Rapid chemical de-N-glycosylation and derivatization for liquid chromatography of immunoglobulin N-linked glycans

Glycan analysis may result in exploitation of glycan biomarkers and evaluation of heterogeneity of glycosylation of biopharmaceuticals. For N-linked glycan analysis, we investigated alkaline hydrolysis of the asparagine glycosyl carboxamide of glycoproteins as a deglycosylation reaction. By adding hydroxylamine into alkaline de-N-glycosylation, we suppressed the degradation of released glycans and obtained a mixture of oximes, free glycans, and glycosylamines. The reaction was completed within 1 h, and the mixture containing oximes was easily tagged with 2-aminobenzamide by reductive amination. Here, we demonstrated N-linked glycan analysis using this method for a monoclonal antibody, and examined whether this method could liberate glycans without degradation from apo-transferrin containing NeuAc and NeuGc and horseradish peroxidase containing Fuc α1-3 GlcNAc at the reducing end. Furthermore, we compared glycan recoveries between conventional enzymatic glycan release and this method. Increasing the reaction temperature and reaction duration led to degradation, whereas decreasing these parameters resulted in lower release. Considering this balance, we proposed to carry out the reaction at 80°C for 1 h for asialo glycoproteins from mammals and at 50°C for 1 h for sialoglycoproteins.

Common conformational changes in flavodoxins induced by FMN and anion binding: the structure of Helicobacter pylori apoflavodoxin

Flavodoxins, noncovalent complexes between apoflavodoxins and flavin mononucleotide (FMN), are useful models to investigate the mechanism of protein/flavin recognition. In this respect, the only available crystal structure of an apoflavodoxin (that from Anabaena) showed a closed isoalloxazine pocket and the presence of a bound phosphate ion, which posed many questions on the recognition mechanism and on the potential physiological role exerted by phosphate ions. To address these issues we report here the X-ray structure of the apoflavodoxin from the pathogen Helicobacter pylori. The protein naturally lacks one of the conserved aromatic residues that close the isoalloxazine pocket in Anabaena, and the structure has been determined in a medium lacking phosphate. In spite of these significant differences, the isoallozaxine pocket in H. pylori apoflavodoxin appears also closed and a chloride ion is bound at a native-like FMN phosphate site. It seems thus that it is a general characteristic of apoflavodoxins to display closed, non-native, isoalloxazine binding sites together with native-like, rather promiscuous, phosphate binding sites that can bear other available small anions present in solution. In this respect, both binding energy hot spots of the apoflavodoxin/FMN complex are initially unavailable to FMN binding and the specific spot for FMN recognition may depend on the dynamics of the two candidate regions. Molecular dynamics simulations show that the isoalloxazine binding loops are intrinsically flexible at physiological temperatures, thus facilitating the intercalation of the cofactor, and that their mobility is modulated by the anion bound at the phosphate site.

Energetics of aliphatic deletions in protein cores

Although core residues can sometimes be replaced by shorter ones without introducing significant changes in protein structure, the energetic consequences are typically large and destabilizing. Many efforts have been devoted to understand and predict changes in stability from analysis of the environment of mutated residues, but the relationships proposed for individual proteins have often failed to describe additional data. We report here 17 apoflavodoxin large-to-small mutations that cause overall protein destabilizations of 0.6-3.9 kcal.mol(-1). By comparing two-state urea and three-state thermal unfolding data, the overall destabilizations observed are partitioned into effects on the N-to-I and on the I-to-U equilibria. In all cases, the equilibrium intermediate exerts a "buffering" effect that reduces the impact of the overall destabilization on the N-to-I equilibrium. The performance of several structure-energetics relationships, proposed to explain the energetics of hydrophobic shortening mutations, has been evaluated by using an apoflavodoxin data set consisting of 14 mutations involving branching-conservative aliphatic side-chain shortenings and a larger data set, including similar mutations implemented in seven model proteins. Our analysis shows that the stability changes observed for any of the different types of mutations (LA, IA, IV, and VA) in either data set are best explained by a combination of differential hydrophobicity and of the calculated volume of the modeled cavity (as previously observed for LA and IA mutations in lysozyme T4). In contrast, sequence conservation within the flavodoxin family, which is a good predictor for charge-reversal stabilizing mutations, does not perform so well for aliphatic shortening ones.

Fluorescence- and luminescence-based methods for the determination of affinity and activity of neuropeptide Y2 receptor ligands

With respect to the discovery and characterization of neuropeptide Y(2) receptor ligands as pharmacological tools or potential drugs, fluorescence- and luminescence-based assays were developed to determine both the affinity and the activity of receptor agonists and antagonists. A flow cytometric binding assay is described for the hY(2) receptor stably expressed in CHO cells using cy5-labeled porcine neuropeptide Y and compared with a radioligand binding assay. Binding of the fluorescent ligand was visualized by confocal microscopy. Stable co-transfection with the chimeric G protein Gq(i5) enabled the establishment of a spectrofluorimetric fura-2 and a flow cytometric fluo-4 calcium assay. Further stable expression of apoaequorin targeted to the mitochondria allowed the establishment of an aequorin assay which could be performed in the 96-well format. The shape of the concentration-response curves of porcine neuropeptide Y in the presence of the Y(2)-selective receptor antagonist BIIE0246, characteristic of either competitive or insurmountable antagonism, depended on the period of incubation with the cells. Functional data of Y(2) receptor agonists and antagonists determined in the fluorescence- and luminescence-based assays were in good agreement.

Structural changes of α-lactalbumin induced by low pH and oleic acid

The effects of low pH and oleic acid on conformation and association state of Ca2+-depleted bovine alpha-lactalbumin (apo-BLA) have been studied by electrospray ionization mass spectrometry, fluorescence spectroscopy, and circular dichroism. The experimental results demonstrate that two structurally distinct species exist in the conformational transition of apo-BLA induced by low pH. One species populates at pH 3.0 characterized as a monomeric molten globule state and the other accumulates at pH 4.0-4.5 which is a partially folded dimer. Oleic acid promotes the formation of the dimeric intermediate at pH 4.0 and 7.0, but increases the content of molten globule state remarkably at pH 3.0 compared with that in the absence of oleic acid, indicating that oleic acid at pH 3.0 plays a different role from those at pH 4.0 and 7.0. Our data provide insight into the mechanism of pH-dependent and oleic acid-dependent structural changes and oligomerization of alpha-lactalbumin, and will be helpful to the understanding of the apoptosis-inducing function of multimeric alpha-lactalbumin in which oleic acid is a necessary cofactor.

Non-covalent modification of the heme-pocket of apomyoglobin by a 1,10-phenanthroline derivative

To expand the repertoire of artificial enzymes that are constructed by replacing the natural prosthetic group of hemoproteins with non-natural cofactors, we examined incorporation of a non-porphyrinic ligand (1) into the heme-pocket of apomyoglobin in a non-covalent fashion. Ligand 1 is a highly conjugated 1,10-phenanthroline derivative, which shares some structural features with protoporphyrin IX; for example, molecular size and arrangement of hydrophobic and anionic parts. Addition of apomyoglobin to a solution of 1 induces clear changes in the absorption spectrum of 1, suggesting one-to-one incorporation of 1 into the heme cavity of apomyoglobin with an affinity of 6.3 x 10(6)M(-1). We found that the hydrolytic activity of apomyoglobin toward p-nitrophenyl hexanoate was greatly suppressed because of the incorporation of 1 into the heme-pocket.

Lipid profile in women over 35 years old using triphasic combined oral contraceptives

Twenty-nine women aged 35 years old or more, using triphasic combined oral contraceptive (COC) were evaluated during six cycles for the following parameters: total cholesterol, low-density lipoprotein cholesterol (LDL-C), high-density lipoprotein cholesterol (HDL-C) and its subfraction HDL2, triglycerides, apoproteins A and B, Castelli risk index I and II (cholesterol/HDL-C and LDL-C/HDL-C) and apoprotein ratio (apoprotein B/apoprotein A). The same laboratory measurements were done in a control group of 49 non-COC-user women. The results showed that there were no differences on most of the studied parameters between user and nonuser women. There was a significant reduction of HDL-C and HDL2-C, although within the normal range. In addition, it was observed a significant increment of triglycerides and apoprotein B at 6 months of follow-up only in user group (p < 0.05), although within the normal range. It is concluded that the use of levonorgestrel triphasic COC appeared to have no additional adverse impact when used by women aged over 35 years. Further studies are needed to obtain conclusive data.

Characterization of non-covalent oligomers of proteins treated with hypochlorous acid

Hypochlorous acid (HOCl) is a potent oxidant produced by myeloperoxidase that causes aggregation of many proteins. Treatment of apohaemoglobin and apomyoglobin with HOCl produced a regular series of oligomer bands when the proteins were separated by SDS/PAGE under reducing conditions. Aggregation was detectable at a HOCl/protein molar ratio of 0.5:1 and was maximal at ratios of 10:1-20:1. Dimers formed within 1 min of adding HOCl, and further aggregation occurred over the next 30 min. No convincing evidence for covalent cross-linking was obtained by amino acid analysis, peptide analysis or electrospray ionization-MS of HOCl-modified apomyoglobin. The latter showed an increase in mass consistent with conversion of the two methionine residues into sulphoxides. A 5-fold excess of HOCl generated approximately three chloramines on the apomyoglobin. These underwent slow decay. Protein carbonyls were formed and were almost entirely located only on the polymer bands. Conversion of positively into negatively charged groups on the protein by succinylation caused preformed aggregates to dissociate. Treatment of apomyoglobin with taurine chloramine generated methionine sulphoxides but few protein carbonyls, and did not result in aggregation. We conclude that aggregation was due to strong, non-covalent interactions between protein chains. We propose that formation of protein carbonyls and possibly chloramines, along with methionine oxidation, alters protein folding to expose hydrophobic areas on neighbouring molecules that associate to form dimers and higher-molecular-mass aggregates. This process could lead to the formation of aggregated proteins at sites of myeloperoxidase activity and contribute to inflammatory tissue injury.

The tobacco apocytochrome b gene predicts sensitivity to the respiratory inhibitors antimycin A and myxothiazol

The potential for use of the cytochrome-pathway electron-transfer inhibitors antimycin A and myxothiazol in the selection of plant mitochondrial genome transformants was investigated. The net growth of Nicotiana tabacum L. (tobacco) suspension-culture cells was reduced by these inhibitors, but complete repression of cell growth occurred only in the presence of both cytochrome and alternative electron-transfer-pathway inhibitors. Antimycin A and myxothiazol bind to and block electron transfer through different sites in the cytochrome b (COB) subunit of the mitochondrial bc1 respiratory complex (complex III). The nucleotide sequence of the tobacco cob gene was determined and found to predict highly conserved glycine and phenylalanine residues that are associated with sensitivity to antimycin A and myxothiazol, respectively. These residues are altered by mutations that confer resistance to antimycin A or myxothiazol in diverse organisms. Tobacco cob cDNA clones were constructed and sequenced, revealing eight full and 11 partial RNA-editing sites. RNA editing did not, however, alter codons for the conserved glycine and phenylalanine residues associated with sensitivity to the respiratory inhibitors. Antimycin A or myxothiazol, in conjunction with a modified cob gene, may therefore be useful in the selection of tobacco cells carrying a genetically transformed mitochondrial genome.

Effects of dexamethasone and linoleic acid on hepatic secretion of biliary lipids and anionic polypeptide factor: In vivo and in vitro studies

Synthetic glucocorticoids, such as dexamethasone, and diets enriched with unsaturated fatty acids have been shown to stimulate hepatic bile salt synthesis. This fact led us to investigate the effects of dexamethasone and linoleic acid supplementation on bile secretion. Cholesterol (Ch) and phospholipid secretions are bile acid dependent. Ch and phospholipid in bile are also highly bound to a small apoprotein, the anionic polypeptide factor (APF). In bile, APF may play a physiological role in stabilizing cholesterol:phospholipid vesicles and might also be important in the regulatory process of bile lipid secretion. In order to study the factors influencing bile secretion, the biliary secretion rates of bile lipids and APF were experimentally modulated in perfused rat liver (PRL) and HepG2 cells. As expected, dexamethasone induced an increase in the biliary secretion rate of bile salts (BS) in the two models (PRL: 34 up to 67 nmol/l/min/g liver; HepG2 cells: 234% vs. 100% in controls). The bile secretion rates for phospholipids (PRL: from 5 down to 1.5 nmol/l/min/g liver; HepG2 cells: 93 vs. 100% in controls) and APF (PRL: from 0.34 down to 0.12 microg/l/min/g liver; cells: 86 vs. 100% in controls) rapidly decreased independently from those of BS. The data from experimental cell models supplemented with linoleic acid indicated a correlation between the BS and APF levels (APF: 71 and 63%; BS: 161 and 197% vs. 100% in controls). The phospholipid level was regulated independently from that of APF and BS and increased (106 and 111% vs. 100% in controls), while Ch remained nevertheless unchanged. Our data showed that dexamethasone induced changes in bile and that linoleic acid clearly impaired the regulation exerted by the dexamethasone on bile lipids.

Magnetic resonance imaging of PLP-induced experimental allergic encephalomyelitis in Lewis rats

An in vivo magnetic resonance (MR) imaging study was performed on experimental allergic encephalomyelitis (EAE) induced in Lewis rats through proteolipid protein (PLP). PLP was solubilized in water or in an aqueous solution of 1% 10-tridecyl ether (TDE), a non-ionic detergent used in membrane protein research. All 16 rats immunized with 500 microg of TDE-solubilized PLP developed clinical signs and MR abnormalities fully comparable to those observed in MBP-induced EAE. Total paraplegia was observed in 12.5% of rats, mild or moderate paraparesis in 68.8% of rats and tail paralysis in the remaining 18.7% of rats. Whereas only 37.5% of the eight rats immunized with 500 microg of water-solubilized PLP developed minor clinical signs (tail weakness or paralysis). Our observations confirm that the difficulties encountered when trying to induce EAE by means of PLP arise from the highly hydrophobic nature of this protein. Accordingly, if a reproducible model is to be developed, it seems more judicious to use non-ionic detergents in both the extraction and solubilization phases of PLP preparation, this would allow maximal solubilization of the protein while avoiding aggregates, which may otherwise form during either of the PLP preparation.

Structure of human apolactoferrin at 2.0 A resolution. Refinement and analysis of ligand-induced conformational change

The three-dimensional structure of a form of human apolactoferrin, in which one lobe (the N-lobe) has an open conformation and the other lobe (the C-lobe) is closed, has been refined at 2.0 A resolution. The refinement, by restrained least-squares methods, used synchrotron radiation X-ray diffraction data combined with a lower resolution diffractometer data set. The final refined model (5346 protein atoms from residues 1-691, two Cl- ions and 363 water molecules) gives a crystallographic R factor of 0.201 (Rfree = 0. 286) for all 51305 reflections in the resolution range 10.0-2.0 A. The conformational change in the N-lobe, which opens up the binding cleft, involves a 54 degrees rotation of the N2 domain relative to the N1 domain. This also results in a small reorientation of the two lobes relative to one another with a further approximately 730 A2 of surface area being buried as the N2 domain contacts the C-lobe and the inter-lobe helix. These new contacts also involve the C-terminal helix and provide a mechanism through which the conformational and iron-binding status of the N-lobe can be signalled to the C-lobe. Surface-area calculations indicate a fine balance between open and closed forms of lactoferrin, which both have essentially the same solvent-accessible surface. Chloride ions are bound in the anion-binding sites of both lobes, emphasizing the functional significance of these sites. The closed configuration of the C-lobe, attributed in part to weak stabilization by crystal packing interactions, has important implications for lactoferrin dynamics. It shows that a stable closed structure, essentially identical to that of the iron-bound form, can be formed in the absence of iron binding.

Trifluoroethanol stabilizes the pH 4 folding intermediate of sperm whale apomyoglobin

2,2,2-Trifluoroethanol (TFE) is known to stabilize peptide helices by strengthening hydrogen bonds. On the other hand, TFE destabilizes native proteins, as we confirm here, presumably by weakening the hydrophobic interaction. The stability of the pH 4 folding intermediate of apomyoglobin is known to depend both on the strength of the individual A, G, and H helices and on hydrophobic interactions between helices. We ask which effect of TFE dominates in this case: strengthening helices or weakening hydrophobic interactions between helices? Protein stability is measured by denaturant-induced unfolding curves, and two-state unfolding is tested by monitoring both far-UV CD and tryptophan fluorescence emission. Low concentrations of TFE strongly stabilize the pH 4 folding intermediate. Moreover, low concentrations of TFE compensate for helix-destabilizing mutations in the A and G helices. Consequently, enhancing helix propensity, rather than weakening the hydrophobic interaction, is the dominant effect of TFE on the folding intermediate. This result agrees with earlier mutational evidence that helix propensities are very important in determining the stability of the pH 4 intermediate. Although TFE destabilizes native holomyoglobin, as well as native lysozyme and ribonuclease A, nevertheless, TFE stabilizes native apomyoglobin.

Keratinocyte growth factor modulates alveolar epithelial cell phenotype in vitro: expression of aquaporin 5

We investigated the role of keratinocyte growth factor (KGF) in regulation of alveolar epithelial cell (AEC) phenotype in vitro. Effects of KGF on cell morphology, expression of surfactant apoproteins A, B, and C (SP-A, -B, and -C), and expression of aquaporin 5 (AQP5), a water channel present in situ on the apical surface of alveolar type I (AT1) cells but not expressed in alveolar type II (AT2) cells, were evaluated in AECs grown in primary culture. Observations were made on AEC monolayers grown in serum-free medium without KGF (control) or grown continuously in the presence of KGF (10 ng/ml) from either Day 0 (i.e., the time of plating) or Day 4 or 6 through Day 8 in culture. AECs monolayers express AQP5 only on their apical surfaces as determined by cell surface biotinylation studies. Control AECs grown in the absence of KGF through Day 8 express increasing levels of AQP5, consistent with transition toward the AT1 cell phenotype. Exposure of AECs to KGF from Day 0 results in decreased AQP5 expression, retention of a cuboidal morphology, and greater numbers of lamellar bodies relative to control on Day 8 in culture. AECs treated with KGF from Day 4 or 6 exhibit a decrease in AQP5 expression through subsequent days in culture, as well as an increase in expression of surfactant apoproteins. These data, showing that KGF both prevents and reverses the increase in AQP5 (and decrease in surfactant apoprotein) expression that accompanies progression of the AT2 toward the AT1 cell phenotype, support the concepts that transdifferentiation between AT2 and AT1 cell phenotypes is at least partially reversible and that KGF may play a major role in modulating AEC phenotype.

Reduced size of retinal ganglion cell axons and hypomyelination in mice lacking brain-derived neurotrophic factor

While brain-derived neurotrophic factor (BDNF) delays the death of axotomized retinal ganglion cells in rodents, it is unclear if it affects any aspect of the normal development of these cells. Here we examined the optic nerve of bdnf-/- mice. Axonal numbers were normal, but their diameter, as well as the proportion of myelinated axons, was reduced at postnatal day 20 (P20). In contrast, the facial nerve was not hypomyelinated. Expression levels of mRNAs coding for the myelin proteins PLP and MBP were substantially reduced in the hippocampus and cortex at P20, but not in the sciatic nerve. Intraventricular injections of BDNF into the ventricles of wild-type mice at P10 and P12 up-regulated expression of PLP in the hippocampus at P14. These results indicate a role of BDNF, discussed as indirect, in the control of myelination in the central nervous system.

Oligomerization state of S100B at nanomolar concentration determined by large-zone analytical gel filtration chromatography

S100B is a Ca(2+)-binding protein known to be a non-covalently associated dimer, S100B(beta beta), at high concentrations (0.2-3.0 mM) under reducing conditions. The solution structure of apo-S100B (beta beta) shows that the subunits associate in an antiparallel manner to form a tightly packed hydrophobic core at the dimer interface involving six of eight helices and the C-terminal loop (Drohat AC, Amburgey JC, Abildgaard F, Starich MR, Baldisseri D, Weber DJ. 1996. Solution structure of rat apo-S100B (beta beta) as determined by NMR spectroscopy. Biochemistry 35:11577-11588). The C-terminal loop, however, is also known to participate in the binding of S100B to target proteins, so its participation in the dimer interface raises questions as to the physiological relevance of dimeric S100B (beta beta). Therefore, we investigated the oligomerization state of S100B at low concentrations (1-10,000 nM) using large-zone analytical gel filtration chromatography with 35S-labeled S100B. We found that S100B exists (> 99%) as a non-covalently associated dimer, S100B (beta beta), at 1 nM subunit concentration (500 pM dimer) in the presence or absence of saturating levels of Ca2+, which implies a dissociation constant in the picomolar range or lower. These results demonstrate for the first time that in reducing environments and at physiological concentrations, S100B exists as dimeric S100B (beta beta) in the presence or absence of Ca2+, and that the non-covalent dimer is most likely the form of S100B presented to target proteins.

The effect of simvastatin on HDL cholesterol in hyperlipidemic patients. Evidence of a relationship with the changes in serum triglyceride level

The main effect of simvastatin is the decrease of serum cholesterol due to the reduction of LDL. A decrease of serum triglycerides and an increase of HDL-C are commonly observed during the treatment. The reduction of triglycerides is accounted for by the increased catabolism of apo B-containing lipoproteins whereas the mechanisms bringing about the increase of HDL-C are still unknown. We treated 318 patients with primary hyperlipidemia (227 with phenotype IIa and 91 with phenotype IIb) with simvastatin 10 mg a day and after 6 weeks we found a mean 3% increase in HDL-C. HDL-C increased only in about half of the patients and the patients in whom HDL-C increased had baseline higher serum triglycerides and had a greater hypotriglyceridemic response than patients in whom HDL-C did not increase. Accordingly, HDL-C increased in type IIb patients who experienced a greater change in triglycerides than type IIa patients, in whom HDL-C did not increase significantly. Apo A-I levels did not change and apo A-I/HDL-C ratio significantly decreased. At a daily dose of 40 mg, administered to 51 treatment-resistant patients, simvastatin produced a marginally greater decrease in serum cholesterol and LDL-C, but not in serum triglycerides and HDL-C, than at the daily dose of 10 mg. An increase in HDL-C was associated with a reduction in serum triglycerides. The decrease in apo A-I/HDL-C ratio suggests that the increase in HDL-C after simvastatin must be regarded as an enrichment of the cholesterol core of HDL particles. The effect is likely to be due to the decrease of the serum concentration of VLDL bringing about a reduction of cholesterol transfer from apo A-I to apo B-containing lipoproteins.

Packing interactions in the apomyglobin folding intermediate

The contribution of specific packing to the stability of the sperm whale apomyoglobin intermediate has been studied by urea denaturation monitored by circular dichroism and fluorescence. Mutations disrupting native packing sites within the subdomain formed by the A, G and H helices destabilize the intermediate, in contrast to the conclusion drawn from earlier studies of pH-induced unfolding. Based on these results, the intermediate is proposed to be stabilized by both partially formed native-like tertiary, and non-specific hydrophobic interactions forming a subdomain folding intermediate. The results help to explain how the intermediate acquires its structure and stability.

Pressure-induced perturbation of apomyoglobin structure: fluorescence studies on native and acidic compact forms

The nature of the structural changes that apomyoglobin undergoes when subjected to hydrostatic pressure, ranging from atmospheric pressure to 2.4 kbar, has been investigated by steady-state fluorescence and frequency domain fluorometry. In particular, we have examined the intrinsic tryptophanyl emission and that of the extrinsic probe 1-anilino-8-naphthalenesulfonate (ANS) bound to apomyoglobin at neutral pH, as well as at strongly acidic high-salt conditions. Apomyoglobin at neutral pH undergoes a pressure-induced structural transition, which causes the disorganization of the heme binding region with a consequent ANS dissociation; a concomitant increase in solvent accessibility to the N-terminus of the macromolecule in which tryptophans are located is also observed. At 2.4 kbar, the tryptophanyl emission is not coincident with that of a fully solvent exposed residue, thus suggesting that the N-terminal region of the apomyoglobin molecule retains elements of organized structure. The spectroscopic properties of the structural state attained at 2.4 kbar and neutral pH are different from those of the acidic compact state. The acidic compact state of apomyoglobin undergoes a pressure-induced structural change that brings the tryptophanyl residues in contact with the solvent, but does not affect the ability to bind ANS.

Preferential self-association of basic fibroblast growth factor is stabilized by heparin during receptor dimerization and activation

Central to signaling by fibroblast growth factors (FGFs) is the oligomeric interaction of the growth factor and its high-affinity cell surface receptor, which is mediated by heparin-like polysaccharides. It has been proposed that the binding of heparin-like polysaccharides to FGF induces a conformational change in FGF, resulting in the formation of FGF dimers or oligomers, and this biologically active form is 'presented' to the FGF receptor for signal transduction. In this study, we show that monomeric basic FGF (FGF-2) preferentially self-associates and forms FGF-2 dimers and higher-order oligomers. As a consequence, FGF-2 monomers are oriented for binding to heparin-like polysaccharides. We also show that heparin-like polysaccharides can readily bind to self-associated FGF-2 without causing a conformational change in FGF-2 or disrupting the FGF-2 self-association, but that the bound polysaccharides only additionally stabilize the FGF-2 self-association. The preferential self-association corresponds to FGF-2 translations along two of the unit cell axes of the FGF-2 crystal structures. These two axes represent the two possible heparin binding directions, whereas the receptor binding sites are oriented along the third axis. Thus, we propose that preferential FGF-2 self-association, further stabilized by heparin, like "beads on a string," mediates FGF-2-induced receptor dimerization and activation. The observed FGF-2 self-association, modulated by heparin, not only provides a mechanism of growth factor activation but also represents a regulatory mechanism governing FGF-2 biological activity.

Transition metal ion activation of DNA binding by the diphtheria tox repressor requires the formation of stable homodimers

The diphtheria tox repressor (DtxR) is a transition metal ion-dependent regulatory element that controls the expression of diphtheria toxin and several genes involved in the synthesis of siderophores in Corynebacterium diphtheriae. In the presence of transition metal ions apo-DtxR becomes activated and specifically binds to its target DNA sequences. We demonstrate by glutaraldehyde cross-linking that monomeric apo-DtxR is in weak equilibrium with a dimeric form and that upon addition of activating metal ions to the reaction mixture a dimeric complex is stabilized. Addition of the DNA-binding-defective mutant apo-DtxR(delta 1-47) to apo-DtxR in the absence of transition metal ions inhibits conversion of the apo-repressor to its activated DNA-binding form. We also show that the binding of Ni2+ to both apo-DtxR and apo-DtxR(delta 1-47) is cooperative and that upon ion binding there is a conformational change in the environment of the indole ring moiety of Trp-104. For the wild-type repressor the consequences of this conformational change include a shift in equilibrium toward dimer formation and activation of target DNA binding by the repressor. We conclude that the formation of DtxR homodimers is mediated through a protein-protein interaction domain that is also activated on metal ion binding.

Periodate modification of human serum transferrin Fe(III)-binding sites. Inhibition of carbonate insertion into Fe(III)- and Cu(II)-chelator-transferrin ternary complexes

Periodate modification of human serum transferrin produces a species that binds Fe(III) weakly at pH 7.4 contrary to previous reports that Fe(III)-binding activity is completely lost. Ternary complexes of periodate-modified transferrin and either Fe(III) with nitrilotriacetate (NTA), oxalate, citrate, or EDTA, or of Cu(II) with oxalate could be formed. Peak wavelength maxima of these spectral bands are identical to those reported for native transferrin in the absence of bicarbonate. No carbonate ternary complexes of periodate-modified transferrin with Fe(III), Al(III), Cu(II), or Zn(II) could be formed. Conditional (Fe(NTA)) binding constants (log K) for C- and N-terminal modified sites are 7.33 and 7.54, respectively. The respective extinction coefficients at 470 nm are decreased 45% compared with the native protein. The electron paramagnetic resonance spectrum of the complex closely resembles that of the Fe(III)-NTA ternary complex formed with native transferrin in the absence of bicarbonate. Anions, including bicarbonate, at high concentrations destabilize formation of this Fe(III)-NTA ternary complex, while Fe(III) chelators readily remove the bound Fe(III). Bicarbonate, sulfate, and pyrophosphate still bind to the modified binding sites in the absence of metal although with slightly lower affinity and with lower molar difference absorptivities. Results are interpreted as an inhibition of a crucial protein conformational change by an intramolecular cross-link, preventing formation of the particularly stable metal-carbonate ternary complex from the less stable metal-chelate ternary complex. The method can be used to produce monosited transferrins.

Trifluoroethanol-induced stabilization of the alpha-helical structure of beta-lactoglobulin: implication for non-hierarchical protein folding

Trifluoroethanol (TFE) is known to stabilize the alpha-helical structure in proteins and their fragments. However, the relationship between the TFE-induced structures and the native structure is not clear. Here we show that beta-lactoglobulin, which consists predominantly of beta-sheets, exhibited a markedly high propensity to form an alpha-helical structure in the presence of TFE, as measured by far-UV circular dichroism. A cooperative transformation from the beta-sheet structure to an alpha-helical structure occurred at a TFE concentration between 10% and 20%. These results were in contrast to a gradual beta-sheet to alpha-helix transition of the constant fragment of the immunoglobulin light chain, which is also a beta-sheet protein. To understand the significance of the high helical propensity of beta-lactoglobulin, we measured the TFE-induced conformational transition of more than 20 proteins of various secondary structural types. Whereas the alpha-helical proteins showed a propensity to form an extensive helical structure in TFE, the helical propensity of proteins with a low helical content in the native state varied. The helical content in TFE was correlated more with the helical content predicted by a secondary structure prediction than with the helical content of the native structure, suggesting that the stability of the helical structure in TFE is determined by local interactions between nearby amino acid residues. Our results suggest that an alpha-helical intermediate can accumulate during the refolding process of beta-lactoglobulin and that a hierarchical model of protein folding is not necessarily true for some beta-sheet proteins including beta-lactoglobulin.

[Effects of pravastatin on serum lipids, apoproteins, and lipoprotein (a) in primary hypercholesterolemia]

The efficacy of pravastatin as reducing plasma cholesterol, LDL-CH and Apo B is widely proved. Other molecules within the Apolipoprotein family are recently emerging to have a predictive and/or causative role in atherosclerosis such as particularly Lp(a). The aim of this study was to evaluate the effects of pravastatin therapy in patients affected by primary hypoercholesterolemia on apoprotein and Lp(a) plasma levels. We investigated the effects of pravastatin on 15 patients, seven female and eight male patients, mean age 50.23 +/- 17.2 (range 21-71 years) with primary hypercholesterolemia, of which 7 patients affected by familial hypercholesterolemia and 8 patients by polygenic hypercholesterolemia, were selected. Five weeks after suspension of lipid-lowering drugs and on a normocaloric-fat diet, were given 20 mg pravastatin/day for 12 weeks. The following parameters were measured basally, on the 6th week and the 12th week on pravastatin therapy and after five weeks from drug withdrawal: cholesterol (CH), triglicerides (TG), high density and low density lipoprotein cholesterol (HDL-CH and LDH-CH) measured enzymatically, apoproteins A1, B, C2, C3, E measured radial immunodiffusion technique (RID) and Lp(a), measured as apoprotein(a) with immunoradiometric assay (RIA). Our data confirm pravastatin efficacy in decreasing CH (from 305.6 +/- 43.4 mg/dl to 266.2 +/- 47.7 mg/dl, p < 0.01) LDL-CH (from 223.9 +/- 56.4 mg/dl to 187.2 +/- 59.8 mg/dl, p < 0.01) and Apo B (from 170.4 +/- 27.5 to 152.4 +/- 25.2, p < 0.02); non influence was observed on HDL-CH and apoproteins A1, C2, E and Lp(a). Pravastatin determined a significant increase only on Apo C3 (from 8.35 +/- 2.7 to 10.3 +/- 3.1, p < 0.04). The above data confirm the beneficial effect of pravastatin in greatly decreasing CH and LDL-CH considered as major risk factors for coronary artery disease, but also point to a role of pravastatin in regulating the apoproteins equilibrium, an aspect that surely merits further studies.

Tryptophan replacements in the trp aporepressor from Escherichia coli: probing the equilibrium and kinetic folding models

Mutants of the dimeric Escherichia coli trp aporepressor are constructed by replacement of the two tryptophan residues in each subunit in order to assess the effects on equilibrium and kinetic fluorescence properties of the folding reaction. The three kinetic phases detected by intrinsic tryptophan fluorescence in refolding of the wild-type aporepressor are also observed in folding of both Trp 19 to Phe and Trp 99 to Phe single mutants, demonstrating that these phases correspond to global rather than local conformational changes. Comparison of equilibrium fluorescence (Royer, C.A., Mann, C.J., & Matthews, C.R., 1993, Protein Sci. 2, 1844-1852) and circular dichroism transition curves induced by urea shows that replacement of either Trp 19 or Trp 99 results in noncoincident behavior. Unlike the wild-type protein (Gittelman, M.S. & Matthews, C.R., 1990, Biochemistry 29, 7011-7020), tertiary and/or quaternary structures are disrupted at lower denaturant concentration than is secondary structure. The equilibrium results can be interpreted in terms of enhancement in the population of a monomeric folding intermediate in which the lone tryptophan residue is highly exposed to solvent, but in which substantial secondary structure is retained. The location of both mutations at the interface between the two subunits (Zhang, R.G., et al., 1987, Nature 327, 591-597) provides a simple explanation for this phenomenon.

Resolution of the fluorescence equilibrium unfolding profile of trp aporepressor using single tryptophan mutants

Single tryptophan mutants of the trp aporepressor, tryptophan 19-->phenylalanine (W19F) and tryptophan 99-->phenylalanine (W99F), were used in this study to resolve the individual steady-state and time-resolved fluorescence urea unfolding profiles of the two tryptophan residues in this highly intertwined, dimeric protein. The wild-type protein exhibits a large increase in fluorescence intensity and lifetime, as well as a large red shift in the steady-state fluorescence emission spectrum, upon unfolding by urea (Lane, A.N. & Jardetsky, O., 1987, Eur. J. Biochem. 164, 389-396; Gittelman, M.S. & Matthews, C.R., 1990, Biochemistry 29, 7011-7020; Fernando, T. & Royer, C.A., 1992, Biochemistry 31, 6683-6691). Unfolding of the W19F mutant demonstrated that Trp 99 undergoes a large increase in intensity and a red shift upon exposure to solvent. Lifetime studies revealed that the contribution of the dominant 0.5-ns component of this tryptophan tends toward zero with increasing urea, whereas the longer lifetime components increase in importance. This lifting of the quenching of Trp 99 may be due to disruption of the interaction between the two subunits upon denaturation, which abolishes the interaction of Trp 99 on one subunit with the amide quenching group of Asn 32 on the other subunit (Royer, C.A., 1992, Biophys. J. 63, 741-750). On the other hand, Trp 19 is quenched in response to unfolding in the W99F mutant. Exposure to solvent of Trp 19, which is buried at the hydrophobic dimer interface in the native protein, results in a large red shift of the average steady-state emission.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulating effects of bile salt hydrophobicity on bile secretion of the major protein of the bile lipoprotein complex

Bile lipids are secreted in association with a newly identified major apoprotein called anionic polypeptide fraction-calcium binding protein (APF-CBP), which is synthesized in the hepatocytes and has been detected in both bile and plasma and characterized. The secretion of the lipids in bile depends both on the concentration and the hydrophobicity of the bile salts (BS) secreted. The present study was undertaken to determine whether the synthesis and the secretion of APF-CBP are similarly regulated by BS, using two methods. The synthesis and secretion of labelled, newly synthesized APF-CBP by isolated rat hepatocytes were monitored by solid-phase immunoassay. For this purpose, hepatocytes were incubated with either glycodeoxycholate (GDC) or taurocholate (TC). The synthesis and secretion of labelled, newly synthesized APF-CBP by perfused rat liver were measured by immunological enzyme-linked assay (ELISA) upon perfusing the liver with either GDC or TC. We found that (i) the synthesis and the secretion of APF-CBP were increased during either TC or GDC perfusion, but the increase was more pronounced with TC; (ii) in GDC perfusion the APF-CBP levels measured were more closely related to the levels of bile salts and not to phospholipid levels, (iii) when the two bile salts were perfused in reverse order, i.e., first GDC and then TC, the secretion of APF-CBP in bile decreased when GDC was perfused, but increased when TC was perfused. Similar results were obtained in experiments with isolated hepatocytes.(ABSTRACT TRUNCATED AT 250 WORDS)

Response of plasma low density lipoprotein subfractions to oestrogen replacement therapy following surgical menopause

OBJECTIVE

Epidemiological studies suggest that postmenopausal oestrogen replacement reduces the incidence of cardiovascular disease. The purpose of this study was to establish the effects of oestrogen replacement therapy on subfractions of plasma low density lipoprotein in bilaterally oophorectomized women.

DESIGN

In a placebo controlled, double-blind study, patients were randomized on a two to one basis to receive either oestradiol valerate (2 mg/day) or placebo respectively for a period of 16 weeks.

PATIENTS

Seventeen women aged 28-51 years who had all had hysterectomy and bilateral oophorectomy at least 2 months before recruitment were assigned to either the active (n = 12) or placebo (n = 5) group.

MEASUREMENTS

Plasma lipids, lipoproteins, apolipoproteins and LDL subfractions were determined immediately before and after the treatment period. LDL subfractions were isolated directly from plasma by density gradient ultracentrifugation within 24 hours. Non-parametric statistical analysis was carried out within each group using Wilcoxon's signed rank test for matched pairs.

RESULTS

After 16 weeks of treatment, HDL cholesterol, apo A-I and HDL-2 were increased in the group receiving oestrogen (HDL cholesterol +12%, P < 0.01; apo A-I +14%, P < 0.01; HDL-2 +24% P < 0.01). While there were no significant changes in serum cholesterol, LDL cholesterol or triglycerides, the proportion and concentration of the least dense LDL-I subfraction was decreased significantly (-27%, P < 0.05). The LDL subfraction of intermediate density (LDL-II) was decreased in eight subjects, while small, dense LDL-III was unaffected. Overall, these changes resulted in an apparent shift in the distribution of LDL subfractions towards small, dense LDL-III, although there was no net increase in the latter.

CONCLUSION

In view of a similar and characteristic response of LDL subfractions to hypolipidaemic drugs that enhance the clearance of LDL via the LDL receptor, the present findings suggest that oestrogen promotes the preferential removal of LDL-I and II by activating LDL receptors. As this effect is normally associated with a reduction in the circulating level of LDL, it should not be regarded as an unfavourable response to oestrogen replacement therapy.

Luminescence studies with trp repressor and its single-tryptophan mutants

Time-resolved and steady-state fluorescence, low-temperature phosphorescence, and optically detected magnetic resonance (ODMR) measurements have been made to resolve the luminescence contributions of the two intrinsic tryptophan residues in the subunits of trp aporepressor from Escherichia coli. Assignments of spectral information have been confirmed by use of the single-tryptophan mutants W19F and W99F. Solute fluorescence quenching studies show that both Trp19 and Trp99 are exposed to acrylamide and iodide, with Trp99 being the more exposed. Time-resolved and steady-state fluorescence measurements show Trp19 to have a bluer emission, a longer mean fluorescence decay time, a higher quantum yield, and essentially no independent rotational motion with respect to the protein. Trp99 is found to have a redder emission, a shorter mean fluorescence decay time, a lower quantum yield, and a significant degree of rotational freedom. Phosphorescence studies show a clear resolution of 0-0 vibronic transitions for each type of residue, with maxima at 407 and 415 nm that are assigned to Trp19 and Trp99, respectively. ODMR measurements show the zero-field splitting parameters to be quite characteristically different for each tryptophan residue. The existence of resonance energy transfer from Trp19 to Trp99, in the wild-type protein, is indicated by three types of data: comparison of the long-lived decay time (attributed to Trp19) in the absence (W99F) and presence (wild type) of the acceptor Trp99, comparison of the fluorescence quantum yield of the wild-type and mutant proteins, and deviations from the expected phosphorescence intensities for Trp19 and Trp99 in the absence of energy transfer.

Guanidine hydrochloride-induced folding of proteins

Guanidine hydrochloride (Gdn-HCl) is the most commonly used denaturant for proteins. Contrary to expectation, we found that Gdn-HCl at low concentrations refolds acid-unfolded apomyoglobin and cytochrome c, stabilizing the molten globule state, i.e. a compact denatured state with a significant amount of secondary structure, but substantially disordered tertiary structure. A further increase in Gdn-HCl concentration, above 1 M, caused co-operative unfolding of the molten globule state. Similar sequential folding and unfolding transitions were also observed at neutral pH with a synthetic amphiphilic peptide consisting of Lys and Leu residues, indicating the generality of the phenomenon. Although the Gdn-HCl-induced refolding and unfolding transitions were puzzling at first glance, we show that they are readily interpreted in terms of the differential action of Gdn-HCl. We also show that the comparison of the unfolding curves for the molten globule and native states provides a measure of the buried surface area upon formation of the molten globule state.

Aggregation and denaturation of apomyoglobin in aqueous urea solutions

The effects of urea on apomyoglobin solubility have been investigated. Apomyoglobin precipitation was found to be a thermodynamically reversible process independent of the pathway of aggregation. A liquid-solid phase diagram was constructed for the precipitation of apomyoglobin as a function of urea and protein concentration. Apomyoglobin solubility decreases by an order of magnitude between 0 and 1.5 M urea, reaching a minimum near 2.4 M urea and increasing at higher urea concentrations (the denaturation midpoint is at approximately 2.6 M urea). This decrease in protein solubility is opposite to that expected based on amino acid solubilities, since both polar and nonpolar molecules become more soluble with increasing urea concentration. Solubility minima for proteins have been rationalized in terms of folding intermediates. However, our structural studies show no evidence for folding intermediates in apomyoglobin under the experimental conditions, apart from small predenaturation changes. Our data are consistent with an alternative hypothesis, namely, that the primary aggregating species are denatured protein molecules, rather than intermediate states. Consistent with recent thermodynamic and statistical mechanical models, the solubility minimum may be described as the result of two competing effects of urea: (1) urea denatures the protein, and (2) urea makes the solvent more favorable for the native and any denatured state. At low urea concentration, solubility decreases with increasing urea concentration due to the domination of the solubility behavior by the increase in the population of aggregation-competent (denatured) protein molecules. However, at high urea concentration, the increasingly favorable nature of the solvent dominates, resulting in increasing solubility with urea concentration. The phase diagram provides guidance for the best experimental conditions (pathway) to use to avoid aggregation during the refolding of denaturant-unfolded protein.

Total synthesis of horse heart cytochrome c. Preparation and characterization of the (1-66)apofragment

A peptide corresponding to the native (1-66) sequence of horse heart cytochrome c has been synthesized by stepwise automated solid-phase methods on PAM resin. The course of the synthesis has been monitored by several analytical methods including quantitative ninhydrin and Edman degradation. After HF cleavage, the peptide has been purified by a combination of semipreparative ion-exchange and RP-HPLC. The homogeneity of the purified synthetic peptide has been determined by different criteria including HPLC, amino-acid composition, electrophoresis, antibody binding, tryptic and chymotryptic peptide mapping. After deprotection of the Acm-Cys residues and CNBr cleavage of the Met65-Glu66 peptide bond with simultaneous transformation of the Met65 residue into the activated C-terminal [Hse65]lactone, this purified synthetic peptide has been utilized for conformation-assisted joining experiments in combination with synthetic (66-104) to produce fully synthetic [Hse65]apocytochrome c. This latter, after mitochondria-mediated stereospecific heme insertion, has given a functional molecule corresponding to native horse heart holocytochrome c.

Effects of metformin on dyslipoproteinemia in non-insulin-dependent diabetes mellitus

A 12-week double-blind placebo-controlled trial to study the effects of metformin on lipoprotein concentration and composition was performed in forty patients with NIDDM and hyperlipoproteinemia. A significant decrease occurred in VLDL-apo B and all lipid components of VLDL, indicating a decreased number of circulating VLDL, while LDL-apo B was unchanged. Moreover in VLDL the relative TG content increased, the cholesterol content decreased, while in LDL the TG content decreased and the cholesterol content increased, indicating a change in the particle distribution over the spectrum VLDL-IDL-LDL. The initially enhanced TG-content in HDL was reduced. While a reduction in VLDL is observed with all methods improving glucose control the observed compositional changes in VLDL and LDL have not been described before and may be a specific effect of metformin.

[The effect of propranolol on the apoprotein spectrum of the blood plasma in patients with ischemic heart disease]

The authors studied the effect of anaprilin monotherapy (dose: 160-200 mg/daily for 10 months) on the level of atherogenic apolipoproteins (apo A, apo B) and fractions of plasma cholesterol in 34 patients with ischemic heart disease, stable exertion angina pectoris (class 2-3). It was established that changes in the plasma lipid and apoprotein spectrum result in an increase of apo B, decrease of apo 1, increase of apo B/apo A1, decrease of high-density lipoprotein cholesterol (HDLCS) and a different dynamics of HDLCS/apo A1. The established changes in the lipid metabolism are of atherogenic character.

One-electron reduction of daunorubicin intercalated in DNA or in a protein: a gamma radiolysis study

The one-electron reduction of daunorubicin, an anthracycline antibiotic, intercalated in DNA or in the apoprotein of the riboflavin binding protein, was studied by gamma radiolysis. The two reduction mechanisms appear very similar to the one found for the non-intercalated drug. Hydrogen peroxide, which oxidizes non-intercalated hydroquinone daunorubicin with two electrons in one step (C. Houée-Levin, M. Gardès-Albert and C. Ferradin, FEBS lett., 173, 27-30, (1984], reacts with daunorubicin hydroquinone in DNA but not in the protein. It appears thus that the site accessibility to hydrogen peroxide in DNA is better than in the protein. Biological consequences are discussed.