Apolipoprotein A-I(Milano) unfolds via an intermediate state as studied by differential scanning calorimetry and circular dichroism.

In this study the thermal and denaturant induced denaturation behaviors of apolipoprotein A-I(Milano) (apo A-IM) have been studied by differential scanning calorimetry and circular dichroism spectroscopy, as well as solution properties by analytical ultracentrifugation. Thermal denaturation is dependent on pH, sodium phosphate concentration and NaCl concentration. The protein is highly self-associated at the protein concentrations used in this study. Denaturation of apo A-IM at pH 7.4 and 8.0 occurs in two steps. The midpoint between the transition is at 37 degrees C. The first step at 31 degrees C involves melting of tertiary structure and rearrangement of protein association complexes, i.e. a transition into an intermediate molten globular-like state. Subsequent melting of this intermediate state into an unfolded state occurs at 52 degrees C. At pH 2.8 the protein lacks all tertiary structure and denaturation occurs over a large temperature interval, indicating the induction of a molten globular-like state at low pH.     

Equilibrium studies of the effect of difference in sequence homology on the mechanism of denaturation of bovine and horse cytochromes-c

We have carried out equilibrium studies of the effect of the amino acid residue difference in the primary structure of bovine cytochrome-c (b-cyt-c) and horse cyt-c (h-cyt-c) on the mechanism of their folding <--> unfolding processes at pH 6.0 and 25 degrees C. It has been observed that guanidinium chloride (GdmCl)-induced denaturation of b-cyt-c follows a two-state mechanism and that of h-cyt-c is not a two-state process. This conclusion is reached from the coincidence and non-coincidence of GdmCl-induced transition curves of bovine and horse proteins, respectively, monitored by measurements of absorbance at 405, 530 and 695 nm and circular dichroism (CD) at 222, 416 and 405 nm. These measurements on h-cyt-c in the presence of GdmCl in the concentration range 0.75-2.0 M also suggest that the protein retains all the native far-UV CD but has slightly perturbed tertiary interaction. The intermediate in the presence of these low denaturant concentrations does not have the structural characteristics of a molten globule as judged by the 8-Anilino-1-napthalene sulfonic acid (ANS) binding and near-UV CD experiments. We have also carried out thermal denaturation studies of bovine and horse cyts-c in the presence of GdmCl monitored by absorbance at 405 nm and far-UV CD at 222 nm. The heat-induced denaturation measurements in the presence of the denaturant show (1) that denaturation of b-cyt-c is a two-state process and that of h-cyt-c does not follow a two-state mechanism, and (2) that the enthalpy change on denaturation of both proteins strongly depends on GdmCl concentration.     

Non enzymatic glycation of apolipoprotein A-I. Effects on its self-association and lipid binding properties

In diabetic patients, hyperglycaemia results in the non enzymatic glycation of many proteins including apolipoprotein A-I. We purified glycated apo A-I and compared its lipid binding properties to those of normal apo A-I. Analysis of tryptophan fluorescence spectra and of fluorescence quenching in the presence of iodine showed that glycation of apo A-I induces a decrease in the stability of the lipid-apoprotein interaction and in that of the apoprotein self-association. Repetitive ultracentrifugations of High Density Lipoprotein (HDL) samples containing radioiodinated apo A-I or glycated apo A-I revealed that glycation of the apoprotein facilitates its dissociation from HDL. These results suggest that the non enzymatic glycation of apo A-I may affect the structural cohesion of HDL particles.     

Difference in the mechanisms of the cold and heat induced unfolding of thioredoxin h from Chlamydomonas reinhardtii: spectroscopic and calorimetric studies

The thermodynamic stability and temperature induced structural changes of oxidized thioredoxin h from Chlamydomonas reinhardtii have been studied using differential scanning calorimetry (DSC), near- and far-UV circular dichroism (CD), and fluorescence spectroscopies. At neutral pH, the heat induced unfolding of thioredoxin h is irreversible. The irreversibly unfolded protein is unable to refold due to the formation of soluble high-order oligomers. In contrast, at acidic pH the heat induced unfolding of thioredoxin h is fully reversible and thus allows the thermodynamic stability of this protein to be characterized. Analysis of the heat induced unfolding at acidic pH using calorimetric and spectroscopic methods shows that the heat induced denaturation of thioredoxin h can be well approximated by a two-state transition. The unfolding of thioredoxin h is accompanied by a large heat capacity change [6.0 +/- 1.0 kJ/(mol.K)], suggesting that at low pH a cold denaturation should be observed at the above-freezing temperatures for this protein. All used methods (DSC, near-UV CD, far-UV CD, Trp fluorescence) do indeed show that thioredoxin h undergoes cold denaturation at pH <2.5. The cold denaturation of thioredoxin h cannot, however, be fitted to a two-state model of unfolding. Furthermore, according to the far-UV CD, thioredoxin h is fully unfolded at pH 2.0 and 0 degrees C, whereas the other three methods (near-UV CD, fluorescence, and DSC) indicate that under these conditions 20-30% of the protein molecules are still in the native state. Several alternative mechanisms explaining these results such as structural differences in the heat and cold denatured state ensembles and the two-domain structure of thioredoxin h are discussed.     

Thermodynamics of protein self-association and unfolding. The case of apolipoprotein A-I

Protein self-association and protein unfolding are two temperature-dependent processes whose understanding is of utmost importance for the development of biological pharmaceuticals because protein association may stabilize or destabilize protein structure and function. Here we present new theoretical and experimental methods for analyzing the thermodynamics of self-association and unfolding. We used isothermal dilution calorimetry and analytical ultracentrifugation to measure protein self-association and introduced binding partition functions to analyze the cooperative association equilibria. In a second type of experiment, we monitored thermal protein unfolding with differential scanning calorimetry and circular dichroism spectroscopy and used the Zimm?Bragg theory to analyze the unfolding process. For α-helical proteins, the cooperative Zimm?Bragg theory appears to be a powerful alternative to the classical two-state model. As a model protein, we chose highly purified human recombinant apolipoprotein A-I. Self-association of Apo A-I showed a maximum at 21 °C with an association constant Ka of 5.6 × 10(5) M(?1), a cooperativity parameter σ of 0.003, and a maximal association number n of 8. The association enthalpy was linearly dependent on temperature and changed from endothermic at low temperatures to exothermic above 21 °C with a molar heat capacity ΔC(p)° of ?2.76 kJ mol(?1) K(?1). Above 45 °C, the association could no longer be measured because of the onset of unfolding. Unfolding occurred between 45 and 65 °C and was reversible and independent of protein concentration up to 160 μM. The midpoint of unfolding (T(0)) as measured by DSC was 52?53 °C; the enthalpy of unfolding (ΔH(N)(U)) was 420 kJ/mol. The molar heat capacity (Δ(N)(U)C(p)) increased by 5.0 ± 0.5 kJ mol(?1) K(?1) upon unfolding corresponding to a loss of 80?85 helical segments, which was confirmed by circular dichroism spectroscopy. Unfolding was highly cooperative with a nucleation parameter σ of 4.4 × 10(?5).     

Characterization of A-I-containing lipoproteins in subjects with A-I Milano variant

The A-I Milano variant of apolipoprotein A-I (A-IM), by virtue of its Arg-173----Cys substitution, is capable of forming a disulfide bond with the 77-amino-acid apolipoprotein A-II polypeptide (A-IIS) as well as with itself to produce dimers, A-IM/A-IIS and A-IM/A-IM, respectively. A-I-containing lipoproteins (Lp): particles with A-II (Lp(A-I with A-11)) and particles without A-II (Lp(A-I without A-II)) in the plasma of two nonhyperlipidemic A-IM carriers were investigated to determine the effect of A-IM on these lipoproteins. Despite the existence of abnormal apolipoprotein dimers and the unusually low HDL cholesterol (17 and 14 mg/dl), A-I (67 and 75 mg/dl), and A-II (18 and 18 mg/dl) levels in the two carriers, the plasma A-I of the carriers was distributed between Lp(A-I with A-II) and Lp(A-I without A-II) in a proportion comparable to that observed in normals. As expected, A-IM/A-IIS mixed dimer was found in carrier Lp(A-I with A-II). However, A-IM/A-IM dimer was located almost exclusively in carrier Lp(A-I without A-II). Chemical (dimethylsuberimidate) crosslinking of the protein moieties of the major subpopulations of Lp(A-I with A-II) and Lp(A-I without A-II) of normal and A-IM carriers showed that Lp(A-I with A-II), which is located predominantly in the 7.8-9.7 nm interval ((HDL2a + 3a + 3b)gge), had an apparent protein molecular weight equivalent to two molecules of A-I and one to two molecules of A-II per particle. Most of the Lp(A-I without A-II) particles, located predominantly in the size intervals of 9.7-12.9 nm (designated (HDL2b)gge) and 8.2-8.8 nm (HDL3a)gge) had protein moieties exhibiting a molecular weight equivalence predominantly of four and three molecules of A-I, respectively. A small quantity of particles with apparent protein content of two molecules of A-I in the 7.2-8.2 nm interval ((HDL3b + 3c)gge) was also detected. These studies showed that in nonhyperlipidemic A-IM carriers, the occurrence of apolipoprotein dimers had not markedly affected the protein stoichiometry of Lp(A-I with A-II) and Lp(A-I without A-II).     

Speciation of human plasma high-density lipoprotein (HDL): HDL stability and apolipoprotein A-I partitioning

The distribution of apolipoprotein (apo) A-I between human high-density lipoproteins (HDL) and water is an important component of reverse cholesterol transport and the atheroprotective effects of HDL. Chaotropic perturbation (CP) with guanidinium chloride (Gdm-Cl) reveals HDL instability by inducing the unfolding and transfer of apo A-I but not apo A-II into the aqueous phase while forming larger apo A-I deficient HDL-like particles and small amounts of cholesteryl ester-rich microemulsions (CERMs). Our kinetic and hydrodynamic studies of the CP of HDL species separated according to size and density show that (1) CP mediated an increase in HDL size, which involves quasi-fusion of surface and core lipids, and release of lipid-free apo A-I (these processes correlate linearly), (2) >94% of the HDL lipids remain with an apo A-I deficient particle, (3) apo A-II remains associated with a very stable HDL-like particle even at high levels of Gdm-Cl, and (4) apo A-I unfolding and transfer from HDL to water vary among HDL subfractions with the larger and more buoyant species exhibiting greater stability. Our data indicate that apo A-I's on small HDL (HDL-S) are highly dynamic and, relative to apo A-I on the larger more mature HDL, partition more readily into the aqueous phase, where they initiate the formation of new HDL species. Our data suggest that the greater instability of HDL-S generates free apo A-I and an apo A-I deficient HDL-S that readily fuses with the more stable HDL-L. Thus, the presence of HDL-L drives the CP remodeling of HDL to an equilibrium with even larger HDL-L and more lipid-free apo A-I than with either HDL-L or HDL-S alone. Moreover, according to dilution studies of HDL in 3 M Gdm-Cl, CP of HDL fits a model of apo A-I partitioning between HDL phospholipids and water that is controlled by the principal of opposing forces. These findings suggest that the size and relative amount of HDL lipid determine the HDL stability and the fraction of apo A-I that partitions into the aqueous phase where it is destined for interaction with ABCA1 transporters, thereby initiating reverse cholesterol transport or, alternatively, renal clearance.     

Self-association of human apolipoproteins A-I and A-II and interactions of apolipoprotein A-I with bile salts: quasi-elastic light scattering studies

We employed quasi-elastic light scattering (QLS) to systematically study the aqueous self-association of human apolipoproteins A-I and A-II (apo A-I and apo A-II) and the interactions of apo A-I with common taurine-conjugated bile salts. Self-association of apo A-I was promoted by increases in apolipoprotein concentration (0.09-2.2 mg/mL) and ionic strength (0.15-2.0 M NaCl), inhibited by increases in temperature (5-50 degrees C) and guanidine hydrochloride concentration (0-2.0 M), and unaffected by hydrostatic pressures up to 500 atm. The mean hydrodynamic radius (Rh) of apo A-I micelles ranged from 38 A to a maximum asymptotic value of 68 A. We examined several possible models of apo A-I self-association; the model that best fitted the Rh values assumed that apo A-I monomers first interacted at low concentrations to form dimers, which then further associated to form ring-shaped limiting octamers. Comparison of the temperature-dependent and ionic strength dependent free energy changes for the formation of octamers from apo A-I dimers suggested that hydrophobic forces strongly favored self-association and that electrostatic repulsive forces were only weakly counteractive. Apo A-II self-association was also promoted by increases in apolipoprotein concentration (0.2-1.8 mg/mL) and inhibited by increases in guanidine hydrochloride concentration (0-1.0 M) but was unaffected by variations in temperature (10-37 degrees C): the largest Rh values observed were consistent with limiting tetramers. As demonstrated by equilibrium dialysis, bile salts in concentrations below their critical micellar concentrations (cmc) bound to apo A-I micelles but had no effect upon apo A-I self-association, as inferred from constant Rh values. When bile salt concentrations exceeded their aqueous cmc values, a dissociation of apo A-I micelles resulted with the formation of mixed bile salt/apo A-I micelles. These studies support the concepts that apo A-I and apo A-II form small dimeric micelles at low concentrations that grow sharply to reach limiting sizes over a narrow concentration range. The influences of bile salt concentration and species upon these micelles have relevance to the plasma transport of bile salts in high-density lipoproteins and to the physical-chemical state of apo A-I and apo A-II molecules in native biles.     

Equilibrium denaturation of insulin and proinsulin

The guanidine hydrochloride induced equilibrium denaturation of insulin and proinsulin was studied by using near- and far-ultraviolet (UV) circular dichroism (CD). The denaturation transition of insulin is reversible, cooperative, symmetrical, and the same whether detected by near- or far-UV CD. These results are consistent with a two-state denaturation process without any appreciable equilibrium intermediates. Analysis of the insulin denaturation data yields a Gibbs free energy of unfolding of 4.5 +/- 0.5 kcal/mol. Denaturation of proinsulin detected by near-UV CD appears to be the same as for insulin, but if detected by far-UV CD appears different. The far-UV CD results demonstrate a multiphasic transition with the connecting peptide portion unfolding at lower concentrations of denaturant. Similar studies with the isolated C-peptide show that its conformation and susceptibility to denaturation are independent of the rest of the proinsulin molecule. After the proinsulin denaturation results were adjusted for the connecting peptide contribution, a denaturation transition identical with that of insulin was obtained. These results show that for proinsulin, the connecting peptide segment is not a random coil; it is an autonomous folding unit, and the portion corresponding to insulin is identical with insulin in terms of conformational stability.     

Induction of cellular cholesterol efflux to lipid-free apolipoprotein A-I by cAMP

In the present study apolipoprotein-mediated free cholesterol (FC) efflux was studied in J774 macrophages having normal cholesterol levels using an experimental design in which efflux occurs in the absence of contributions from cholesteryl ester hydrolysis. The results show that cAMP induces both saturable apolipoprotein (apo) A-I-mediated FC efflux and saturable apo A-I cell-surface binding, suggesting a link between these processes. However, the EC50 for efflux was 5-7-fold lower than the Kd for binding in both control and cAMP-stimulated cells. This dissociation between apo A-I binding and FC efflux was also seen in cells treated for 1 h with probucol which completely blocked FC efflux without affecting apo A-I specific binding. Thus, cAMP-stimulated FC efflux involves probucol-sensitive processes distinct from apo A-I binding to its putative cell surface receptor. FC efflux was also dramatically stimulated in elicited mouse peritoneal macrophages, suggesting that cAMP-regulated apolipoprotein-mediated FC efflux may be important in cholesterol homeostasis in normal macrophages. The presence of a cAMP-inducible cell protein that interacts with lipid-free apo A-I was investigated by chemical cross-linking of 125I-apo A-I with J774 cell surface proteins which revealed a Mr 200 kDa component when the cells were treated with cAMP.     

The primary structure of apolipoprotein A-I from rabbit high-density lipoprotein

The amino acid sequence of rabbit apolipoprotein A-I (apo A-I) has been determined by degradation and alignment of two overlapping sets of peptides obtained from tryptic and staphylococcal digestions. All of the peptides of rabbit apo A-I resulting from digestion by staphylococcal protease were isolated and sequenced except residues 33-37. A digestion with trypsin was employed to find overlapping and missing peptides. The N-terminus of rabbit apo A-I was confirmed by sequencing the intact protein up to 20 residues while the C-terminus was identified through its homology with human apo A-I. The protein contains 241 residues in its single chain. Its primary structure is highly homologous to the reported canine apo A-I (80%) and human apo A-I (78%), but exhibits less similarity with rat apo A-I (60%). Like human apo A-I, rabbit apo A-I contains very little histidine (2) and methionine (1); it does however have two residues of isoleucine. Based on a comparison of the hydrophobic-hydrophilic character of apo A-I residues with that of the two synthetic peptides that activated lecithin: cholesterol acyltransferase (Pownall et al. and Yokoyama et al.), we found that the five segments with the highest corresponding homologies on the protein are located within the N-terminal half. This suggests that the N-terminal half of apo A-I contains the major portion of regions activating lecithin: cholesterol acyltransferase.     

A natively unfolded toxin domain uses its receptor as a folding template

Natively unfolded proteins range from molten globules to disordered coils. They are abundant in eukaryotic genomes and commonly involved in molecular interactions. The essential N-terminal translocation domains of colicin toxins from Escherichia coli are disordered bacterial proteins that bind at least one protein of the Tol or Ton family. The colicin N translocation domain (ColN-(1-90)), which binds to the C-terminal domain of TolA (TolA-(296-421)), shows a disordered far-UV CD spectrum, no near-UV CD signal, and non-cooperative thermal unfolding. As expected, TolA-(296-421) displays both secondary structure in far-UV CD and tertiary structure in near-UV CD. Furthermore it shows a cooperative unfolding transition at 65 degrees C. CD spectra of the 1:1 complex show both increased secondary structure and colicin N-specific near-UV CD signals. A new cooperative thermal transition at 35 degrees C is followed by the unchanged unfolding behavior of TolA-(296-421). Fluorescence and surface plasmon resonance confirm that the new unfolding transition accompanies dissociation of ColN-(1-90). Hence upon binding the disordered structure of ColN-(1-90) converts to a cooperatively folded domain without altering the TolA-(296-421) structure.     

Oxidation of methionine residues affects the structure and stability of apolipoprotein A-I in reconstituted high density lipoprotein particles.

To determine the effect of oxidative damage to lipid-bound apolipoprotein A-I (apo A-I) on its structure and stability that might be related to previously observed functional disorders of oxidized apo A-I in high density lipoproteins (HDL), we prepared homogeneous reconstituted HDL (rHDL) particles containing unoxidized apo A-I and its commonly occurring oxidized form (Met-112, 148 bis-sulfoxide). The size of the obtained discoidal rHDL particles ranged from 9.0 to 10.0 nm and did not depend upon the content of the oxidized protein. Using circular dichroism methods, no change in the secondary structure of lipid-bound oxidized apo A-I was found. Isothermal and thermal denaturation experiments showed a significant destabilization of the oxidized protein to denaturation by guanidine hydrochloride or heat. This effect was observed with and without co-reconstituted apolipoprotein A-II. Limited tryptic digestion indicated that the central region of oxidatively damaged apo A-I becomes exposed to proteolysis in the rHDL particles. Implications of these data for apolipoprotein function are discussed.     

Cholesterol-induced alteration of apolipoprotein A-I conformation in reassembled high density lipoprotein.

Recent reports have shown that apolipoprotein A-I (apo A-I), the major protein of high density lipoprotein (HDL) may exist in different conformational states. We studied the effects of apolipoprotein A-II and/or cholesterol on the conformation of apo A-I in reassembled HDL. Analysis of tryptophan fluorescence quenching in the presence of iodine suggested that cholesterol increased the number of apo A-I tryptophan residues accessible to the aqueous phase, but decreased their mean degree of hydration. These observations cannot be totally explained on the basis of the effect of cholesterol on phospholipid viscosity as determined by fluorescence anisotropy of diphenyl hexatriene. We did not observe any effect of apo A-II on the conformation of apo A-I.     

Stimulation of lecithin:cholesterol acyltransferase activity by apolipoprotein A-II in the presence of apolipoprotein A-I

Various combinations of incorporation and addition of apolipoprotein A-I (apo A-I) and apolipoprotein A-II (apo A-II) individually or together to a defined lecithin-cholesterol (250/12.5 molar ratio) liposome prepared by the cholate dialysis procedure were used to study the effect of apo A-II on lecithin:cholesterol acyltransferase (LCAT, EC 2.3.1.43) activity of both purified enzyme preparations and plasma. When apo A-I (0.1-3.0 nmol/assay) alone was incorporated or added to the liposome, apo A-I effectively activated the enzyme. By contrast, when apo A-II (0.1-3.0 nmol/assay) alone was incorporated into or added to the liposome, apo A-II exhibited minimal activation of LCAT activity, approximately 1% of the activity obtained by an equal amount of apo A-I. Addition of apo A-II (0.1-3.0 nmol/assay) together with apo A-I (0.8 nmol/assay) to the liposome reduced the LCAT activity to approximately 30% of the level obtained with addition of apo A-I alone. On the other hand, addition of apo A-II (0.1-3.0 nmol/assay) or addition of lecithin-cholesterol liposome containing apo A-II (0.1-3.0 nmol/assay) to lecithin-cholesterol liposome containing apo A-I (0.8 nmol/assay) did not significantly alter apo A-I activation of LCAT activity. However, when the same amounts (0.1-3.0 nmol/assay) of apo A-II were incorporated together with apo A-I (0.8 nmol/assay) into the liposome, apo A-II significantly stimulated LCAT activity as compared to activity obtained with incorporation of apo A-I alone. The maximal stimulation was obtained with 0.4 nmol apo A-II/assay for both purified and plasma enzyme. At this apo A-II concentration, approximately 4-fold and 1.8-fold stimulation was observed for purified enzyme and plasma enzyme, respectively. These results indicated that apo A-II must be incorporated together with apo A-I into lecithin-cholesterol liposomes to exert its stimulatory effect on LCAT activity and that apo A-II in high-density lipoprotein may play an important role in the regulation of LCAT activity.     

Mechanism of dissociation of human apolipoprotein A-I from complexes with dimyristoylphosphatidylcholine as studied by guanidine hydrochloride denaturation

The reversibility of the binding of human apolipoprotein A-I (apo A-I) to phospholipid has been monitored through the influence of guanidine hydrochloride (Gdn-HCl) on the isothermal denaturation and renaturation of apo A-1/dimyristoylphosphatidylcholine (DMPC) complexes at 24 degree C. Denaturation was studied by incubating discoidal 1:100 and vesicular 1:500 mol/mol apo A-I/DMPC complexes with up to 7 M Gdn-HCl for up to 72 h. Unfolding of apo A-I molecules was observed from circular dichroism spectra while the distribution of protein between free and lipid-associated states was monitored by density gradient ultracentrifugation. The ability of apo A-I to combine with DMPC in the presence of Gdn-HCl at 24 degrees C was also investigated by similar procedures. In both the denaturation and renaturation of 1:100 and 1:500 complexes, the final values of the molar ellipticity and the ratio of free to bound apo A-I at various concentrations of Gdn-HCl are dependent on the initial state of the lipid and protein; apo A-I is more resistant to denaturation when Gdn-HCl is added to existing complexes than to a mixture of apo A-I and DMPC. There is an intermediate state in the denaturation pathway of apo A-I/DMPC complexes which is not present in the renaturation; the intermediate comprises partially unfold apo A-I molecules still associated with the complex by some of their apolar residues. Complete unfolding of the alpha helix and subsequent desorption of the apo A-I molecules from the lipid/water interface involve cooperative exposure of these apolar residues to the aqueous phase. The energy barrier associated with this desorption step makes the binding of apo A-I to DMPC a thermodynamically irreversible process. Consequently, binding constants of apo A-I and PC cannot be calculated simply from equilibrium thermodynamic treatments of the partitioning of protein between free and bound states. Apo A-I molecules do not exchange freely between the lipid-free and lipid-bound states, and extra work is required to drive protein molecules off the surface. The required increased in surface pressure can be achieved by a net mass transfer of protein to the surface; in vivo, increases in the surface pressure of lipoproteins by lipolysis can cause protein desorption.     

Restriction fragment length polymorphism of the apolipoprotein A-I gene among inhabitants of Novosibirsk]

Restriction fragments' length polymorphism in the region of apolipoprotein A-I (apo A-I) gene was investigated in Novosibirsk (Siberia, USSR) population. Correlation between PstI apo A-I alleles (2,2 kb-P1; 3,3 kb-P2) and total cholesterol, triglycerides, high density polyproteins cholesterol, and apolipoprotein A-I level was analysed. A tendency to increase in cholesterol index of atherogenicity and to decrease in high density lipoproteins cholesterol as well as apolipoprotein A-I level was shown to occur for P1P2 genotype patients.     

Kinetic stability plays a dominant role in the denaturant-induced unfolding of Erythrina indica lectin

The urea-induced denaturation of dimeric Erythrina indica lectin (EIL) has been studied at pH 7.2 under equilibrium and kinetic conditions in the temperature range of 40-55 degrees C. The structure of EIL is largely unaffected in this temperature range in absence of denaturant, and also in 8 M urea after incubation for 24 h at ambient temperature. The equilibrium denaturation of EIL exhibits a monophasic unfolding transition from the native dimer to the unfolded monomer as monitored by fluorescence, far-UV CD, and size-exclusion FPLC. The thermodynamic parameters determined for the two-state unfolding equilibrium show that the free energy of unfolding (DeltaGu, aq) remains practically same between 40 and 55 degrees C, with a value of 11.8 +/- 0.6 kcal mol(-1) (monomer units). The unfolding kinetics of EIL describes a single exponential decay pattern, and the apparent rate constants determined at different temperatures indicate that the rate of the unfolding reaction increases several fold with increase in temperature. The presence of probe like external metal ions (Mn2+, Ca2+) does not influence the unfolding reaction thermodynamically or kinetically; however, the presence of EDTA affects only kinetics. The present results suggest that the ability of EIL to preserve the structural integrity against the highly denaturing conditions is linked primarily to its kinetic stability, and the synergic action of heat and denaturant is involved in the unfolding of the protein.     

Human apolipoprotein A-I forms small high density lipoprotein particles in rats in vivo.

The effects of injection of purified human or rat apolipoprotein (apo) A-I (1.7 mg/100 g body weight) on the size and composition of rat high density lipoprotein (HDL) particles have been investigated. The injection of human apo A-I results in the formation (over a period of 3 to 6 h) of a population of smaller HDL particles resembling human HDL3. This population of smaller particles contains human apo A-I and rat apo A-IV but lacks rat apo A-I and rat apo E. Small HDL3-like particles are not detected in rat plasma following the injection of rat apo A-I. Associated with the injection of either human or rat apo A-I is a gradual increase of plasma cholesterol levels of 20 to 50% (over 24 h) and the appearance of larger HDL particles. The results suggest that the smaller HDL particles in human plasma compared to rat plasma are not simply due to the action of lipid modifying enzymes or lipid transfer proteins but a specific property of human apo A-I.     

Human apolipoprotein A-I forms thermally stable complexes with anionic but not with zwitterionic phospholipids

The mechanism of the association of human plasma apolipoprotein A-I (apo A-I) with the acidic phospholipids, dimyristoylphosphatidylglycerol (DMPG), egg yolk phosphatidylglycerol, and dioleoylphosphatidylserine as well as with the zwitterionic dimyristoylphosphatidylcholine (DMPC) has been studied using turbidimetry, circular dichroism, high-sensitivity differential scanning calorimetry, and electron microscopy. The association of apo A-I with multilamellar liposomes of acidic phospholipids is rapid over a broad temperature range at and above the temperature of the lipid gel to liquid crystalline transition, Tc. This is in contrast to zwitterionic phosphatidylcholine which recombines with apo A-I only over a narrow temperature range around Tc. The complex of apo A-I with DMPC denatures at elevated temperatures giving rise to a calorimetrically detectable transition. The temperature range and width of this transition is shown to be markedly dependent on the heating rate. This is again in contrast to apo A-I recombinants with DMPG which show no calorimetrically detectable thermal denaturation, at least in a temperature range up to 100 degrees C. Also circular dichroism data indicate high resistance of apo A-I to thermal unfolding in the presence of DMPG. It is concluded that the complexes of apo A-I with DMPC are thermodynamically stable only at temperatures near Tc, whereas above and below this temperature range the stability of these recombinants is determined by kinetic factors. In contrast, complexes of apo A-I with DMPG and other acidic phospholipids may be thermodynamically stable over a wide temperature range greater than or equal to Tc. In spite of these fundamental differences between zwitterionic and acidic phospholipids in their mode of association with apo A-I, the binding affinity and the morphology of the recombinants are similar. Both apo A-I X DMPC and apo A-I X DMPG complexes form lipoprotein particles having a discoidal shape.