Human serum albumin: from bench to bedside

Unraveling the Interaction between FcRn and Albumin: Opportunities for Design of Albumin-Based Therapeutics

The neonatal Fc receptor (FcRn) was first found to be responsible for transporting antibodies of the immunoglobulin G (IgG) class from the mother to the fetus or neonate as well as for protecting IgG from intracellular catabolism. However, it has now become apparent that the same receptor also binds albumin and plays a fundamental role in homeostatic regulation of both IgG and albumin, as FcRn is expressed in many different cell types and organs at diverse body sites. Thus, to gain a complete understanding of the biological function of each ligand, and also their distribution in the body, an in-depth characterization of how FcRn binds and regulates the transport of both ligands is necessary. Importantly, such knowledge is also relevant when developing new drugs, as IgG and albumin are increasingly utilized in therapy. This review discusses our current structural and biological understanding of the relationship between FcRn and its ligands, with a particular focus on albumin and design of albumin-based therapeutics.

Induced lung inflammation and dietary protein supply affect nitrogen retention and amino acid metabolism in growing pigs

It is hypothesised that during immune system activation, there is a competition for amino acids (AA) between body protein deposition and immune system functioning. The aim of the present study was to quantify the effect of immune system activation on N retention and AA metabolism in growing pigs, depending on dietary protein supply. A total of sixteen barrows received an adequate (Ad) or restricted (Res) amount of dietary protein, and were challenged at day 0 with intravenous complete Freund's adjuvant (CFA). At days - 5, 3 and 8, an irreversible loss rate (ILR) of eight AA was determined. CFA successfully activated the immune system, as indicated by a 2- to 4-fold increase in serum concentrations of acute-phase proteins (APP). Pre-challenge C-reactive protein concentrations were lower (P< 0·05) and pre- and post-challenge albumin tended to be lower in Res-pigs. These findings indicate that a restricted protein supply can limit the acute-phase response. CFA increased urinary N losses (P= 0·04) and tended to reduce N retention in Ad-pigs, but not in Res-pigs (P= 0·07). The ILR for Val was lower (P= 0·05) at day 8 than at day 3 in the post-challenge period. The ILR of most AA, except for Trp, were strongly affected by dietary protein supply and positively correlated with N retention. The correlations between the ILR and APP indices were absent or negative, indicating that changes in AA utilisation for APP synthesis were either not substantial or more likely outweighed by a decrease in muscle protein synthesis during immune system activation in growing pigs.

Kinetic Analyses of Data from a Human Serum Albumin Assay Using the liSPR System

We used the interaction between human serum albumin (HSA) and a high-affinity antibody to evaluate binding affinity measurements by the bench-top ^liSPR system (capitalis technology GmbH). HSA was immobilized directly onto a carboxylated sensor layer, and the mechanism of interaction between the antibody and HSA was investigated. The bivalence and heterogeneity of the antibody caused a complex binding mechanism. Three different interaction models (1:1 binding, heterogeneous analyte, bivalent analyte) were compared, and the bivalent analyte model best fit the curves obtained from the assay. This model describes the interaction of a bivalent analyte with one or two ligands (A + L ↔ LA + L ↔ LLA). The apparent binding affinity for this model measured 37 pM for the first reaction step, and 20 pM for the second step.

Phosphonate monoesters on a thiacalix[4]arene framework as potential inhibitors of protein tyrosine phosphatase 1B

Monoester derivatives of thiacalix[4]arene tetrakis(methylphosphonic) acid were found to be capable of inhibiting protein tyrosine phosphatase 1B. In addition, these compounds can strongly bind to human serum albumin.

Quantum molecular modelling of ibuprofen bound to human serum albumin

The total interaction energies of the ibuprofen complexed with FA3/FA4 and FA6 binding sites of human serum albumin are in agreement with the hypothesis that the Sudlow's site II is the main binding pocket for ibuprofen.

Interaction of the anticancer gallium(III) complexes of 8-hydroxyquinoline and maltol with human serum proteins

Tris(8-quinolinolato)gallium(III) (KP46) and tris(maltolato)gallium(III) (GaM) are promising orally active antitumor metallodrugs currently undergoing clinical trials. Their interaction with human serum albumin (HSA) and transferrin (Tf) was studied in detail in aqueous solution by the combination of various methods such as spectrofluorometry, UV-vis spectrophotometry, (1)H and saturation transfer difference NMR spectroscopy, and ultrafiltration-UV-vis spectrophotometry. Binding data were evaluated quantitatively. Tf was found to replace the original ligand much less efficiently in KP46 than in GaM, whereas a significant noncovalent binding of KP46 with HSA (log K' = 4.04) retaining the coordination environment around gallium(III) was found. The interaction between HSA and KP46 was also confirmed by protein-complex modeling calculations. On the basis of the conditional stability constants, the distribution of gallium(III) in serum was computed and compared for these metallodrugs under physiological conditions, and revealed the prominent role of HSA in the case of KP46 and that of Tf for GaM.

Posttranscriptional changes of serum albumin: clinical and prognostic significance in hospitalized patients with cirrhosis

Beside the regulation of fluid distribution, human serum albumin (HSA) carries other activities, such as binding, transport, and detoxification of many molecules. In patients with cirrhosis, HSA exhibits posttranscriptional alterations that likely affect its functions. This study aimed at identifying the structural HSA alterations occurring in cirrhosis and determining their relationship with specific clinical complications and patient survival. One hundred sixty-eight patients with cirrhosis, 35 with stable conditions and 133 hospitalized for acute clinical complications, and 94 healthy controls were enrolled. Posttranscriptional HSA molecular changes were identified and quantified by using a high-performance liquid chromatography/electrospray ionization mass spectrometry technique. Clinical and biochemical parameters were also recorded and hospitalized patients were followed for up to 1 year. Seven HSA isoforms carrying one or more posttranscriptional changes were identified. Altered HSA isoforms were significantly more represented in patients than in healthy controls. Conversely, the native, unchanged HSA isoform was significantly reduced in cirrhosis. Native HSA and most altered isoforms correlated with both Child-Pugh and Model for End-Stage Liver Disease scores. In hospitalized patients, oxidized and N-terminal truncated isoforms were independently associated with ascites, renal impairment, and bacterial infection. Finally, the native HSA and cysteinylated/N-terminal truncated isoforms were predictors of 1-year survival, with greater prognostic accuracy than total serum albumin concentration.

CONCLUSIONS

Extensive posttranscriptional changes of HSA, involving several molecular sites and increasing in parallel with disease severity, occur in patients with cirrhosis. Altered isoforms are independently associated with specific clinical complications, whereas the residual, native HSA isoform independently predicts patient survival. These findings support the concept of the "effective albumin concentration," which implies that the global HSA function is related not only to its serum concentration, but also to the preservation of its structural integrity.

Characterization of the species-dependent ketoprofen/albumin binding modes by induced CD spectroscopy and TD-DFT calculations

The stereospecificity of high-affinity biorecognition phenomena at the basis of the activity of drugs is an important topic of active research in medicinal chemistry. The binding of drugs to their targets or to carrier proteins may lead to the onset of an induced circular dichroism (ICD) signal, which can be detected experimentally. Quantum mechanical (QM) calculations based on density functional theory (DFT) and its time-dependent formulation (TD-DFT) can be used to determine the theoretical chiroptical response of all the possible conformations of drugs bound to their hosts; by comparison with the experimental ICD spectra of drug-host complexes, this approach can lead to the identification of possible binding modes in the absence of X-ray crystallography or NMR data. The present article reports the application of experimental electronic circular dichroism (ECD) spectroscopy, DFT conformational analysis and TD-DFT calculations to the investigation of the binding modes of (S)-ketoprofen to serum albumins. The peculiar species-dependent ICD spectra observed for the binding of (S)-ketoprofen to different serum albumins can be explained by the selection of different mutual arrangements of the phenyl moieties inside the binding pocket. Such structural elucidations contribute to a better understanding of the changes in the pharmacokinetic and pharmacodynamic profiles of drugs among different species.

An in-depth view of human serum albumin corona on gold nanoparticles

Upon entering biological systems, such as the blood stream, nanoparticles form molecular complexes with the proteins encountered called protein coronas, which shield the surface of the exogenous nanoparticle. The most abundant blood proteins, such as albumin, initially occupy the surface of the nanoparticle. Owing to the widespread applications of gold nanoparticles in medicine, in this study, the docking of human serum albumin to gold nanoparticles was examined and the changes in protein structure were investigated by a molecular dynamic simulation and GOLP force field. The results showed that after the adsorption of albumin on the gold nanoparticle, human serum albumin was denatured and the amount of alpha-helix significantly decreased. Domain III, which has a large cavity of fatty acids binding sites, plays an important role in the adsorption on the gold nanoparticles. Lys464, Thr504, Phe505, and Leu581 are critical amino acids in HSA adsorption on the GNPs. After the adsorption of albumin on the surface of gold nanoparticles, the fluctuations in some of the domains of the protein increased. Variations in the helix properties, such as helix length, dipole, radius, average phi and psi angles, and the length of hydrogen bonds, were calculated in detail.

Albumin hydrogels formed by electrostatically triggered self-assembly and their drug delivery capability

Biological hydrogels are fundamentally biocompatible and have intrinsic similarities to extracellular matrices in medical applications and drug delivery systems. Herein we demonstrate the ability to form drug-eluting protein hydrogels using a novel mechanism that involves the electrostatically triggered partial denaturation and self-assembly of the protein via changes in pH. Partial denaturation increases the protein's solvent exposed hydrophobic surface area, which then drives self-assembly of the protein into a hydrogel within 10 min at 37 °C. We describe the properties of an albumin hydrogel formed by this mechanism. Intrinsic drug binding properties of albumin to all-trans retinoic acid (atRA) are conserved through the partial denaturation process, as confirmed by fluorescence quenching. atRA released from the hydrogel inhibited smooth muscle cell migration as per an in vitro scratch wound assay. Atomistic molecular dynamics and potential of mean force calculations show the preservation and potential creation of new atRA binding sites with a binding energy of -41 kJ/mol. The resulting hydrogel is also biocompatible and exhibits rapid postgelation degradation after its implantation in vivo. This interdisciplinary work provides a new tool for the development of biocompatible protein hydrogel drug delivery systems.

Determination of supplier-to-supplier and lot-to-lot variability in glycation of recombinant human serum albumin expressed in Oryza sativa

The use of different expression systems to produce the same recombinant human protein can result in expression-dependent chemical modifications (CMs) leading to variability of structure, stability and immunogenicity. Of particular interest are recombinant human proteins expressed in plant-based systems, which have shown particularly high CM variability. In studies presented here, recombinant human serum albumins (rHSA) produced in Oryza sativa (Asian rice) (OsrHSA) from a number of suppliers have been extensively characterized and compared to plasma-derived HSA (pHSA) and rHSA expressed in yeast (Pichia pastoris and Saccharomyces cerevisiae). The heterogeneity of each sample was evaluated using size exclusion chromatography (SEC), reversed-phase high-performance liquid chromatography (RP-HPLC) and capillary electrophoresis (CE). Modifications of the samples were identified by liquid chromatography-mass spectrometry (LC-MS). The secondary and tertiary structure of the albumin samples were assessed with far U/V circular dichroism spectropolarimetry (far U/V CD) and fluorescence spectroscopy, respectively. Far U/V CD and fluorescence analyses were also used to assess thermal stability and drug binding. High molecular weight aggregates in OsrHSA samples were detected with SEC and supplier-to-supplier variability and, more critically, lot-to-lot variability in one manufactures supplied products were identified. LC-MS analysis identified a greater number of hexose-glycated arginine and lysine residues on OsrHSA compared to pHSA or rHSA expressed in yeast. This analysis also showed supplier-to-supplier and lot-to-lot variability in the degree of glycation at specific lysine and arginine residues for OsrHSA. Both the number of glycated residues and the degree of glycation correlated positively with the quantity of non-monomeric species and the chromatographic profiles of the samples. Tertiary structural changes were observed for most OsrHSA samples which correlated well with the degree of arginine/lysine glycation. The extensive glycation of OsrHSA from multiple suppliers may have further implications for the use of OsrHSA as a therapeutic product.

A label-free untethered approach to single-molecule protein binding kinetics

Single molecule approaches provide rich real-time dynamics of molecular interactions that are not accessible to ensemble measurements. Previous single molecule studies have relied on labeling and tethering, which alters the natural state of the protein. Here we use the double-nanohole (DNH) optical tweezer approach to measure protein binding kinetics at the single molecule level in a label-free, free-solution (untethered) way. The binding kinetics of human serum albumin (HSA) to tolbutamide and to phenytoin are in quantitative agreement with previous measurements, and our single-molecule approach reveals a biexponential behavior characteristic of a multistep process. The DNH optical tweezer is an inexpensive platform for studying the real-time binding kinetics of protein-small molecule interactions in a label-free, free-solution environment, which will be of interest to future studies including drug discovery.

Development of an improved fractionation of the human plasma proteome by a combination of abundant proteins depletion and multi-lectin affinity chromatography

Aim: Current analytical tools lack the required capacity to reduce the complexity of the plasma proteome and identify low-level proteins of clinical interest. Hence, the need to develop a fractionation approach to provide adequate throughput for a clinical study and minimize the loss and improve the detection of low abundance proteins. Materials & methods: We present the development of an analytical platform that combines the depletion of 12 high abundance proteins and multi-lectin affinity chromatography (12P-M-LAC) fractionation. Results & conclusion: We validated the highly specific, stable and robust 12P-M-LAC platform using human plasma. An improved enrichment of low abundance proteins and glycoproteins with minimum sample loss was achieved demonstrating the suitability of this platform in future biomarker discovery studies.

Protein modification by adenine propenal

Base propenals are products of the reaction of DNA with oxidants such as peroxynitrite and bleomycin. The most reactive base propenal, adenine propenal, is mutagenic in Escherichia coli and reacts with DNA to form covalent adducts; however, the reaction of adenine propenal with protein has not yet been investigated. A survey of the reaction of adenine propenal with amino acids revealed that lysine and cysteine form adducts, whereas histidine and arginine do not. N^ε-Oxopropenyllysine, a lysine–lysine cross-link, and S-oxopropenyl cysteine are the major products. Comprehensive profiling of the reaction of adenine propenal with human serum albumin and the DNA repair protein, XPA, revealed that the only stable adduct is N^ε-oxopropenyllysine. The most reactive sites for modification in human albumin are K190 and K351. Three sites of modification of XPA are in the DNA-binding domain, and two sites are subject to regulatory acetylation. Modification by adenine propenal dramatically reduces XPA’s ability to bind to a DNA substrate.

Studies of metabolite-protein interactions: a review

The study of metabolomics can provide valuable information about biochemical pathways and processes at the molecular level. There have been many reports that have examined the structure, identity and concentrations of metabolites in biological systems. However, the binding of metabolites with proteins is also of growing interest. This review examines past reports that have looked at the binding of various types of metabolites with proteins. An overview of the techniques that have been used to characterize and study metabolite-protein binding is first provided. This is followed by examples of studies that have investigated the binding of hormones, fatty acids, drugs or other xenobiotics, and their metabolites with transport proteins and receptors. These examples include reports that have considered the structure of the resulting solute-protein complexes, the nature of the binding sites, the strength of these interactions, the variations in these interactions with solute structure, and the kinetics of these reactions. The possible effects of metabolic diseases on these processes, including the impact of alterations in the structure and function of proteins, are also considered.

Role of protein-protein interactions in cytochrome P450-mediated drug metabolism and toxicity

Through their unique oxidative chemistry, cytochrome P450 monooxygenases (CYPs) catalyze the elimination of most drugs and toxins from the human body. Protein–protein interactions play a critical role in this process. Historically, the study of CYP–protein interactions has focused on their electron transfer partners and allosteric mediators, cytochrome P450 reductase and cytochrome b5. However, CYPs can bind other proteins that also affect CYP function. Some examples include the progesterone receptor membrane component 1, damage resistance protein 1, human and bovine serum albumin, and intestinal fatty acid binding protein, in addition to other CYP isoforms. Furthermore, disruption of these interactions can lead to altered paths of metabolism and the production of toxic metabolites. In this review, we summarize the available evidence for CYP protein–protein interactions from the literature and offer a discussion of the potential impact of future studies aimed at characterizing noncanonical protein–protein interactions with CYP enzymes.

Multifaceted interplay between lipophilicity, protein interaction and luminescence parameters of non-intercalative ruthenium(II) polypyridyl complexes controlling cellular imaging and cytotoxic properties

Here, we examine the photophysical properties of five ruthenium(II) complexes comprising two 4,7-diphenyl-1,10-phenanthroline (dip) ligands and functionalized bipyridine (R₁bpy-R₂, where R₁ = H or CH₃, R₂ = H, CH₃, COO⁻,4-[3-(2-nitro-1H-imidazol-1-yl)propyl] or 1,3-dicyclohexyl-1-carbonyl-urea) towards development of luminescence probes for cellular imaging. These complexes have been shown to interact with albumin and the formed adducts exhibited up to eightfold increase in the luminescence quantum yield as well as the average lifetime of emission. It was demonstrated that they cannot bind to DNA through the intercalation mode and its luminescence in the presence of DNA is quenching. Cell viability experiments indicated that all complexes possess significant dose-dependent cytotoxicity (with IC₅₀ 5–19 μM) on 4T1 breast cancer cell line and their anti-proliferative activity correlates very well with their lipophilicity. Cellular uptake was studied by measuring the ruthenium content in cells using ICP-MS technique. As expected, the better uptake is directly related to higher lipophilicity of doubly charged ruthenium complexes while uptake of monocationic one is much lower in spite of the highest lipophilicity. Additionally staining properties were assessed using flow cytometry and fluorescence microscopy. These experiments showed that complex with 1,3-dicyclohexyl-1-carbonyl-urea substituent exhibits the best staining properties in spite of the lowest luminescence quantum yield in buffered solution (pH 7.4). Our results point out that both the imaging and cytotoxic properties of the studied ruthenium complexes are strongly influence by the level of internalization and protein interaction.

The five-to-six-coordination transition of ferric human serum heme-albumin is allosterically-modulated by ibuprofen and warfarin: a combined XAS and MD study

Human serum albumin (HSA) is involved physiologically in heme scavenging; in turn, heme-albumin (HSA-heme-Fe) displays globin-like properties. Here, the allosteric effect of ibuprofen and warfarin on the local atomic structure around the ferric heme-Fe (heme-Fe(III)) atom of HSA-heme-Fe (HSA-heme-Fe(III)) has been probed by Fe-K edge X-ray absorption spectroscopy (XAS). The quantitative analysis of the Fe-K edge extended X-ray absorption fine structure (EXAFS) signals and modeling of the near edge (XANES) spectral features demonstrated that warfarin and ibuprofen binding modify the local structure of the heme-Fe(III). Combined XAS data analysis and targeted molecular dynamics (MD) simulations provided atomic resolution insights of protein structural rearrangements required to accommodate the heme-Fe(III) upon ibuprofen and warfarin binding. In the absence of drugs, the heme-Fe(III) atom is penta-coordinated having distorted 4+1 configuration made by the nitrogen atoms of the porphyrin ring and the oxygen phenoxy atom of the Tyr161 residue. MD simulations show that ibuprofen and warfarin association to the secondary fatty acid (FA) binding site 2 (FA2) induces a reorientation of domain I of HSA-heme-Fe(III), this leads to the redirection of the His146 residue providing an additional bond to the heme-Fe(III) atom, providing the 5+1 configuration. The comparison of Fe-K edge XANES spectra calculated using MD structures with those obtained experimentally confirms the reliability of the proposed structural model. As a whole, combining XAS and MD simulations it has been possible to provide a reliable model of the heme-Fe(III) atom coordination state and to understand the complex allosteric transition occurring in HSA-heme-Fe(III) upon ibuprofen and warfarin binding.

Human serum albumin, systemic inflammation, and cirrhosis

Human serum albumin (HSA) is one of the most frequent treatments in patients with decompensated cirrhosis. Prevention of paracentesis-induced circulatory dysfunction, prevention of type-1 HRS associated with bacterial infections, and treatment of type-1 hepatorenal syndrome are the main indications. In these indications treatment with HSA is associated with improvement in survival. Albumin is a stable and very flexible molecule with a heart shape, 585 residues, and three domains of similar size, each one containing two sub-domains. Many of the physiological functions of HSA rely on its ability to bind an extremely wide range of endogenous and exogenous ligands, to increase their solubility in plasma, to transport them to specific tissues and organs, or to dispose of them when they are toxic. The chemical structure of albumin can be altered by some specific processes (oxidation, glycation) leading to rapid clearance and catabolism. An outstanding feature of HSA is its capacity to bind lipopolysaccharide and other bacterial products (lipoteichoic acid and peptidoglycan), reactive oxygen species, nitric oxide and other nitrogen reactive species, and prostaglandins. Binding to NO and prostaglandins are reversible, so they can be transferred to other molecules at different sites from their synthesis. Through these functions, HSA modulates the inflammatory reaction. Decompensated cirrhosis is a disease associated systemic inflammation, which plays an important role in the pathogenesis of organ or system dysfunction/failure. Although, the beneficial effects of HAS have been traditionally attributed to plasma volume expansion, they could also relate to its effects modulating systemic and organ inflammation.

Nitrosylation mechanisms of Mycobacterium tuberculosis and Campylobacter jejuni truncated hemoglobins N, O, and P

Truncated hemoglobins (trHbs) are widely distributed in bacteria and plants and have been found in some unicellular eukaryotes. Phylogenetic analysis based on protein sequences shows that trHbs branch into three groups, designated N (or I), O (or II), and P (or III). Most trHbs are involved in the O₂/NO chemistry and/or oxidation/reduction function, permitting the survival of the microorganism in the host. Here, a detailed comparative analysis of kinetics and/or thermodynamics of (i) ferrous Mycobacterium tubertulosis trHbs N and O (Mt-trHbN and Mt-trHbO, respectively), and Campylobacter jejuni trHb (Cj-trHbP) nitrosylation, (ii) nitrite-mediated nitrosylation of ferrous Mt-trHbN, Mt-trHbO, and Cj-trHbP, and (iii) NO-based reductive nitrosylation of ferric Mt-trHbN, Mt-trHbO, and Cj-trHbP is reported. Ferrous and ferric Mt-trHbN and Cj-trHbP display a very high reactivity towards NO; however, the conversion of nitrite to NO is facilitated primarily by ferrous Mt-trHbN. Values of kinetic and/or thermodynamic parameters reflect specific trHb structural features, such as the ligand diffusion pathways to/from the heme, the heme distal pocket structure and polarity, and the ligand stabilization mechanisms. In particular, the high reactivity of Mt-trHbN and Cj-trHbP reflects the great ligand accessibility to the heme center by two protein matrix tunnels and the E7-path, respectively, and the penta-coordination of the heme-Fe atom. In contrast, the heme-Fe atom of Mt-trHbO the ligand accessibility to the heme center of Mt-trHbO needs large conformational readjustments, thus limiting the heme-based reactivity. These results agree with different roles of Mt-trHbN, Mt-trHbO, and Cj-trHbP in vivo.

Albumin administration in the acutely ill: what is new and where next?

Albumin solutions have been used worldwide for the treatment of critically ill patients since they became commercially available in the 1940s. However, their use has become the subject of criticism and debate in more recent years. Importantly, all fluid solutions have potential benefits and drawbacks. Large multicenter randomized studies have provided valuable data regarding the safety of albumin solutions, and have begun to clarify which groups of patients are most likely to benefit from their use. However, many questions remain related to where exactly albumin fits within our fluid choices. Here, we briefly summarize some of the physiology and history of albumin use in intensive care before offering some evidence-based guidance for albumin use in critically ill patients.

Metal ions bound to the human milk immunoglobulin A: metalloproteomic approach

The presence of calcium, iron, and zinc bound to human milk secretory IgA (sIgA) was investigated. The sIgA components were first separated by two-dimensional polyacrylamide gel electrophoresis and then identified by electrospray ionization-tandem mass spectrometry (ESI MS MS). The metal ions were detected by flame atomic absorption spectrometry after acid mineralization of the spots. The results showed eight protein spots corresponding to the IgA heavy chain constant region. Another spot was identified as the transmembrane secretory component. Calcium was bound to both the transmembrane component and the heavy chain constant region, while zinc was bound to the heavy chain constant region and iron was not bound with the identified proteins. The association of a metal ion with a protein is important for a number of reasons, and therefore, the findings of the present study may lead to a better understanding of the mechanisms of action and of additional roles that sIgA and its components play in human milk.

Tandem mass spectral libraries of peptides in digests of individual proteins: Human Serum Albumin (HSA)

This work presents a method for creating a mass spectral library containing tandem spectra of identifiable peptide ions in the tryptic digestion of a single protein. Human serum albumin (HSA(1)) was selected for this purpose owing to its ubiquity, high level of characterization and availability of digest data. The underlying experimental data consisted of ∼3000 one-dimensional LC-ESI-MS/MS runs with ion-trap fragmentation. In order to generate a wide range of peptides, studies covered a broad set of instrument and digestion conditions using multiple sources of HSA and trypsin. Computer methods were developed to enable the reliable identification and reference spectrum extraction of all peptide ions identifiable by current sequence search methods. This process made use of both MS2 (tandem) spectra and MS1 (electrospray) data. Identified spectra were generated for 2918 different peptide ions, using a variety of manually-validated filters to ensure spectrum quality and identification reliability. The resulting library was composed of 10% conventional tryptic and 29% semitryptic peptide ions, along with 42% tryptic peptide ions with known or unknown modifications, which included both analytical artifacts and post-translational modifications (PTMs) present in the original HSA. The remaining 19% contained unexpected missed-cleavages or were under/over alkylated. The methods described can be extended to create equivalent spectral libraries for any target protein. Such libraries have a number of applications in addition to their known advantages of speed and sensitivity, including the ready re-identification of known PTMs, rejection of artifact spectra and a means of assessing sample and digestion quality.

Identification of Staphylococcus aureus colony-spreading stimulatory factors from mammalian serum

Staphylococcus aureus forms giant colonies on soft-agar surfaces, which is called colony-spreading. In the present study, we searched for host factors that influence S. aureus colony-spreading activity. The addition of calf serum, porcine serum, or silkworm hemolymph to soft-agar medium stimulated S. aureus colony-spreading activity. Gel filtration column chromatography of calf serum produced a high molecular weight fraction and a low molecular weight fraction, both of which exhibited colony-spreading stimulatory activity. In the low molecular weight fraction, we identified the stimulatory factor as bovine serum albumin. The stimulatory fraction in the high molecular weight fraction was identified as high-density lipoprotein (HDL) particles. Delipidation of HDL abolished the stimulatory activity of HDL. Phosphatidylcholine, which is the major lipid component in HDL particles, stimulated the colony-spreading activity. Other phosphatidylcholine-containing lipoprotein particles, low-density lipoprotein and very low-density lipoprotein, also showed colony-spreading stimulatory activity. These findings suggest that S. aureus colony-spreading activity is stimulated by albumin and lipoprotein particles in mammalian serum.

Role of the flavan-3-ol and galloyl moieties in the interaction of (-)-epigallocatechin gallate with serum albumin

The principal green tea polyphenol, (-)-epigallocatechin-3-O-gallate (EGCg), may provide chemoprotection against conditions ranging from cardiovascular disease to cancer. Binding to plasma proteins stabilizes EGCg during its transport to targeted tissues. This study explored the details EGCg binding to bovine serum albumin. Both fluorescence lifetime and intensity data showed that the hydrophobic pocket between subdomains IIA and IIIA is the binding site for EGCg. Fluorescence and circular dichroism were used to establish the roles of the flavan-3-ol and galloyl moieties of the EGCg in binding and to demonstrate a binding-dependent conformational change in the protein. Competitive binding experiments confirmed the location of binding, and molecular modeling identified protein residues that play key roles in the interaction. This model of EGCg-BSA interactions improves the understanding of the likely physiological fate of this green tea-derived bioactive polyphenol.

The role of caprylate ligand ion on the stabilization of human serum albumin

Sodium caprylate was added to a pharmaceutical-grade human serum albumin (HSA) to stabilize the product. In this study we have aimed to establish how caprylate ligand protects HSA from thermal degradation. The fatty acid stabilizer was first removed from commercial HSA by charcoal treatment. Cleaned HSA was made to 10% w/v in pH 7.4 buffered solutions and doped with sodium caprylate in serial concentrations up to 0.16 mmol/g-protein. These solutions as well as a commercial HSA, human serum, and enriched-albumin fraction were subjected to differential scanning calorimetry (DSC) within the temperature range of 37-90°C at a 5.0°C/min scanning rate. The globular size of the cleaned HSA solutions was measured by dynamic light scattering. The denaturing temperatures for albumin with sodium caprylate and a commercial one were significantly higher than for albumin only. It was found that the protein globules of cleaned HSA were not as stable as that of the native one due to aggregation, and the caprylate ion may reduce the aggregation by enlarging the globules' electrical double layer. A rational approximation of the Lumry-Eyring protein denaturation model was used to treat DSC denaturing endotherms. The system turned from irreversible dominant Scheme: N (k3K)→ P to reversible dominant Scheme:N (k1)→ P with the increase in caprylate concentration from null to ~0.08 mmol/g-protein. It was postulated that the caprylate ligand may decrease the rate of reversible unfolding as it binds to the IIIA domain which is prone to reversible unfolding/refolding and causes further difficulty for irreversible denaturation which, in turn, HSA can be stabilized.

A novel tumor targeting drug carrier for optical imaging and therapy

Human serum albumin (HSA), a naturally abundant protein in blood plasma and tissue fluids, has an extraordinary ligand-binding capacity and is advocated as a drug carrier to facilitate drug delivery. To render it tumor targeting specificity, we generated a recombinant HSA fused with the amino-terminal fragment (ATF) of urokinase, allowing the fusion protein to bind to urokinase receptor (uPAR), which is shown to have a high expression level in many tumors, but not in normal tissues. To test the efficacy of this bifunctional protein (ATF-HSA), a hydrophobic photosensitizer (mono-substituted β-carboxy phthalocyanine zinc, CPZ) was chosen as a cytotoxic agent. A dilution-incubation-purification (DIP) strategy was developed to load the ATF-HSA with this CPZ, forming a 1:1 molecular complex (ATF-HSA:CPZ). We demonstrated that CPZ was indeed embedded inside ATF-HSA at the fatty acid binding site 1 (FA1) of HSA, giving a hydrodynamic radius of 7.5 nm, close to HSA's (6.5 nm). ATF-HSA:CPZ showed high stability and remarkable optical and photophysical properties in aqueous solution. In addition, the molecular complex ATF-HSA:CPZ can bind to recombinant uPAR in vitro and uPAR on tumor cell surfaces, and was efficient in photodynamic killing of tumor cells. The tumor-killing potency of this molecular complex was further demonstrated in a tumor-bearing mouse model at a dose of 0.080 μmol / kg, or 0.050 mg CPZ / kg of mouse body weight. Using fluorescent molecular tomography (FMT), ATF-HSA:CPZ was shown to accumulate specifically in tumors, and importantly, such tumor retention was higher than that of HSA:CPZ. Together, these results indicate that ATF-HSA:CPZ is not only an efficient tumor-specific cytotoxic agent, but also an useful tumor-specific imaging probe. This bifunctional protein ATF-HSA can also be used as a drug carrier for other types of cytotoxic or imaging agents to render them specificity for uPAR-expressing tumors.

Interactive association of drugs binding to human serum albumin

Human serum albumin (HSA) is an abundant plasma protein, which attracts great interest in the pharmaceutical industry since it can bind a remarkable variety of drugs impacting their delivery and efficacy and ultimately altering the drug’s pharmacokinetic and pharmacodynamic properties. Additionally, HSA is widely used in clinical settings as a drug delivery system due to its potential for improving targeting while decreasing the side effects of drugs. It is thus of great importance from the viewpoint of pharmaceutical sciences to clarify the structure, function, and properties of HSA–drug complexes. This review will succinctly outline the properties of binding site of drugs in IIA subdomain within the structure of HSA. We will also give an overview on the binding characterization of interactive association of drugs to human serum albumin that may potentially lead to significant clinical applications.