Structure of 1 -acid glycoprotein. The complete amino acid sequence, multiple amino acid substitutions, and homology with the immunoglobulins

Comprehending binding features between ibrutinib and Human Alpha-1 acid glycoprotein: Combined experimental approaches and theoretical simulations

Human alpha-1 acidic glycoprotein (HAG) is one of the proteins widely present in the blood, and the level of HAG in patients with cancer and inflammation is significantly increased. As one of transport proteins in the blood, the ability of HAG to bind with a drug, especially alkaline drugs, affects significantly the drug content at the target site, which in turn affects the efficacy of the drug. In this study, the interaction mechanism between HAG and the first generation Bruton's tyrosine kinase (BTK) inhibitor namely ibrutinib was explored by a combination of multi-spectroscopic techniques and theoretical calculations. The findings revealed that the quenching and binding constants of the HAG-ibrutinib system both reduced as the temperature rose, demonstrating that ibrutinib quenched the intrinsic fluorescence of HAG in a static manner. It was confirmed that HAG and ibrutinib formed a 1:1 complex with moderate affinity due to the binding constant of around 10⁵ M^-1 and accompanied by Förster resonance energy transfer. It was verified by thermodynamic parameter analysis and competition assays as well as molecular simulation that the existence of hydrogen bonds, van der Waals forces, and hydrophobic forces in the complexation of HAG and ibrutinib.The findings from theoretical calculations including molecular docking and theoretical calculation simulation confirmed that ibrutinib bound to the barrel hydrophobic pocket of HAG with a binding energy of -41.9 kJ∙mol^-1, and the the binding constant of around 10⁵ M^-1 and the contribution of each residue in the complexation of ibrutinib and HAG. Additionally, it can be confirmed that metal ions affected the binding interaction of ibrutinib with HAG, among them, some promoted binding while others inhibited it.

An overview of albumin and alpha-1-acid glycoprotein main characteristics: highlighting the roles of amino acids in binding kinetics and molecular interactions

Although Albumin (ALB) and alpha-1-acid glycoprotein (AGP) have distinctive structural and functional characteristics, they both play a key role in binding a large variety of endogenous and exogenous ligands. An extensive binding to these plasma proteins could have a potential impact on drugs disposition (e.g. bioavailability, distribution and clearance), on their innocuity and their efficacy. This review summarizes the common knowledge about the structural and molecular characteristics of both ALB and AGP in humans, and about the most involved amino acids in their high-affinity binding pockets. However, the variability in residues found in binding pockets, for the same species, allows each plasma protein to interact differently with the ligands. The protein-ligand interaction influences differently the disposition of drugs that bind to either of these plasma proteins. The content of this review is useful for the design of new drug entities with high-binding characteristics, in qualitative and quantitative modelling (e.g. in vitro-in vivo extrapolations, 3D molecular docking, interspecies extrapolations), and for other interdisciplinary research.

The Immune Functions of α1 Acid Glycoprotein

α1-acid glycoprotein (orosomucoid, AGP) is an Acute Phase Protein produced by liver and peripheral tissues in response to systemic reaction to inflammation. AGP functions have been studied mostly in human, cattle and fish, although the protein has been also found in many mammalian species and birds. AGP fulfils at least two set of functions, which are apparently different from each other but in fact intimately linked. On one hand, AGP is an immunomodulatory protein. On the other hand, AGP is one of the most important binding proteins in plasma and, beside modulating pharmacokinetics and pharmacodynamics of many drugs, it is also able to bind and transport several endogen ligands related to inflammation. The focus of this review is the immunomodulatory activity of AGP. This protein regulates every single event related to inflammation, including binding of pathogens and modulating white blood cells activity throughout the entire leukocyte attacking sequence. The regulation of AGP activity is complex: the inflammation induces not only an increase in AGP serum concentration, but also a qualitative change in its carbohydrate moiety, generating a multitude of glycoforms, each of them with different, and sometimes opposite and contradictory, activities. We also present the most recent findings about the relationship between AGP and adipose tissue: AGP interacts with leptin receptor and, given its immunomodulatory function, it may be included among the potential players in the field of immunometabolism.

Scrutinizing the interactions between bisphenol analogues and plasma proteins: Insights from biomimetic liquid chromatography, molecular docking simulations and in silico predictions

The interactions between human serum albumin (HSA) and α₁- acid glycoprotein (AGP), the main plasma proteins binding drugs/xenobiotics, and some endocrine disrupting chemicals (EDCs), such as bisphenol A (BPA) and some of its structural analogues, bisphenol S (BPS), bisphenol F (BPF), bisphenol E (BPE), bisphenol B (BPB), bisphenol AF (BPAF), bisphenol A diglycidyl ether (BADGE) and bisphenol M (BPM), were characterized by biomimetic liquid chromatography (LC). The interactions between bisphenols (BPs) and either HSA or AGP protein was found to be non-specific and essentially lipophilicity-driven. To get more information on the binding of BPs and plasma proteins, in silico predictions and molecular docking simulations were exploited, and the results achieved in silico were compared to those observed in vitro. BPM was the one exhibiting the highest affinity on both plasma proteins according to these data. Our findings clarified the binding of these EDCs to plasma proteins and offered insights into the biodistribution and bioaccumulation processes underlying their toxicity.

A combined photophysical and computational study on the binding of mycophenolate mofetil and its major metabolite to transport proteins

Binding of the immunosuppressive agent mycophenolate mofetil (MMP) and its pharmacologically active metabolite mycophenolic acid (MPA) to human serum albumin (HSA) and α₁-acid glycoprotein (HAAG) has been investigated by means of an integrated approach involving selective excitation of the drug fluorophore, following their UV-A triggered fluorescence and docking studies. The formation of the protein/ligand complexes was evidenced by a dramatic enhancement of the fluorescence intensity and a hypsochromic shift of the emission band. In HSA, competitive studies using oleic acid as site I probe revealed site I as the main binding site of the ligands. Binding constants revealed that the affinity of the active metabolite by HSA is four-fold higher than its proactive form. Moreover, the affinity of MMP by HSA is three-fold higher than by HAAG. Docking studies revealed significant molecular binding differences in the binding of MMP and MPA to sub-domain IIA of HSA (site 1). For MPA, the aromatic moiety would be in close contact to Trp214 with the flexible chain pointing to the other end of the sub-domain; on the contrary, for MMP, the carboxylate group of the chain would be fixed nearby Trp214 through electrostatic interactions with residues Arg218 and Arg222.

Human plasma lipocalins and serum albumin: Plasma alternative carriers?

Lipocalins are an evolutionarily conserved family of proteins that bind and transport a variety of exogenous and endogenous ligands. Lipocalins share a conserved eight anti-parallel β-sheet structure. Among the different lipocalins identified in humans, α-1-acid glycoprotein (AGP), apolipoprotein D (apoD), apolipoprotein M (apoM), α1-microglobulin (α1-m) and retinol-binding protein (RBP) are plasma proteins. In particular, AGP is the most important transporter for basic and neutral drugs, apoD, apoM, and RBP mainly bind endogenous molecules such as progesterone, pregnenolone, bilirubin, sphingosine-1-phosphate, and retinol, while α1-m binds the heme. Human serum albumin (HSA) is a monomeric all-α protein that binds endogenous and exogenous molecules like fatty acids, heme, and acidic drugs. Changes in the plasmatic levels of lipocalins and HSA are responsible for the onset of pathological conditions associated with an altered drug transport and delivery. This, however, does not necessary result in potential adverse effects in patients because many drugs can bind both HSA and lipocalins, and therefore mutual compensatory binding mechanisms can be hypothesized. Here, molecular and clinical aspects of ligand transport by plasma lipocalins and HSA are reviewed, with special attention to their role as alterative carriers in health and disease.

Human plasma protein N-glycosylation

Glycosylation is the most abundant and complex protein modification, and can have a profound structural and functional effect on the conjugate. The oligosaccharide fraction is recognized to be involved in multiple biological processes, and to affect proteins physical properties, and has consequentially been labeled a critical quality attribute of biopharmaceuticals. Additionally, due to recent advances in analytical methods and analysis software, glycosylation is targeted in the search for disease biomarkers for early diagnosis and patient stratification. Biofluids such as saliva, serum or plasma are of great use in this regard, as they are easily accessible and can provide relevant glycosylation information. Thus, as the assessment of protein glycosylation is becoming a major element in clinical and biopharmaceutical research, this review aims to convey the current state of knowledge on the N-glycosylation of the major plasma glycoproteins alpha-1-acid glycoprotein, alpha-1-antitrypsin, alpha-1B-glycoprotein, alpha-2-HS-glycoprotein, alpha-2-macroglobulin, antithrombin-III, apolipoprotein B-100, apolipoprotein D, apolipoprotein F, beta-2-glycoprotein 1, ceruloplasmin, fibrinogen, immunoglobulin (Ig) A, IgG, IgM, haptoglobin, hemopexin, histidine-rich glycoprotein, kininogen-1, serotransferrin, vitronectin, and zinc-alpha-2-glycoprotein. In addition, the less abundant immunoglobulins D and E are included because of their major relevance in immunology and biopharmaceutical research. Where available, the glycosylation is described in a site-specific manner. In the discussion, we put the glycosylation of individual proteins into perspective and speculate how the individual proteins may contribute to a total plasma N-glycosylation profile determined at the released glycan level.

Orosomucoid 1 drives opportunistic infections through the polarization of monocytes to the M2b phenotype

Orosomucoid (ORM, composed of two isoforms, ORM1 and ORM2) has been described as an inducer of M2 macrophages, which are cells that decrease host antibacterial innate immunities. However, it is unknown which phenotypes of M2 macrophages are induced by ORM. In this study, healthy donor monocytes stimulated with ORM (ORM-monocytes) were characterized phenotypically and biologically. CCL1 (a biomarker of M2b macrophages) and IL-10 were detected in monocyte cultures supplemented with ORM1; however, CCL17 (a biomarker of M2a macrophages) and CXCL13 (a biomarker of M2c macrophages) were not produced in these cultures. All of these soluble factors were not detected in the culture fluids of monocytes stimulated with ORM2. Monocytes stimulated with ORM1 were characterized as CD64(-)CD209(-)CD163(+)CCL1(+) cells. MRSA and Enterococcus faecalis infections were accelerated in chimeras (NOD/scid IL-2Rγ(null) mice reconstituted with white blood cells) after inoculation with monocytes stimulated with ORM1 or treatment with ORM1; however, the infections were greatly mitigated in both chimeras inoculated with ORM1-stimulated monocytes and treated with ORM1, after an additional treatment with an inhibitor of M2b macrophages (CCL1 antisense ODN). These results indicate that ORM1 stimulates quiescent monocytes to polarize to M2b monocytes. The regulation of M2b macrophages may be beneficial in controlling opportunistic infections in patients with a large amount of plasma ORM1.

Photoactive assemblies of organic compounds and biomolecules: drug–protein supramolecular systems

Modification of the drug excited state properties within proteins provides information on binding and may result in a different photoreactivity.

EndoS2 is a unique and conserved enzyme of serotype M49 group A Streptococcus that hydrolyses N-linked glycans on IgG and α1-acid glycoprotein

Many bacteria have evolved ways to interact with glycosylation functions of the immune system of their hosts. Streptococcus pyogenes [GAS (group A Streptococcus)] secretes the enzyme EndoS that cleaves glycans on human IgG and impairs the effector functions of the antibody. The ndoS gene, encoding EndoS, has, until now, been thought to be conserved throughout the serotypes. However, in the present study, we identify EndoS2, an endoglycosidase in serotype M49 GAS strains. We characterized EndoS2 and the corresponding ndoS2 gene using sequencing, bioinformatics, phylogenetic analysis, recombinant expression and LC-MS analysis of glycosidic activity. This revealed that EndoS2 is present exclusively, and highly conserved, in serotype M49 of GAS and is only 37% identical with EndoS. EndoS2 showed endo-β-N-acetylglucosaminidase activity on all N-linked glycans of IgG and on biantennary and sialylated glycans of AGP (α1-acid glycoprotein). The enzyme was found to act only on native IgG and AGP and to be specific for free biantennary glycans with or without terminal sialylation. GAS M49 expression of EndoS2 was monitored in relation to carbohydrates present in the culture medium and was linked to the presence of sucrose. We conclude that EndoS2 is a unique endoglycosidase in serotype M49 and differs from EndoS of other GAS strains by targeting both IgG and AGP. EndoS2 expands the repertoire of GAS effectors that modify key glycosylated molecules of host defence.

MALDI mass spectrometry-based sequence analysis of arginine-containing glycopeptides: improved fragmentation of glycan and peptide chains by modifying arginine residue

This paper describes an improved method for the sequence analysis of Arg-containing glycopeptide by MALDI mass spectrometry (MS). The method uses amino group derivatization (4-aza-6-(2,6-dimethyl-1-piperidinyl)-5-oxohexanoic acid N-succinimidyl ester) and removal (carboxypeptidase B) or modification (peptidylarginine deiminase 4) of the arginine residue of the peptide. The derivatization attaches a basic tertiary amine moiety onto the peptides, and the enzymatic treatment removes or modifies the arginine residue. Fragmentation of the resulting glycopeptide under low-energy collision-induced dissociation yielded a simplified ion series of both the glycan and the peptide that can facilitate their sequencing. The feasibility of the method was studied using α1 -acid glycoprotein-derived N-linked glycopeptides, and glycan and peptide in each glycopeptide were successfully sequenced by MALDI tandem MS (MS/MS).

Drug-Drug Interactions within Protein Cavities Probed by Triplet-Triplet Energy Transfer

A new direct and noninvasive methodology based on transient absorption spectroscopy has been developed to probe the feasibility of drug-drug interactions within a common protein binding site. The simultaneous presence of (R)-cinacalcet (CIN) and (S)-propranolol (PPN) within human or bovine α1-acid glycoproteins (AAGs) is revealed by detection of (3)CIN* as the only transient species after laser flash photolysis of CIN/PPN/AAG mixtures at 308 nm. This is the result of triplet-triplet energy transfer from (3)PPN* to CIN, which requires close contact between the two drugs within the same biological compartment. Similar results are obtained with nabumetone and CIN as donor/acceptor partners. This new methodology can, in principle, be extended to a variety of drug/drug/biomolecule combinations.

Drug-binding energetics of human α-1-acid glycoprotein assessed by isothermal titration calorimetry and molecular docking simulations

This study utilizes sensitive, modern isothermal titration calorimetric methods to characterize the microscopic thermodynamic parameters that drive the binding of basic drugs to α-1-acid glycoprotein (AGP) and thereby rationalize the thermodynamic data in relation to docking models and crystallographic structures of the drug-AGP complexes. The binding of basic compounds from the tricyclic antidepressant series, together with miaserine, chlorpromazine, disopyramide and cimetidine, all displayed an exothermically driven binding interaction with AGP. The impact of protonation/deprotonation events, ionic strength, temperature and the individual selectivity of the A and F1*S AGP variants on drug-binding thermodynamics was characterized. A correlation plot of the thermodynamic parameters for all of the test compounds revealed that an enthalpy-entropy compensation is in effect. The exothermic binding energetics of the test compounds were driven by a combination of favorable (negative) enthalpic (∆Hº) and favorable (positive) entropic (∆Sº) contributions to the Gibbs free energy (∆Gº). Collectively, the data imply that the free energies that drive drug binding to AGP and its relationship to drug serum residency evolve from the complex interplay of enthalpic and entropic forces from interactions with explicit combinations of hydrophobic and polar side-chain sub-domains within the multi-lobed AGP ligand binding cavity.

The effect of robenacoxib on the concentration of C-reactive protein in synovial fluid from dogs with osteoarthritis

Background

Robenacoxib is a novel and highly selective inhibitor of COX-2 in dogs and cats and because of its acidic nature is regarded as being tissue-selective. Thirty four dogs with stifle osteoarthritis secondary to failure of the cranial cruciate ligament were recruited into this study. Lameness, radiographic features, synovial cytology and C-reactive protein concentrations in serum and synovial fluid were assessed before and 28 days after commencing a course of Robenacoxib at a dose of 1 mg/kg SID.

Results

There was a significant reduction in the lameness score (P < 0.01) and an increase in the radiographic score (P < 0.05) between pre- and post-treatment assessments. There was no difference between pre- (median 1.49 mg/l; Q1-Q3 0.56-4.24 mg/L) and post – (1.10 mg/L; 0.31-1.78 mg/L) treatment serum C-reactive protein levels although synovial fluid levels were significantly reduced (pre- : 0.44 mg/L; 0.23-1.62 mg/L; post- : 0.17 mg/L; 0.05-0.49 mg/L) (P < 0.05). There was no correlation between C-reactive protein concentrations in serum and matched synovial fluid samples.

Conclusions

Robenacoxib proved effective in reducing lameness in dogs with failure of the cranial cruciate ligament and osteoarthritis of the stifle joint. The drug also reduced levels of C-reactive protein in the synovial fluid taken from the affected stifle joint. Robenacoxib appears to reduce articular inflammation as assessed by C-reactive protein which supports the concept that Robenacoxib is a tissue-selective non-steroidal anti-inflammatory drug.

α(1)-Acid glycoprotein up-regulates CD163 via TLR4/CD14 protein pathway: possible protection against hemolysis-induced oxidative stress

CD163, a scavenger receptor that is expressed at high levels in the monocyte-macrophage system, is a critical factor for the efficient extracellular hemoglobin (Hb) clearance during hemolysis. Because of the enormous detrimental effect of liberated Hb on our body by its ability to induce pro-inflammatory signals and tissue damage, an understanding of the molecular mechanisms associated with CD163 expression during the acute phase response is a central issue. We report here that α(1)-acid glycoprotein (AGP), an acute phase protein, the serum concentration of which is elevated under various inflammatory conditions, including hemolysis, up-regulates CD163 expression in both macrophage-like differentiated THP-1 (dTHP-1) cells and peripheral blood mononuclear cells in a time- and concentration-dependent manner. Moreover, the subsequent induction of Hb uptake was also observed in AGP-treated dTHP-1 cells. Among representative acute phase proteins such as AGP, α(1)-antitrypsin, C-reactive protein, and haptoglobin, only AGP increased CD163 expression, suggesting that AGP plays a specific role in the regulation of CD163. Consistently, the physiological concentrations of AGP induced CD163, and the subsequent induction of Hb uptake as well as the reduction of oxidative stress in plasma were observed in phenylhydrazine-induced hemolytic model mice, confirming the in vivo role of AGP. Finally, AGP signaling through the toll-like receptor-4 (TLR4) and CD14, the common innate immune receptor complex that normally recognizes bacterial components, was identified as a crucial stimulus that induces the autocrine regulatory loops of IL-6 and/or IL-10 via NF-κB, p38, and JNK pathways, which leads to an enhancement in CD163 expression. These findings provide possible insights into how AGP exerts anti-inflammatory properties against hemolysis-induced oxidative stress.

Relation between proteins tertiary structure, tryptophan fluorescence lifetimes and tryptophan S(o)→(1)L(b) and S(o)→(1)L(a) transitions. Studies on α1-acid glycoprotein and β-lactoglobulin

We measured fluorescence lifetimes and fluorescence spectra (excitation and emission) of tryptophan residues of α(1)-acid glycoprotein (three Trp residues) and β-lactoglobulin (two Trp residues) in absence and presence of 450 μM progesterone. Progesterone binds only to α(1)-acid glycoprotein. In absence of progesterone, each of the two proteins displays three fluorescence lifetimes. Addition of progesterone induces a partial inhibition of the S(o) → (1)L(a) transition without affecting fluorescence lifetimes. The same experiments performed in presence of denatured proteins in 6 M guanidine show that addition of progesterone inhibits partially the S(o) → (1)L(a) transition and its peak is 15 nm shifted to the red compared to that obtained for native proteins. However, the S(o) → (1)L(b) transition position peak is not affected by protein denaturation. Thus, the tertiary structure of the protein plays an important role by modulating the tryptophan electronic transitions. Fluorescence emission decay recorded in absence and presence of progesterone yields three fluorescence lifetimes whether proteins are denatured or not. Thus, protein tertiary structure is not responsible for the presence of three fluorescence lifetimes. These characterize tryptophan substructures reached at the excited states and which population (pre-exponential values) depend on the tryptophan residues interaction with their microenvironment(s) and thus on the global conformation of the protein.

Impact of protein binding on the analytical detectability and anticancer activity of thymoquinone

Thymoquinone (TQ), an active component of Nigella sativa L., is known to have anti-cancer and anti-inflammatory effects; however, no studies on its analytical detection in serum and its protein binding have been published. Using high performance liquid chromatography analysis, we show that the average recovery of TQ from serum is 2.5% at 10 μg/ml of TQ and 72% at 100 μg/ml. The low recovery of TQ from serum is due to its extensive binding to plasma proteins, as more than 99% of TQ was bound within 30 min of incubation. The binding of TQ to the major plasma proteins, bovine serum albumin (BSA) and alpha -1 acid glycoprotein (AGP), was studied and found to be 94.5 ± 1.7% for BSA and 99.1 ± 0.1% for AGP. Mass spectrometric analysis revealed that TQ was bound covalently to BSA, specifically on Cyst-34. Using WST-1 proliferation assay, we showed that BSA plays a protective role against TQ-induced cell death; pre-incubation with BSA prevented TQ from exerting its anti-proliferative effects against DLD-1 and HCT-116 human colon cancer cells. On the other hand, binding of TQ to AGP did not alter its anti-proliferative activity against both cell lines. When TQ was pre-incubated with AGP prior to the addition of BSA, the activity of TQ against DLD-1 was maintained, suggesting that AGP prevented the binding of TQ to BSA. This is the first time the covalent binding and inhibitory effect of BSA on TQ is documented. These data offer new grounds for TQ future pharmacokinetic analysis in vivo.

ELECTRONIC SUPPLEMENTARY MATERIAL

The online version of this article (doi:10.1007/s12154-010-0052-4) contains supplementary material, which is available to authorized users.

Membrane-induced conformational change of alpha1-acid glycoprotein characterized by vacuum-ultraviolet circular dichroism spectroscopy

The tertiary structure of alpha1-acid glycoprotein (AGP) remains unresolved despite its novel function because AGP is a hard target in X-ray and NMR analyses. To elucidate the membrane-induced conformational change of AGP, the vacuum-ultraviolet circular dichroism (VUVCD) spectra of AGP and its constituent sugars were measured down to 160 nm in the presence or absence of phosphoglyceride liposome using a synchrotron-radiation VUVCD spectrophotometer. The secondary-structure contents and numbers of segments of AGP were estimated from the VUVCD spectra of the protein moiety obtained by subtracting the contributions of the glycan moiety. Further, the positions of secondary structures on the amino acid sequence were predicted by combining the VUVCD data with a neural network algorithm. These comprehensive secondary-structure analyses revealed that AGP consists of 11.4% alpha-helices (3 segments) and 39.9% beta-strands (12 segments) in the absence of liposome (pH 4.5), which are close to the proportions in the secondary structure of native AGP (pH 7.4) predicted by homology modeling, and that it consists of 47.5% alpha-helices (7 segments) and 2.7% beta-strands (2 segments) in the presence of liposome (pH 4.5). Detailed characterization of these alpha-helices of AGP bound to liposome suggested that two alpha-helices (residues 15-27 and 161-175) in the N- and C-terminal regions strongly interact with liposome. Most of the progesterone-binding residues of AGP were involved in the sequences transferring from beta-strands to alpha-helices or unordered structures, which coincided with the large decrease in progesterone-binding capacity of liposome-bound AGP. These results provide the first sequence-level information on the membrane-binding mechanism and structure-function relationship of AGP.

[Characterization of expression of alpha1-acid glycoprotein gene in Beijing fatty chicken (Gallus gallus)]

The specific expression of alpha1-AGP gene in eight different tissues of Beijing fatty chicken was investigated by RT-PCR. The full-length cDNA of alpha1-AGP was inserted into pEGFP-C1 multi-cloning sites to construct recombinant eukaryotic expression vector pEGFP-alpha1-AGP. The lipofectin method was used to transfect the pEGFP-alpha1-AGP into Beijing fatty chicken fibroblast cells. The open reading frame of Beijing fatty chicken alpha1-AGP gene was 612 base pairs in length, which was expressed higher in liver and lung than in muscle. This gene did not express in heart and kidney. The expression efficiency ranged from 31.3% to 47.6% in 24, 48, and 72 h after transformation. The green fluorescence mainly concentrated in the nucleus. With the increase of the expression of green fluorescence, granula was observed in the nucleus. RT-PCR and Western blotting analyses showed that pEGFP-alpha1-AGP had been integrated into the genome of Beijing chicken fibroblast cell with normal expression level. In optimized condition, there was no significant effect (P>0.05) on apoptosis ratio, positive cell shape, growth and reduplication state comparing with the control group. This research established the foundation for further function research of alpha1-AGP gene and application in transgenic animal cloning.

The 1.8-A crystal structure of alpha1-acid glycoprotein (Orosomucoid) solved by UV RIP reveals the broad drug-binding activity of this human plasma lipocalin

Alpha(1)-acid glycoprotein (AGP) is an important drug-binding protein in human plasma and, as an acute-phase protein, it has a strong influence on pharmacokinetics and pharmacodynamics of many pharmaceuticals. We report the crystal structure of the recombinant unglycosylated human AGP at 1.8 A resolution, which was solved using the new method of UV-radiation-damage-induced phasing (UV RIP). AGP reveals a typical lipocalin fold comprising an eight-stranded beta-barrel. Of the four loops that form the entrance to the ligand-binding site, loop 1, which connects beta-strands A and B, is among the longest observed so far and exhibits two full turns of an alpha-helix. Furthermore, it carries one of the five N-linked glycosylation sites, while a second one occurs underneath the tip of loop 2. The branched, partly hydrophobic, and partly acidic cavity, together with the presumably flexible loop 1 and the two sugar side chains at its entrance, explains the diverse ligand spectrum of AGP, which is known to vary with changes in glycosylation pattern.

Development of an enhanced proteomic method to detect prognostic and diagnostic markers of healing in chronic wound fluid

BACKGROUND

Chronic venous leg ulcers are a significant cause of pain, immobility and decreased quality of life for patients with these wounds. In view of this, research efforts are focused on multiple factors in the wound environment to obtain information regarding the healing of ulcers.

OBJECTIVES

Chronic wound fluid (CWF), containing a complex mixture of proteins, is an important modulator of the wound environment, and therefore we hypothesized that these proteins may be indicators of the status of wounds and their potential to heal or otherwise. To explore this we developed and validated a proteomic approach to analyse CWF.

METHODS

In this study, pooled CWF was depleted of high abundant proteins using immunoaffinity chromatography. The flow-through and bound fractions were collected, concentrated, desalted and analysed using a range of techniques. Each fraction was further separated using two-dimensional (2D) gel electrophoresis and 2D liquid chromatography and analysed using mass spectrometry (MS).

RESULTS

Western blot analysis against three high abundant proteins confirmed the selective removal of these proteins from CWF. Critically, one-dimensional and 2D gel electrophoresis indicated that subsequent removal of these proteins enhanced the ability to detect proteins in low abundance in CWF. Further, MS demonstrated that depletion of these abundant proteins increased the detection of other proteins in these samples.

CONCLUSIONS

Results obtained indicate that this approach significantly improves separation of proteins present in low concentrations in CWF. This will facilitate the identification of biomarkers in samples collected from patients with ulcers and lead to improved patient therapies and wound care approaches.

Acute-phase protein alpha-1-acid glycoprotein mediates neutrophil migration failure in sepsis by a nitric oxide-dependent mechanism

The reduction of circulating neutrophil migration to infection sites is associated with a poor outcome of severe sepsis. alpha-1-Acid glycoprotein (AGP) was isolated from the sera of severely septic patients by HPLC and acrylamide gel electrophoresis and identified by mass spectrometry. Both the isolated protein and commercial AGP inhibited carrageenin-induced neutrophil migration into the rat peritoneal cavity when administered i.v. at a dose of 4.0 microg per rat (95 pmol per rat). Analysis by intravital microscopy demonstrated that both proteins inhibited the rolling and adhesion of leukocytes in the mesenteric microcirculation. The inhibitory activity was blocked by 50 mg/kg aminoguanidine, s.c., and was not demonstrable in inducible nitric oxide synthase (iNOS) knockout mice. Incubation of AGP with neutrophils from healthy subjects induced the production of NO and inhibited the neutrophil chemotaxis by an iNOS/NO/cyclic guanosine 3,5-monophosphate-dependent pathway. In addition, AGP induced the l-selectin shedding by neutrophils. The administration of AGP to rats with mild cecal ligation puncture sepsis inhibited neutrophil migration and reduced 7-day survival from approximately 80% to 20%. These data demonstrate that AGP, an acute-phase protein, inhibits neutrophil migration by an NO-dependent process and suggest that AGP also participates in human sepsis.

Alpha1-acid glycoprotein suppresses rat acute inflammatory paw edema through the inhibition of neutrophils activation and prostaglandin E2 generation

Alpha(1)-acid glycoprotein (AGP) is an acute phase protein. Whereas the expression of AGP in an inflammatory state is enhanced by inflammatory cytokines including interleukin-1, 6 (IL-1 and IL-6), and tumor necrosis factor-alpha (TNF-alpha), the biological significance of AGP remains unclear. In the current study, the anti-inflammatory effect of AGP on the acute inflammatory state was examined in vivo and in vitro. AGP suppressed carrageenan-, dextran- and kaolin-induced paw edema and vascular permeability in rat. These results suggest that both initial inflammatory mediators (serotonin and histamine) and later inflammatory mediators (prostaglandin and bradykinin) are involved in the anti-inflammatory effects of AGP. In fact, prostaglandin E(2) (PGE(2)) generation in plasma was significantly inhibited by AGP. Moreover, AGP inhibited the migration of neutrophils treated with N-formyl-methionyl-leucyl-phenylalanine (fMLP) through membrane filter. In addition, AGP significantly suppressed superoxide generation from neutrophils that has been treated with fMLP or phorbol 12-myristate 13-acetate. These results imply that the anti-inflammatory effect of AGP may involve the inhibition of neutrophils migration. The data obtained in this study support a scenario in which an increase in AGP concentration in pathological conditions suppresses inflammation reactions induced by autacoids and neutrophils activities and that AGP plays an important role in the maintenance in the body.

Progesterone binding to the tryptophan residues of human alpha1-acid glycoprotein

Binding studies between progesterone and alpha1-acid glycoprotein allowed us to demonstrate that the binding site of progesterone contains one hydrophobic tryptophan residue and that the structure of the protein is not altered upon binding. The data obtained at saturated concentrations of progesterone clearly reveal the type of interaction at physiological levels.

Anticalins in Drug Development

Anticalins are a novel class of engineered ligand-binding proteins that are prepared from lipocalins--conventional plasma proteins in humans--via targeted random mutagenesis and selection against prescribed haptens or antigens. The first anticalins were selected to bind to small ligands, such as the cardioactive drug digoxin. Recently, libraries that also permit the generation of anticalins with high affinities and specificities for protein targets, especially disease-related cell-surface receptors, have been constructed. Anticalins are much smaller than antibodies or their antigen-binding fragments, lack glycosylation as well as immunological effector functions, and consist of a single, stably folded polypeptide chain. Thus, they offer benefits as biopharmaceuticals in several areas of medical therapy, for example as receptor antagonists or as effective antidotes against toxic compounds.

Identification of disulfide bonds and site-specific glycosylation in chicken α1-acid glycoprotein by matrix-assisted laser desorption ionization time-of-flight mass spectrometry

Recently, we reported the amino acid sequence of chicken alpha1-acid glycoprotein (chicken alpha1-AGP) [Biochem. Biophys. Res. Commun. 295 (2002) 587]. In this study, we located the disulfide bonds and site-specific glycosylation in chicken alpha1-AGP using tryptic digests of carbamidomethylated chicken alpha1-AGP, carbamidomethylated completely deglycosylated chicken alpha1-AGP (cd-alpha1-AGP), and nonreduced denatured cd-alpha1-AGP by matrix-assisted laser desorption ionization time-of-flight mass spectrometry. Based on the detection of peptides mlz 3037.4 (amino acid sequences 69-76 plus 161-183) and 3453.3 (amino acid sequences 69-80 plus 161-183), the two disulfide bonds of chicken alpha1-AGP were determined to be located at Cys 6-Cys 146 and Cys 73-Cys 163. The results also showed that Asn 16, 70, 77, and 87 were fully glycosylated and that Asn 62 was partially glycosylated.

Förster energy-transfer studies between Trp residues of alpha1-acid glycoprotein (orosomucoid) and the glycosylation site of the protein

Energy-transfer studies between Trp residues of alpha(1)-acid glycoprotein and the fluorescent probe Calcofluor White were performed. Calcofluor White interacts with carbohydrate residues of the protein, while the three Trp residues are located at the surface (Trp-160) and in hydrophobic domains of the protein (Trp-25 and Trp-122). Binding of Calcofluor to the protein induces a decrease in the fluorescence intensity of the Trp residues accompanied by an increase of that of Calcofluor White. Efficiency (E) of Trp fluorescence quenching was determined to be equal to 45%, and the Förster distance R(o), at which the efficiency of energy transfer is 50%, was calculated to be 18.13 A. This low distance and the value of the efficiency clearly indicate that energy transfer between Trp residues and Calcofluor White is weak.

Tertiary structure of human α1-acid glycoprotein (orosomucoid). Straightforward fluorescence experiments revealing the presence of a binding pocket

Binding of hemin to alpha1-acid glycoprotein has been investigated. Hemin binds to the hydrophobic pocket of hemoproteins. The fluorescent probe 2-(p-toluidino)-6-naphthalenesulfonate (TNS) binds to a hydrophobic domain in alpha1-acid glycoprotein with a dissociation constant equal to 60 microM. Addition of hemin to an alpha1-acid glycoprotein-TNS complex induces the displacement of TNS from its binding site. At saturation (1 hemin for 1 protein) all the TNS has been displaced from its binding site. The dissociation constant of hemin-alpha1-acid glycoprotein was found equal to 2 microM. Thus, TNS and hemin bind to the same hydrophobic site: the pocket of alpha1-acid glycoprotein. Energy-transfer studies performed between the Trp residues of alpha1-acid glycoprotein and hemin indicated that efficiency (E) of Trp fluorescence quenching was equal to 80% and the Förster distance, R0 at which the efficiency of energy transfer is 50% was calculated to be 26 A, revealing a very high energy transfer.

Differential binding of disopyramide and warfarin enantiomers to human alpha(1)-acid glycoprotein variants

AIMS

The F1S and A genetic variants of alpha1-acid glycoprotein (AAG) change under various physiological and pathological conditions. They also vary in their drug binding abilities. We have studied the stereoselective binding ability of each of the AAG variants using enantiomers of disopyramide (DP) and warfarin (WR).

METHODS

The AAG variants were separated by hydroxyapatite chromatography. Binding of drug enantiomers to the AAG variants was studied by the Hummel-Dreyer method. The characteristics of the binding activities were examined by Scatchard plot analysis. The first five amino-terminal amino acids (residues 112-116) of the cyanogen bromide (CNBr) fragment (residues 112-181) of each of the separated AAG fractions were elucidated by Edman degradation.

RESULTS

Commercial AAG was separated into two main fractions. Residues 112-116 of fraction 2 were identical to the amino acid sequences predicted from the AAG A gene, LAFDV, and encode the F1S variant. In fraction 3, the deduced amino acid sequence of the AAG B gene, FGSYL, was established, and encodes the A variant. The binding affinities of both DP enantiomers in fraction 3 were significantly higher than those in fraction 2. The differences between dissociation constants (Kd) in fractions 2 and 3 were 5.2-fold for (S)-DP (P < 0.05) and 3.7-fold for (R)-DP (P < 0.001). The dissociation constant of (S)-DP (0.39 +/- 0.08 micro m) was lower than that of (R)-DP (0.53 +/- 0.10 micro m) in fraction 3 [95% confidence interval (CI) - 0.282, - 0.010; P < 0.05], although the binding activities of the DP enantiomers were almost the same in fraction 2. By contrast WR enantiomers had a higher binding affinity in fraction 2 than in fraction 3, the differences in dissociation constants between fractions 2 and 3 being 12.6-fold for (S)-WR (P < 0.001) and 8.3-fold for (R)-WR (P < 0.001). The dissociation constant of (S)-WR (0.28 +/- 0.10 microm) was significantly lower than that of (R)-WR (0.48 +/- 0.08 microm) in fraction 2 (95% CI - 0.369, - 0.028; P < 0.05), but there were no significant differences between the binding activities of WR enantiomers in fraction 3.

CONCLUSIONS

DP and WR enantiomers bind preferentially to fraction 3 and fraction 2, respectively. Fractions 2 and 3 are encoded by the AAG A and the AAG B genes, respectively.

Relation between the secondary structure of carbohydrate residues of alpha1-acid glycoprotein (orosomucoid) and the fluorescence of the protein

We studied in this work the relation that exists between the secondary structure of the glycans of alpha(1)-acid glycoprotein and the fluorescence of the Trp residues of the protein. We calculated for that the efficiency of quenching and the radiative and non-radiative constants. Our results indicate that the glycans display a spatial structure that is modified upon asialylation. The asialylated conformation is closer to the protein matrix than the sialylated form, inducing by that a decrease in the fluorescence parameters of the Trp residues. In fact, the mean quantum yield of Trp residues in sialylated and asialylated alpha(1)-acid glycoprotein are 0.0645 and 0.0385, respectively. Analysis of the fluorescence emission of alpha(1)-acid glycoprotein as the result of two contributions (surface and hydrophobic domains) indicates that quantum yields of both classes of Trp residues are lower when the protein is in the asialylated form. Also, the mean fluorescence lifetime of Trp residues decreases from 2.285 ns in the sialylated protein to 1.948 ns in the asialylated one. The radiative rate constant k(r) of the Trp residues in the sialylated alpha(1)-acid glycoprotein is higher than that in the asialylated protein. Thus, the carbohydrate residues are closer to the Trp residues in the absence of sialic acid. The modification of the spatial conformation of the glycans upon asialylation is confirmed by the decrease of the fluorescence lifetimes of Calcofluor, a fluorophore that binds to the carbohydrate residues. Finally, thermal intensity quenching of Calcofluor bound to alpha(1)-acid glycoprotein shows that the carbohydrate residues have slower residual motions in the absence of sialic acid residues.

One step purification of alpha(1)-acid glycoprotein from human plasma. Fractionation of its polymorphic allele products

Alpha(1)-acid glycoprotein is a plasma protein that exhibits both microheterogeneity and polymorphism. Its purification from human plasma is usually performed using a sequence of different fractionation steps. Here we report a one-step isolation technique of this protein based upon pseudo-ligand affinity chromatography on immobilized Cibacron Blue F3GA at acidic pH. In addition, the use of two narrow pH elution buffers allows us to separate the two genetic products of this protein, which differ from each other by 21 amino acid substitutions. This technique will facilitate the study of the structural, biological and pharmacokinetic properties of each individual allele product.

Thermal stability of the human blood serum acid alpha(1)-glycoprotein in acidic media

Thermal stability of human alpha(1)-acid glycoprotein and its desialyzed form were studied in the pH range of 1.5-5.2, i.e. about its pI. Circular dichroism, fluorescence and UV-absorption were used to determine the conformational changes and their reversibility in the temperature range 25-80 degrees C. These changes were tested in a three step process-heating, cooling and a second heating. Principal component analysis was applied for analyzing the spectral sets obtained in these experiments. Fully reversible behavior of Trp residues, as characterized by fluorescence spectroscopy, was observed during the heating process at all pH values. Nevertheless, three different types of the protein motion (reversible, irreversible and rearrangement of the protein core) were determined by UV-absorption spectroscopy. Thus, an environment of Tyr and Phe is modified or reversibly rearranged during the heating process in acid media. These types of alpha(1)-acid glycoprotein behavior were not significantly affected by desialyzation.

Structure of human alpha1-acid glycoprotein and its high-affinity binding site

Secondary and tertiary structures of human blood alpha(1)-acid glycoprotein, a member of the lipocalin family, have been studied for the first time by infrared and Raman spectroscopies. Vibrational spectroscopy confirmed details of the secondary structure and the structure content predicted by homology modeling of the protein moiety, i.e., 15% alpha-helices, 41% beta-sheets, 12% beta-turns, 8% bands, and 24% unordered structure at pH 7.4. Our model shows that the protein folds as a highly symmetrical all-beta protein dominated by a single eight-stranded antiparallel beta-sheet. Thermal dynamics in the range 20-70 degrees C followed by Raman spectroscopy and analyzed by principle component analysis revealed full reversibility of the protein motion upon heating dominated by decreasing of beta-sheets. Raman difference spectroscopy confirmed the proximity of Trp(122) to progesterone binding.