% and 6.17%, respectively; both, P < 0.05). The combination of glimepiride with pioglitazone or rosiglitazone significantly improved glycemic control in the study patients. At 12 months, we observed a 1.3% improvement in mean values for plasma HbA(1c) concentration (P < 0.01) 19.3% in FPG (P < 0.01), 16.3% in PPG (P < 0.01), 42.4% in FPI ), and 23.3% in PPI (P <0.05); no significant differences were found between treatment groups. Although the G + P group experienced a significant improvement at 12 months in almost all variables of lipid metabolism from baseline (TC, - 11%; LDL-C, -12%; HDL-C, 15%; and apo B, - 10.6% [all, P , 0.05]), the G + R group experienced a significant increase in mostly the lipid risk factors for cardiovascular disease (TC, 14.9%; LDL-C, 16.5%; TG, 17.9%; and apo B, 10.3% [all, P , 0.05]). Overall, no statistically significant changes in plasma aminotransferase activities were observed. Of the 87 patients who completed the study, 6.7% (3/45) of patients in the G + P group and 11.9% (5/42) of patients in the G + R group had transient, mild to moderate AEs that did not cause withdrawal from the trial.

CONCLUSION

In this study of patients with type 2 DM and metabolic syndrome who did not respond adequately to, or experienced AEs with, diet and either a sulfonylurea or metformin previously, the combination of glimepiride plus pioglitazone was associated with a significant improvement in lipid and lipoprotein variables, whereas the combination of glimepiride plus rosiglitazone appears to not have had any clinically significant effect on lipid metabolism.

A randomized, double-blind, controlled, parallel-group comparison of perindopril and candesartan in hypertensive patients with type 2 diabetes mellitus

BACKGROUND

When choosing an antihypertensive drug for patients with hypertension and diabetes mellitus (DM), the metabolic side effects, possibility of improving some metabolic parameters, and need for adequate blood pressure control must all be considered.

OBJECTIVE

The goal of this study was to compare the impacts of perindopril and candesartan on blood pressure, glucose metabolism, serum lipid profile, and metabolic parameters in patients with mild hypertension and type 2 DM during therapy and after a 1-month washout period.

METHODS

Type 2 DM patients with mild hypertension and good glucose control who were not taking hypercholesterolemic drugs were enrolled. Perindopril 4 mg QD or candesartan 16 mg QD was administered for 12 months in this randomized, double-blind, controlled, parallel-group clinical trial. Fasting plasma glucose (FPG), fasting plasma insulin (FPI), glycosylated hemoglobin, homeostasis model assessment (HOMA) index, systolic blood pressure (SBP), diastolic blood pressure (DBP), total cholesterol (TC), low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol, triglycerides, lipoprotein(a) (Lp[a]), plasminogen activator inhibitor 1 (PAI-1), homocysteine, body mass index (BMI), and albumin excretion rate (AER) were assessed.

RESULTS

Ninety-six patients (49 women and 47 men; mean [SD] ages, 53 [10] years [perindopril] and 55 [9] years [candesartan]) were enrolled. Mean (SD) body weight, height, and BMI were 78.2 (9.4) kg, 1.69 (0.05) m, and 27.2 (2.0) kg/m(2) in the perindopril group and 77.5 (8.6) kg, 1.70 (0.06) m, and 26.8 (2.5) kg/m(2) in the candesartan group. A significant change occurred from baseline to month 12 during treatment with perindopril in SBP and DBP (both P < 0.01), FPG (P < 0.05), FPI (P < 0.05), TC (P < 0.05), LDL-C (P < 0.05), Lp(a) (P < 0.05), PAM (P < 0.05), and AER (P < 0.05). Significant changes from baseline to month 12 occurred with candesartan in SBP and DBP (both P < 0.01) and AER (P < 0.05). The HOMA index was significantly lower at month 12 in the perindopril group than in the candesartan group (P < 0.05). When we interrupted perindopril and candesartan therapy for a 1-month washout period, changes in SBP and DBP values were significant compared with month 12 in both groups (all P < 0.05). Changes in TC and LDL-C from month 12 to the end of washout were significant only in the perindopril group (both P < 0.05).

CONCLUSIONS

Perindopril and candesartan both effectively lowered blood pressure in this group of patients with mild hypertension and type 2 DM. Perindopril showed an improvement on some metabolic parameters compared with candesartan. However, the inclusion/exclusion criteria could limit the ability to extrapolate the results to a general population.

The effect of L-carnitine on plasma lipoprotein(a) levels in hypercholesterolemic patients with type 2 diabetes mellitus

BACKGROUND

A previous study has demonstrated that L-carnitine reduces plasma lipoprotein(a) (Lp[a]) levels in patients with hypercholesterolemia.

OBJECTIVE

To test a tolerable Lp(a)-reducing agent in diabetic patients, we assessed the effect of a dietary supplementation of L-carnitine on plasma lipid levels, particularly Lp(a), of patients with type 2 diabetes mellitus (DM) and hypercholesterolemia.

METHODS

In this 6-month, randomized, double-masked, placebo-controlled clinical trial, patients were enrolled, assessed, and followed up at the Diabetic and Metabolic Diseases Center of the Department of Internal Medicine and Therapeutics at the University of Pavia, Pavia, Italy. All study patients had newly diagnosed type 2 DM that was managed through dietary restriction alone throughout the study, as well as hypercholesterolemia. Patients were randomized to 1 of 2 groups. One group received L-carnitine, one 1-g tablet BID. The other group received a corresponding placebo. We assessed body mass index, fasting plasma glucose, postprandial plasma glucose, glycosylated hemoglobin, fasting plasma insulin, total cholesterol, low-density lipoprotein cholesterol, high-density lipoprotein cholesterol, triglycerides, apolipoprotein (apo) A-I, apo B, and Lp(a) at baseline and at 1, 3, and 6 months of treatment.

RESULTS

This study included 94 patients. The treatment group included 24 men and 22 women (mean [SD] age, 52 [6] years). The placebo group included 23 men and 25 women (mean [SD] age, 50 [7] years). The baseline characteristics of the groups did not differ significantly. The mean (SD) body weight, height, and body mass index were 78.2 (5.8) kg, 1.70 (0.04) m, and 27.3 (2.5) kg/m(2), respectively, in the L-carnitine group and 77.6 (6.4) kg, 1.71 (0.05) m, and 26.8 (2.2) kg/m(2), respectively, in the placebo group. In the treatment group, Lp(a) was significantly reduced at 3 and 6 months compared with baseline (P < 0.05) and P < 0.01, respectively). We observed a significant improvement after 6 months (P < 0.05) in the Lp(a) value in patients taking L-carnitine compared with those taking placebo. Between-group differences in other variables did not reach a level of significance at months 3 and 6. No drug-related adverse events were reported or observed.

CONCLUSION

In this preliminary study, after 3 and 6 months, L-carnitine significantly lowered the plasma Lp(a) level compared with placebo in selected hypercholesterolemic patients with newly diagnosed type 2 DM.

Comparison between repaglinide and glimepiride in patients with type 2 diabetes mellitus: a one-year, randomized, double-blind assessment of metabolic parameters and cardiovascular risk factors

BACKGROUND

Repaglinide and glimepiride are relatively new oral hypoglycemic agents. Few data are available concerning their effects on metabolic parameters other than measures of glycemic control.

OBJECTIVES

In addition to assessing the effects of repaglinide and glimepiride on glycemic control in patients with type 2 diabetes mellitus, this study also examined the effects of these agents on 3 metabolic parameters known to be cardiovascular risk factors--lipoprotein(a) (Lp[a]), plasminogen activator inhibitor-1 (PAI-1), and homocysteine (Hcy).

METHODS

This randomized, placebo-controlled, double-blind trial was conducted at a single center in Italy. Eligible patients were nonsmokers; had no hypertension or coronary heart disease; were taking no hypolipidemic drugs, diuretics, beta-blockers, or thyroxin; and had normal renal function. After an initial 4-week placebo washout period, patients were randomized to receive repaglinide 1 mg/d or glimepiride 1 mg/d. The dose of study drug was optimized over an 8-week titration period, which was followed by a 12-month treatment period. Measures of glycemic control (glycated hemoglobin [HbA1c], fasting plasma glucose [FPG], postprandial plasma glucose [PPG], fasting plasma insulin [FPI], postprandial plasma insulin [PPI]) and the other metabolic parameters of interest were assessed after 6 and 12 months of treatment.

RESULTS

One hundred twenty-four patients (63 women, 61 men) completed the study, 62 in each treatment group. There were no significant differences in demographic characteristics between groups. After 6 and 12 months of treatment, FPG levels and HbA1c values were significantly reduced from baseline in both groups (6 months, P < 0.05; 12 months, P < 0.01). After 6 months, PPG levels were significantly decreased only in the repaglinide group (P < 0.05 vs baseline); at 12 months, however, PPG levels were significantly reduced from baseline in both groups (P < 0.01 repaglinide, P < 0.05 glimepiride). No significant changes from baseline in FPI or PPI levels were seen in either group at 6 months, although FPI levels were significantly increased in the repaglinide group at 12 months (P < 0.05). Repaglinide significantly lowered levels of Lp(a), PAI-1, and Hcy after 12 months (all, P < 0.05 vs baseline). Glimepiride significantly lowered levels of Lp(a) and Hcy after 6 months (both, P < 0.05 vs baseline) and levels of Lp(a) (P < 0.01 vs baseline), Hcy (P < 0.01 vs baseline), and PAI-1 (P < 0.05 vs baseline) after 12 months.

CONCLUSIONS

Repaglinide and glimepiride improved glycemic control and reduced levels of other metabolic parameters of interest in this population of patients with type 2 diabetes. It is possible that the reductions in Lp(a), PAI-1, and Hcy were the result of improved glucose metabolism; however, the possibility that repaglinide and glimepiride may have a direct effect on these parameters should not be excluded.

Lipoprotein A-I structure

High density lipoproteins are produced by the liver as protein-lipid complexes with a characteristic discoidal shape. A crystal structure is available for the chief protein component of these complexes, apolipoprotein A-I, but controversy about how this protein is situated with respect to the lipid components has flourished for lack of experimental techniques that can characterize protein structure in a lipid environment. New spectroscopic techniques developed to address this problem now indicate that apolipoprotein A-I is arranged as a helical belt around a bilayer of phospholipids. This is an important step towards understanding how these lipoproteins regulate cholesterol transport.

Identification of domains in apolipoprotein B100 that confer a high requirement for the microsomal triglyceride transfer protein

The microsomal triglyceride transfer protein (MTP) is required for the assembly and secretion of apoB-containing lipoproteins. To investigate the role of MTP in lipoprotein assembly, we determined the ability of carboxyl-terminally truncated forms of apoB to be secreted from cells treated with the MTP inhibitor 4'-bromo-3'-methylmetaqualone (Benoist, F., Nicodeme, E., and Grand-Perret, T. (1996) Eur. J. Biochem. 240, 713-720). In Caco-2 and mhAT3F cells that produce apoB100 and apoB48, the inhibitor preferentially blocked apoB100 secretion. When the inhibitor was tested on McA-RH7777 cells stably transfected with cDNAs encoding human apoB100, apoB72, apoB53, apoB29, and apoB18, the secretion of apoB100, apoB72, and apoB53 was preferentially impaired relative to apoB48 and shorter forms. To delineate the region between apoB48 and apoB53 that has a high requirement for MTP, we used puromycin to generate a range of truncated forms of apoB in HepG2 cells. The secretion of apoB53 and longer forms of apoB was markedly affected by low concentrations of the MTP inhibitor (approximately 1 microM), whereas apoB51 and smaller forms of apoB were only affected at higher concentrations (> 10 microM). The size-related sensitivity to MTP inhibitor was not due to late processing or retention, since the same result was observed when nascent lipoproteins were isolated from the endoplasmic reticulum. The MTP inhibitor did not alter the density of the secreted lipoproteins, indicating that each apoB polypeptide requires a minimally defined amount of lipid to attain a secretable conformation. Our results suggest that the folding of the domain between apoB51 and apoB53 has a high requirement for lipid. This domain is predicted to form amphipathic alpha-helices and to bind lipid reversibly. It proceeds and is followed by rigid amphipathic beta-sheets that are predicted to associate with lipid irreversibly. We speculate that these domains enable apoB to switch from a stable lipid-poor conformation in apoB48 to another lipid-rich conformation in apoB100 during lipoprotein assembly.

The mechanism of inhibition of factor III (thromboplastin) activity by apolipoprotein B-100. Protein-protein interactions

Factor III (thromboplastin) activity is inhibited by apoB-100, but the mechanism of inhibition is unknown. By examining the effect of purified apoB-100 on factor III activity, we showed that apoB-100 can inhibit factor III via a different mechanism from that caused by the issue-factor pathway-inhibitor, which is mainly carried on the surface of lipoproteins. Although the presence of calcium ions and factors X and VII may enhance the rate of inhibition, they are not a prerequisite for the inhibition of factor III by apoB-100. In addition, by investigating the changes in the UV spectra of apoB-100 on interaction with factor III and factors X and VII and by assigning the shifts in absorption spectra to particular amino acids, we showed that these interactions involve negative and positive residues within these proteins. By following the rates of interactions between apoB-100 and either factors III, X, VII, a two-step mechanism for the inhibition process involving factors X and VII was postulated. In this mechanism, the primary interaction of apoB-100 with factor III is followed by a rate-limiting step that can be accelerated by the presence of either factor X or VII and leads to the inhibition of factor III. Furthermore, a computer-based analysis of the sequences of factor III revealed a possible binding site for apoB-100.

Helix-helix interactions in reconstituted high-density lipoproteins

In this work we calculated the ionic interactions between adjacent amphipathic helices of apo A-I and apo A-IV. The calculation of the electrostatic potential around the helices helps identify the charged residues susceptible to form salt bridges between adjacent helices. An estimation of the stability of the different pairs of helices is derived from the calculation of the energy of interaction between contiguous helices at a water/lipid interface after energy minimization. The most stable energetic conformation corresponds to the 17-residue helices oriented anti-parallel and separated by a stretch of 5 residues in an extended beta-strand conformation, as calculated through the 'stereo alphabet' calculation procedure. In a pair of helices, the hydrophobic faces are directed towards the lipid core of the discoidal phospholipid-apolipoprotein complex and the hydrophobic lipid-protein interactions are major determinants for the stability of the complex. Interactions between polar residues located on the opposite face of the helix and water molecules can also contribute to the overall energy of the system. Finally, salt bridge formation between residues of opposite charge along the edge of the helical segments contribute to the cooperativity of the phospholipid-apolipoprotein complex formation. The mode of assembly of the amphipathic helical repeats of the apolipoproteins around the edge of a discoidal complex is therefore determined both by the hydrophobic character of the residues and by the charge complementarity along the edge of the helices which increases the structural stability and determines the relative orientation of the helices.(ABSTRACT TRUNCATED AT 250 WORDS)

Presence of an extended duplication in the putative low-density-lipoprotein receptor-binding domain of apolipoprotein B. Cloning and characterization of the domain in salmon

The sequence of the C-terminal 1058 amino acids of atlantic salmon (Salmo salar) apolipoprotein (apo) B was deduced from the nucleotide sequence of cloned cDNA. In comparison with chicken or mammals apoB-100, salmon apoB is C-terminally truncated and extended gaps are found. The two clusters of positively charged residues, previously identified as part of the putative low-density-lipoprotein (LDL) receptor-binding domain of apoB, are brought into close proximity in salmon apoB. This is achieved by the absence between the two clusters of the proline-rich area with the potential to form an amphipathic beta sheet, present in higher vertebrates. In addition, analysis of apoB amino acid sequences currently available in vertebrates revealed the presence of an extended internal duplication in the putative LDL receptor-binding domain. Thus, the two basic clusters would have been duplicated resulting in the presence, except for salmon apoB, of two homologous sites in the C-terminal part of the molecule. The results described here together with earlier biochemical and genetic evidence support the view that Arg3500, a residue mutated in familial defective apoB-100, could be included in a folded critical region of the putative LDL receptor-binding domain of human apoB-100. This region possibly brings the two sub-domains that arise from the duplication close to each other.

Lipoprotein lipase association with lipoproteins involves protein-protein interaction with apolipoprotein B

Lipoprotein lipase (LPL) hydrolyzes chylomicron and very low density lipoprotein (VLDL) triglycerides and potentiates the cellular uptake of lipoproteins. These LPL-lipoprotein associations could involve only protein-lipid interaction, or they could be modulated by apolipoproteins (apo). ApoB is the major protein component of chylomicrons, VLDL, and low density lipoprotein (LDL). ApoB100, a large glycoprotein with a molecular mass of 550 kDa, is composed of several functional domains. A carboxyl-terminal region of the protein is the ligand for the LDL receptor. There are several hydrophobic domains that are believed to be important in lipid binding. The relatively hydrophilic amino-terminal region of apoB, however, has no known function. Using solid phase assays we quantified LPL-lipoprotein complex formation. On a molar basis, severalfold greater amounts of LPL bound to LDL and VLDL than to high density lipoprotein at all the concentrations of LPL tested (0.9-55 nM). To assess the roles of LDL protein versus lipid, we performed competition and ligand blotting experiments. LDL and an amino-terminal fragment of apoB competed better for 125I-LPL binding to LDL than did lipid emulsion particles. Delipidation of LDL-coated plates did not alter LPL binding. On ligand blots, LPL bound to amino-terminal fragments of apoB generated by thrombin digestion but not to apoA1, apoE, or carboxyl-terminal fragments of apoB. Further evidence for LPL interaction with the amino-terminal region of apoB was obtained using anti-apoB monoclonal antibodies. Antibodies directed against the amino-terminal regions of apoB blocked LPL interaction with LDL, whereas those against the carboxyl-terminal region of apoB did not inhibit LPL interaction with LDL. Thus, we conclude that a specific interaction between LPL and the amino-terminal region of apoB may facilitate LPL association with circulating lipoproteins.

apoB-100 has a pentapartite structure composed of three amphipathic alpha-helical domains alternating with two amphipathic beta-strand domains. Detection by the computer program LOCATE

Due to the great length of apolipoprotein (apo) B-100, the localization of lipid-associating domains in this protein has been difficult. To address this question, we developed a computer program called Locate that searches amino acid sequences to identify potential amphipathic alpha-helixes and beta-strands by using sets of rules for helix and strand termination. A series of model chimeric protein test datasets were created by tandem linking of amino acid sequences of multiple proteins containing four different secondary structural motifs: motif A (exchangeable plasma apolipoproteins); motif G (globular alpha-helical proteins); motif C (coiled-coil alpha-helical proteins); and motif B (beta pleated-sheet proteins). These four test datasets, as well as randomly scrambled sequences of each dataset, were analyzed by Locate using increasingly stringent parameters. Using intermediately stringent parameters under which significant numbers of amphipathic helixes were found only in the unscrambled motif A, two dense clusters of putative lipid-associating amphipathic helixes were located precisely in the middle and at the C-terminal end of apoB-100 (a sparse cluster of class G* helixes is located at the N-terminus). The dense clusters are located between residues 2103 through 2560 and 4061 through 4338 and have densities of 2.4 and 2.2 amphipathic helixes per 100 residues, respectively; under these conditions, motif A has a density of 1.4 amphipathic helixes per 100 residues. These two domains correspond closely to the two major apoB-100 lipid-associated domains at residues 2100 through 2700 and 4100 through 4500 using the principle of releasability of tryptic peptides from trypsin-treated intact low-density lipoprotein. The classes of amphipathic helixes identified within these two putative lipid-associating domains are considerably more diverse than those found in the exchangeable plasma apolipoproteins. Interestingly, apoB-48 terminates at the N-terminal edge of the middle cluster. By using a similar strategy for analysis of amphipathic beta-strands, we discovered that the two gap regions between the three amphipathic helix clusters are highly enriched in putative amphipathic beta-strands, while the three amphipathic helical domains are essentially devoid of this putative lipid-associating motif. We propose, therefore, that apoB-100 has a pentapartite structure, NH2-alpha 1-beta 1-alpha 2-beta 2-alpha 3-COOH, with alpha 1 representing a globular domain.

Synthesis and secretion of hepatic apolipoprotein B-containing lipoproteins

Human apolipoprotein (apo) B-100 is required for the synthesis and secretion of hepatic triacyglycerol-rich lipoproteins. This review summarizes recent developments in understanding the interaction of cis-acting DNA sequences and trans-acting protein factors in regulation of apo B gene expression and apo B mRNA editing, and the role of structural determinants of apo B-100 in the assembly and secretion of hepatic lipoproteins. In particular, experimental results obtained from cell culture studies using techniques of molecular and cellular biology are described and discussed. The relationship between apo B length and its ability to recruit lipids is presented, and the involvement of factors other than apo B in hepatic triacylglycerol-rich lipoprotein production is discussed.

Apolipoprotein B and low-density lipoprotein structure: implications for biosynthesis of triglyceride-rich lipoproteins

ApoB100 is a very large glycoprotein essential for triglyceride transport in vertebrates. It plays functional roles in lipoprotein biosynthesis in liver and intestine, and is the ligand recognized by the LDL receptor during receptor-mediated endocytosis. ApoB100 is encoded by a single gene on chromosome 2, and the message undergoes a unique processing event to form apoB48 message in the human intestine, and, in some species, in liver as well. The primary sequence is relatively unique and appears unrelated to the sequences of other serum apolipoproteins, except for some possible homology with the receptor recognition sequence of apolipoprotein E. From its sequence, structure prediction shows the presence of both sheet and helix scattered along its length, but no transmembrane domains apart from the signal sequence. The multiple carbohydrate attachment sites have been identified, as well as the locations of most of its disulfides. ApoB is the single protein found on LDL. These lipoproteins are emulsion particles, containing a core of nonpolar cholesteryl ester and triglyceride oil, surrounded by an emulsifying agent, a monolayer of phospholipid, cholesterol, and a single molecule of apoB100. An emulsion particle model is developed to predict accurately the physical and compositional properties of an LDL of any given size. A variety of techniques have been employed to map apoB100 on the surface of the LDL, and all yield a model in which apoB surrounds the LDL like a belt. Moreover, it is concluded that apoB100 folds into a long, flexible structure with a cross-section of about 20 x 54 A2 and a length of about 585 A. This structure is embedded in the surface coat of the LDL and makes contact with the core. During lipoprotein biosynthesis in tissue culture, truncated fragments of apoB100 are secreted on lipoproteins. Here, it was found that the lipoprotein core circumference was directly proportional to the apoB fragment size. A cotranslational model has been porposed for the lipoprotein assembly, which includes these structural features, and it is concluded that in permanent hepatocyte cell lines, apoB size determines lipoprotein core circumference.

Influence of triacylglycerol biosynthesis rate on the assembly of apoB-100-containing lipoproteins in Hep G2 cells

Apolipoprotein B-100 (apoB-100) appears in three forms in the endoplasmic reticulum of Hep G2 cells: (1) tightly bound to the membrane, ie, not extractable by sodium carbonate. This form is glycosylated but protease sensitive when present in intact microsomes, suggesting that it is only partially translocated to the microsomal lumen; (2) extractable by sodium carbonate and present on low-density lipoprotein-very-low-density lipoprotein (LDL-VLDL)-like particles. This form is glycosylated and secreted into the medium; and (3) extractable by sodium carbonate but having a higher density than the LDL-VLDL-like particles. This form, referred to as Fraction I, is glycosylated and protected against proteases when present in intact microsomal vesicles, indicating that it is completely translocated to the luminal side of the microsomal membrane. Fraction I is not secreted into the medium, but it disappears with time from the cell, suggesting that it is degraded. Oleic acid induced a 2.7-fold increase in the rate of the biosynthesis of triacylglycerol but not of phosphatidylcholine in Hep G2 cells. Incubation of the cells with oleic acid had no significant effect on the rate of initiation of the apoB-100-containing lipoproteins, nor did it influence the amount of apoB-100 that was associated with the membrane or the turnover of apoB-100 in the membrane. Instead, it increased the proportion of the nascent apoB polypeptides on initiated lipoproteins that was converted into full-length apoB-100 on LDL-VLDL-like particles, giving rise to an increased amount of these particles in the lumen of the secretory pathway. Pulse-chase experiments showed that incubation with oleic acid gave rise to an increased formation of LDL-VLDL-like particles on behalf of the formation of Fraction I. This effect of oleic acid could partially explain the protective effect of the fatty acid on apoB-100, preventing it from undergoing posttranslational degradation.

Lipoprotein-thyroid hormone interactions

A small but significant portion of the thyroid hormones that circulate in human plasma is associated with lipoproteins. Although the major lipoprotein carrier is HDL, the role of these interactions on T(4) entry into cells was tested first with LDL and human fibroblasts because of the well-characterized LDL receptors and the availability of cells with genetically absent receptors. It was shown that LDL enhanced the uptake of T(4) by cultured fibroblasts in which the LDL receptors were expressed. The T(4) binding sites on apolipoproteins B-100 and E, as well as apoA-I, have been partially characterized, and they exhibit considerable homology. A number of possible physiologic consequences of thyroid hormonelipoprotein interactions have been put forward as topics for further investigation.

Molecular modeling of the amphipathic helices of the plasma apolipoproteins

In this paper we propose a classification of the amphipathic helical repeats occurring in the plasma apolipoprotein sequences. It is based upon the calculation of the molecular hydrophobicity potential around the helical segments. The repeats were identified using a new autocorrelation matrix, based upon similarities of hydrophobic and hydrophilic properties of the amino acid residues within the apolipoprotein sequences. The helices were constructed by molecular modeling, the molecular hydrophobicity potential was calculated, and isopotential contour lines drawn around the helices yielded a three-dimensional visualization of the hydrophobicity potential. Two classes of apolipoproteins could be differentiated by comparing the hydrophobic angles obtained by projection of the isopotential contour lines on a plane perpendicular to the long axis of the helix. The isopotential contour lines around apo AI, AIV, and E are more hydrophilic than hydrophobic, whereas they are of similar intensity for apo AII, CI, and CIII. In both cases discoidal lipid-protein complexes are generated, with the amphipathic helices around the edge of the lipid core. The long axis of the helices is oriented parallel to the phospholipid acyl chains and the hydrophilic side of the helix toward the aqueous phase. As a result of the differences in hydrophobicity potential, the contact between the hydrophobic side of the helices and the phospholipid acyl chains is larger for apo AII, CI, and CIII than for the other apolipoproteins. This might account for the greater stability of the discoidal complexes generated between phospholipids and these apoproteins.

Apolipoprotein B gene mutations affecting cholesterol levels

In the past 5 years, many different mutations in the apolipoprotein (apo) B gene have been described that affect plasma cholesterol levels. More than 20 different mutations in the apoB gene have been shown to cause familial hypobetalipoproteinaemia, a condition characterized by abnormally low plasma concentrations of apoB and LDL cholesterol. Almost all of the mutations are nonsense or frameshift mutations that interfere with the translation of a full-length apoB100 molecule. Many, but not all, of these apoB gene mutations result in the synthesis of a truncated species of apoB that can be detected within the plasma lipoproteins. Familial hypobetalipoproteinaemia heterozygotes are almost always asymptomatic and have LDL cholesterol levels about one-quarter to one-third of those of unaffected family members. Several homozygotes and compound heterozygotes for familial hypobetalipoproteinaemia have been described. In these individuals, the LDL cholesterol levels are extremely low, usually less than 5 or 10 mg dl-1, and the clinical phenotype is variable, ranging from completely asymptomatic to severe problems related to intestinal fat malabsorption. One missense mutation in the apoB gene (an Arg----Gln substitution at apoB amino acid 3500) is associated with very poor binding of apoB100 to the cellular LDL receptor. This syndrome has been designated familial defective apolipoprotein B (FDB). The amino-acid substitution at residue 3500 delays the clearance of LDL from the plasma and results in hypercholesterolaemia. In some Western populations, the frequency of FDB heterozygotes appears to be as high as 1 in 500 individuals.

Vitellogenin--homologs of mammalian apolipoproteins?

1. To determine if a relationship exists between vertebrate vitellogenins and mammalian plasma proteins the EMBL and NBRF computer databases were searched with two partial amino acid sequences from Xenopus laevis and Gallus gallus vitellogenin. 2. A significant relationship was found between vitellogenin and human apolipoprotein B-100 genes, and confirmed using homology-determination programs. 3. Further analysis shows that unique multiple proline consensus regions found in apolipoprotein B-100 are significantly similar to proline dominant regions in vitellogenin. 4. This work suggests that these proteins are functionally and structurally related and should be categorized as a functional group of hepatic lipid transport and metabolism proteins.

Recent progress in understanding apolipoprotein B

For the past 5 years, investigators from many different laboratories have contributed to a greatly increased understanding of two very important lipid-carrying proteins in plasma--apo B-100 and apo B-48. Apo B-100, an extremely large protein composed of 4,536 amino acids, is synthesized by the liver and is crucial for the assembly of triglyceride-rich VLDL particles. Apo B-100 is virtually the only protein of LDL, a cholesteryl ester-enriched class of lipoproteins that are metabolic products of VLDL. The apo B-100 of LDL serves as a ligand for the LDL receptor-mediated uptake of LDL particles by the liver and extrahepatic tissues. The LDL receptor-binding region of apo B-100 is located in the carboxyterminal portion of the molecule, whereas its lipid-binding regions appear to be broadly dispersed throughout its length. Apo B-48 contains the amino-terminal 2,152 amino acids of apo B-100 and is produced by the intestine as a result of editing of a single nucleotide of the apo B mRNA, which changes the codon specifying apo B-100 amino acid 2,153 to a premature stop codon. Apo B-48 has an obligatory structural role in the formation of chylomicrons; therefore, its synthesis is essential for absorption of dietary fats and fat-soluble vitamins. Both apo B-48 and apo B-100 are encoded on chromosome 2 by a single gene that contains 29 exons and 28 introns. An elevated level of apo B-100 in the plasma is a potent risk factor for developing premature atherosclerotic disease. In the past 3 years, many different apo B gene mutations that affect the concentrations of both apo B and cholesterol in the plasma have been characterized. A missense mutation in the codon for apo B-100 amino aid 3,500 is associated with hypercholesterolemia. This mutation results in poor binding of apo B-100 to the LDL receptor, thereby causing the cholesteryl ester-enriched LDL particles to accumulate in the plasma. This disorder is called familial defective apo B-100, and it is probably a cause of premature atherosclerotic disease. Familial hypobetalipoproteinemia is a condition associated with abnormally low levels of apo B and cholesterol; affected individuals may actually have a reduced risk of atherosclerotic disease.(ABSTRACT TRUNCATED AT 400 WORDS)

Mode of assembly of amphipathic helical segments in model high-density lipoproteins

The structure of discoidal apo A-I-phospholipid complexes, representing the metabolic precursors of mature high-density lipoprotein particles, was studied by a combination of both a theoretical and an experimental approach. The secondary structure of the complex was determined by circular dichroic measurements, while the relative orientation of the apo A-I helical segments and of the phospholipid acyl chains was determined by ATR infrared measurements. Fluorescence energy transfer between the tryptophan residues of apo A-I and fluorescent phospholipid probes yielded an estimation of the relative topography of the lipid and apolipoprotein components in discoidal and spherical particles. The theoretical approach consisted of the identification of the helical segments in various apo A-I species. These segments were then oriented at a lipid/water interface by minimization of their hydrophobic and hydrophilic transfer energies. The calculation of the hydrophobicity profiles along the axis of the helices leads to the identification of specific interactions between pairs of helices. The helices were further assembled together with the phospholipids by computer modelling, enabling an estimation of the dimensions of the complex. The combination of the experimental and theoretical results yielded a model for discoidal apolipoprotein-phospholipid complexes, in which the amphipathic helical segments are oriented along the edges of the discs. Such a model can be extended to the conversion of these complexes into mature spherical HDL, through the formation of a cholesteryl ester core.

Evaluation of the secondary structure of apo B-100 in low-density lipoprotein (LDL) by infrared spectroscopy

The secondary structure of the apo B-100 protein present in human low density lipoprotein has been investigated by transmission and attenuated total reflection infrared spectroscopy. The amount of beta-sheet (41%) is significantly higher than that determined by CD spectroscopy in the present study (12%) and elsewhere (15-16%). The high percentage of beta-sheet structure in apo B-100 supports the importance of such segments in maintaining the lipid-protein assembly in LDL. Polarized infrared spectroscopy indicates that the beta-sheet component of apo B-100 adopts a preferential orientation with respect to the phospholipid monolayer surrounding the LDL, whereas no such orientation is observed for the other secondary structure components.

Topo-dynamic characteristics of human plasma VLDL apolipoproteins and efficiency of triacylglycerol hydrolysis by lipoprotein lipase

A lower accessibility to water-soluble quenchers of tryptophanyls of VLDL apolipoproteins B, E, C as compared to LDL apoB chromophores has been detected by a fluorescence quenching technique. The dynamic behaviour of the tryptophanyls of VLDL amphipathic apolipoproteins E and C did not change in the presence of a detergent, Tween-20, at sub-lytic concentrations. However, a reversible structural transition registered by the 'red' shift of the emission spectrum maximum and the changes in the quenching pattern by I- occurred under these conditions. The increase in the VLDL tryptophanyl accessibility to acrylamide and the decrease in the quenching constant were observed at partial and complete solubilization of the VLDL particles by the detergent. Dissociation of apolipoproteins from VLDL occurred after their treatment with Tween-20 or lipoprotein lipase isolated from bovine milk, and the tryptophanyl population not participating in fluorescence energy transfer on lipid phase-localized fluorescent probe pyrene appeared. In the presence of Tween-20, the relative affinity of apoE for the lipid matrix of VLDL was lower than that of apoC. Besides, the uncompetitive mode of inhibition of the LPL activity by apoC-III has been demonstrated. It is suggested that: (1) the amphipathic apolipoproteins E and C are organized as clusters on the VLDL surface and/or partially shielded by apolipoprotein B: (2) self-regulation of lypolysis may exist involving detergent-like reaction product accumulation and changes in relative apolipoprotein contents.

The apolipoprotein multigene family: structure, expression, evolution, and molecular genetics

The plasma apolipoproteins can be classified into two subgroups: the soluble apolipoproteins including apolipoprotein (apo) A-I, A-II, A-IV, C-I, C-II, C-III, and E, and the apoBs including apoB-100 and apoB-48. The soluble apolipoproteins have very similar genomic structures, each having a total of three introns at the same locations; apoA-IV is an exception in that it has lost its first intron. Using the exon/intron junctions as reference points, we can obtain an alignment of the coding regions of all the soluble apolipoprotein genes. The mature peptide regions of the genes are almost completely made up of tandem repeats of 11 codons. The part of mature peptide region encoded by exon 3 contains a common block of 33 codons, whereas the part encoded by exon 4 contains a much more variable number of internal repeats of 11 codons. On the basis of the degree of homology of the various sequences, and the pattern of the internal repeats in these genes, an evolutionary tree has been proposed for the soluble apolipoprotein genes. ApoB-100 differs considerably from the soluble apolipoproteins. It is the largest apolipoprotein containing 4536 amino acid residues. Two types of internal repeats are identified in apoB-100: amphipathic alpha-helical repeats and proline-containing repeats with high beta-sheet content. The apoB gene contains 29 exons and 28 introns. Its evolutionary relationship to the soluble apolipoprotein genes is unclear. The 3' end of the apoB gene contains a region of variable number of tandem 12-16-base pair repeats.(ABSTRACT TRUNCATED AT 250 WORDS)