Protein binding investigation by difference circular dichroism: Native and acetylated human serum albumins

A modified albumin was prepared by selective reaction of Lys₁₉₉ with acetyl salicylic acid. Protein binding investigation was carried out on native and modified proteins by difference circular dichroism (ΔCD). Acetylation of Lys₁₉₉ reduces significantly the effects of the interaction between drugs in the stereoselective HSA binding at specific binding areas. © 1995 Wiley-Liss, Inc.     

Drug binding to human serum albumin: abridged review of results obtained with high-performance liquid chromatography and circular dichroism

The drug binding to plasma and tissue proteins are fundamental factors in determining the overall pharmacological activity of a drug. Human serum albumin (HSA), together with alpha1-acid glycoprotein (AGP), are the most important plasma proteins, which act as drug carriers, with drug pharmacokinetic implications, resulting in important clinical impacts for drugs that have a relatively narrow therapeutic index. This review focuses on the combination of biochromatography and circular dichroism as an effective approach for the characterization of albumin binding sites and their enantioselectivity. Furthermore, their applications to the study of changes in the binding properties of the protein arising by the reversible or covalent binding of drugs are discussed, and examples of physiological relevance reported. Perspectives of these studies reside in supporting the development of new drugs, which require miniaturization to facilitate the screening of classes of compounds for their binding to the target protein, and a deeper characterization of the mechanisms involved in the molecular recognition processes.     

Improved chromatographic performance of a modified human albumin based stationary phase

Derivatization of the free cys₃₄ in human serum albumin (HSA) anchored to a silica matrix has been performed by in situ reaction with ethacrynic acid. This modification, which is reported to occur under physiological conditions, gives rise in practice to a new column with different binding properties with respect to the column based on the native protein. Significant differences were observed in the binding of drugs known to bind to site I, (R)-(S)-warfarin and phenylbutazone, and to site II, 1,4-benzodiazepin-2-ones and nonsteroidal anti-inflammatory agents. In particular, the chromatographic retentions markedly decreased for most of the drugs, and, in the case of chiral compounds, significant differences were often observed in the behavior of the two enantiomers, with higher values of enantioselectivity obtained for some of the examined compounds. Furthermore, the noncovalent binding of ethacrynic acid to the protein modifies the binding properties of the albumin. Chirality 9:335–340, 1997. © 1997 Wiley-Liss, Inc.     

Flavonoid binding to human serum albumin

Dietary flavonoid may have beneficial effects in the prevention of chronic diseases. However, flavonoid bioavailability is often poor probably due to their interaction with plasma proteins. Here, the affinity of daidzein and daidzein metabolites as well as of genistein, naringenin, and quercetin for human serum albumin (HSA) has been assessed in the absence and presence of oleate. Values of the dissociation equilibrium constant (K) for binding of flavonoids and related metabolites to Sudlow’s site I range between 3.3 × 10⁻⁶ and 3.9 × 10⁻⁵ M, at pH 7.0 and 20.0 °C, indicating that these flavonoids are mainly bound to HSA in vivo. Values of K increase (i.e., the flavonoid affinity decreases) in the presence of saturating amounts of oleate by about two folds. Present data indicate a novel role of fatty acids as allosteric inhibitors of flavonoid bioavailability, and appear to be relevant in rationalizing the interference between dietary compounds, food supplements, and drugs.     

Reversible human serum albumin binding of lipocrine: a circular dichroism study

Lipocrine has been selected as an effective candidate for in vivo investigation because of its multiple biological properties, namely inhibition of AChE and BChE activities, inhibition of AChE-induced Aβ aggregation, and ability to protect cells against reactive oxygen species. To evaluate the possibility for lipocrine to become a lead and to be developed as a multipotent drug for the treatment of Alzheimer's disease, ADMET (absorption, distribution, metabolism, excretion, and toxicity) parameters need to be determined. Among ADMET parameters, distribution plays a key role in determining the lead drugability, and the drug binding to plasma proteins greatly influences the drug distribution. Here, the human serum albumin (HSA) binding of lipocrine has been studied by circular dichroism (CD) spectroscopy. The reversible binding of lipocrine is stereoselective as shown by the well-defined induced CD spectrum in its binding to HSA. The intensity of the CD signal changes upon changing the [drug]/[HSA] molar ratio, showing a different behavior for a [drug]/[HSA] up to 2/1 or over this molar ratio, suggesting a binding to multiple sites. Competition experiments show that lipocrine interacts significantly with all the main binding sites on the serum carrier. A direct competition has been monitored for site II and bilirubin-binding site, whereas a noncooperative binding should better describe the displacement observed at site I. Rac-lipocrine and its enantiomers are characterized by two different binding modes. Almost the same induced CD spectra were obtained for both (R)- and (S)-lipocrine complexed to HSA, suggesting a similar stereochemistry for the bound enantiomers.     

Stereoselective binding of carbenicillin epimers to human serum albumin

Binding of carbenicillin (CBPC) epimers to human serum albumin (HSA) was found to be stereoselective. Epimer-epimer interaction was also observed in the binding to HSA. There were at least three binding sites on HSA for CBPC epimers, one of which (stereoselective site) was more in favor of S-CBPC than R-CBPC. At the stereoselective site, the binding constant of S-CBPC was approximately 4-fold greater than that of R-CBPC. The affinities to other binding sites (non-stereoselective sites) were similar between the epimers, and the affinity of S-CBPC of the non-stereoselective sites was much smaller than that for the stereoselective site. R-CBPC and S-CBPC appeared to displace each other at all the binding sites, i.e., the binding of the epimers was competitive at the non-stereoselective sites as well as at the stereoselective site. By using site marker ligands, it was revealed that CBPC epimers may bind to Site I (warfarin binding site), but not to Site II (diazepam binding site). A binding model with an assumption of competitive interactions at all the binding sites simulated the binding characteristics of CBPC epimers fairly well. © 1996 Wiley-Liss, Inc.     

Unveiling the binding mode of perfluorooctanoic acid to human serum albumin

Perfluorooctanoic acid (PFOA) is a toxic compound that is absorbed and distributed throughout the body by noncovalent binding to serum proteins such as human serum albumin (hSA). Though the interaction between PFOA and hSA has been already assessed using various analytical techniques, a high resolution and detailed analysis of the binding mode is still lacking. We report here the crystal structure of hSA in complex with PFOA and a medium‐chain saturated fatty acid (FA). A total of eight distinct binding sites, four occupied by PFOAs and four by FAs, have been identified. In solution binding studies confirmed the 4:1 PFOA‐hSA stoichiometry and revealed the presence of one high and three low affinity binding sites. Competition experiments with known hSA‐binding drugs allowed locating the high affinity binding site in sub‐domain IIIA. The elucidation of the molecular basis of the interaction between PFOA and hSA might provide not only a better assessment of the absorption and elimination mechanisms of these compounds in vivo but also have implications for the development of novel molecular receptors for diagnostic and biotechnological applications.     

Luminescence,circular dichroism and in silico studies of binding interaction of synthesized naphthylchalcone derivatives with bovine serum albumin

Chalcones possess various biological properties, for example, antimicrobial, anti‐inflammatory, analgesic, antimalarial, anticancer, antiprotozoal and antitubercular activity. In this study, naphthylchalcone derivatives were synthesized and characterized using ¹H NMR ¹³C NMR, Fourier transform infrared and mass techniques. Yields for all derivatives were found to be >90%. Protein–drug interactions influence the absorption, distribution, metabolism and excretion (ADME) properties of a drug. Therefore, to establish whether the synthesized naphthylchalcone derivatives can be used as drugs, their binding interaction toward a serum protein (bovine serum albumin) was investigated using fluorescence, circular dichroism and molecular docking techniques under physiological conditions. Fluorescence quenching of the protein in the presence of naphthylchalcone derivatives, and other derived parameters such as association constants, number of binding sites and static quenching involving confirmed non‐covalent binding interactions in the protein–ligand complex were observed. Circular dichroism clearly showed changes in the secondary structure of the protein in the presence of naphthylchalcones, indicating binding between the derivatives and the serum protein. Molecular modelling further confirmed the binding mode of naphthylchalcone derivatives in bovine serum albumin. A site‐specific molecular docking study of naphthylchalcone derivatives with serum albumin showed that binding took place primarily in the aromatic low helix and then in subdomain II. The dominance of hydrophobic, hydrophilic and hydrogen bonding was clearly visible and was responsible for stabilization of the complex.     

Chain length-dependent binding of fatty acid anions to human serum albumin studied by site-directed mutagenesis

Human serum albumin is the most abundant protein in the circulatory system, and one of its principal functions is to transport fatty acids. Binding of octanoate, decanoate, laurate and myristate was studied by a rate-of-dialysis technique. The primary association constants increased, but not linearly, with chain length. The number of high-affinity sites also increased with chain length; octanoate and decanoate bind to one such site, whereas laurate and myristate most probably bind to two sites. Albumin is composed of three homologous helical domains (I-III), which can be subdivided into two subdomains (A and B). For getting information about the positions of the high-affinity sites we produced 13 recombinant isoforms mutated in four different subdomains. Results obtained with these albumins are in accordance with the following model: octanoate and decanoate bind to a single site in subdomain IIIA, laurate binds to sites in subdomains IIIA and IIIB, whereas myristate binds in subdomains IB and IIIB. The results also showed that primary fatty acid binding is sensitive to amino acid substitutions in other parts of the protein. This is in contrast to the effect of amino acid substitutions of genetic albumin variants (alloalbumins). Usually these substitutions, which are situated at the surface of the protein, have no effect on fatty acid binding. Binding of fatty acid anions to different high-affinity sites and the sensitivity of these sites to amino acid substitutions elsewhere in the protein (and perhaps also to other types of modifications) are important factors that could effect simultaneous binding of other ligands, e.g. in patients treated with albumin-binding drugs.     

Binding of amifostine to human serum albumin: A biophysical study

The aim of this present work is to investigate the interaction between amifostine and human serum albumin (HSA) in simulated physiological conditions by spectroscopic methods to reveal potential toxic effects of the drug. The results reflected that amifostine caused fluorescence quenching of HSA through a static quenching process, which was further confirmed by the electrochemical experiments. The binding constants at 290, 297 and 304 K were obtained as 2.53 × 10⁵/M, 8.13 × 10⁴/M and 3.59 × 10⁴/M, respectively. There may be one binding site of amifostine on HSA. The thermodynamic parameters indicated that the interaction between amifostine and HSA was driven mainly by hydrogen bonding and electrostatic forces. Synchronous fluorescence spectra, circular dichroism and Fourier transform infrared spectroscopy results showed amifostine binding slightly changed the conformation of HSA with secondary structural content changes. Förster resonance energy transfer study revealed high possibility of energy transfer with amifostine‐Trp‐214 distance of 3.48 nm. The results of the present study may provide valuable information for studying the distribution, toxicological and pharmacological mechanisms of amifostine in vivo. Copyright © 2014 John Wiley & Sons, Ltd.     

Enantioselective inhibition of the binding of rac-profens to human serum albumin induced by lithocholate

The reversible binding of lithocholate to human serum albumin determines a decrease of the binding of rac-ketoprofen. The process was followed by displacement chromatography using increasing concentrations of the competitor, i.e., lithocholate, in the mobile phase. The inhibition of rac-ketoprofen binding resulting was enantioselective and greater displacement was observed for the (S) enantiomer. The displacement process resulting was competitive in nature, the two enantiomers of ketoprofen binding to the same binding site as the modifier. The investigation was extended to other nonsteroidal antiinflammatory drugs. The enantioselective binding inhibition was larger in the case of rac-naproxen and rac-suprofen with respect to the phenomenon observed in the case of rac-ketoprofen. The difference in circular dichroism spectroscopy was also used to characterize the binding of lithocholate to human serum albumin. This bile acid was proven to bind to site II on human serum albumin. The results, as obtained by displacement chromatography and difference circular dichroism spectroscopy, strongly support the hypothesized role of bile acids in inducing the enantioselective inhibition of ketoprofen binding to human serum albumin in patients suffering from liver diseases.     

Studies of human transcortin at different pHs: circular dichroism, polymerisation and binding affinity.

The transcortin we have used in this work is extremely pure. This was shown by the polymerisation observed at pH 4. This polymerisation is never observed with an impure form of transcortin [4]. Moreover, since it is known that the presence of cortisol in the binding site is an essential condition to the activity of purified transcortin [5], it appears that a correlation between the secondary structure and the biological activity of the transcortin exists. The results we have obtained are summarized below: (1) The inhibition of the transcortin binding capacity essentially takes place between pH 5 and 4. (2) A reorganisation of the structure of the protein moiety is observed between pH 6.5 and 5.9. (3) A decrease of the helicity ratio is observed between pH 5 and 4. It appears therefore that, in the limits of experimental accuracy of CD measurements to determine the amount of beta-structure, no appreciable change of binding activity is taking place after the appearance of a large percentage of beta-structure between pH 6.5 and 6. On the other hand, the sudden decrease of protein activity at low pH is likely to be correlated with the disappearance of a well-defined helical region. Other biochemical and physical experiments would be of course necessary, in order to precise this first observation of a structure-function relationship in transcortin.     

Surface characterization of human serum albumin and sodium perfluorooctanoate mixed solutions by pendant drop tensiometry and circular dichroism

The interfacial behavior of mixed human serum albumin (HSA)/sodium perfluorooctanoate (C8FONa) solutions is examined by using two experimental techniques, pendant drop tensiometry and circular dichroism spectroscopy. Through the analysis of the surface tension of the mixed solutions, surface competitive adsorption at the air-water interface between C8FONa and HSA is detected. The dynamic adsorption curves exhibit the distinct regimes in their time-dependent surface tension. The nature of these regimes is further analyzed in terms of the variation of the molecules surface areas. As a consequence, a compact and dense structure was formed where protein molecules were interconnected and overlapped. Thus, a reduction of the area occupied per molecule from 100 to 0.2 nm(2) is interpreted as a gel-like structure at the surface. The presence of the surfactant seems to favor the formation of this interfacial structure. Finally, measurements of circular dichroism suggests a compaction of the protein due to the association with the surfactant given by an increase of alpha-helix structure in the complexes as compared to that of pure protein.     

Stereoselectivity and enantiomer-enantiomer interactions in the binding of ibuprofen to human serum albumin

Binding of ibuprofen (IB) enantiomers to human serum albumin (HSA) was studied using a chiral fluorescent derivatizing reagent, which enabled the measurement of IB enantiomers at a concentration as low as 5 × 10⁻⁸ M. Scatchard analyses revealed that there were two classes of binding sites for both enantiomers. For the high affinity site, the number of the binding sites was one for both enantiomers, and the binding constant of R-IB was 2.3-fold greater than that of S-IB. The difference in the affinity at the high affinity site may result in the stereoselective binding of IB enantiomers at therapeutic concentrations. It was confirmed that the high affinity site of IB enantiomers is Site II (diazepam binding site) by using site marker ligands. Also, significant enantiomer-enantiomer interactions were observed in the binding. The binding data were quantitatively analyzed and a binding model with an assumption of competitive interactions only at the high affinity site simulated the binding characteristics of IB enantiomers fairly well. Chirality 9:643–649, 1997. © 1997 Wiley-Liss, Inc.     

Spectroscopic and molecular modeling approaches to investigate the binding of proton pump inhibitors to human serum albumin

The interaction between two proton pump inhibitors viz., omeprazole (OME) and esomeprazole (EPZ) with human serum albumin (HSA) was studied by fluorescence, absorption, circular dichroism (CD), Fourier transform infrared spectroscopy (FT-IR), voltammetry, and molecular modeling approaches. The Stern–Volmer quenching constants (K_sv) for OME-HSA and EPZ-HSA systems obtained at different temperatures revealed that both OME and EPZ quenched the intensity of HSA through dynamic mode of quenching mechanism. The binding constants of OME-HSA and EPZ-HSA increased with temperature, indicating the increased stability of these systems at higher temperatures. Thermodynamic parameters viz., ?H^°, ?S^°, and ?G^° were determined for both systems. These values revealed that both systems were stabilized by hydrophobic forces. The competitive displacement and molecular docking studies suggested that OME/EPZ was bound to Sudlow’s site I in subdomain IIA in HSA. The extent of energy transfer from HSA to OME/EPZ and the distance of separation in tryptophan (Trp214) Trp214-OME and Trp214-EPZ was determined based on the theory of fluorescence resonance energy transfer. UV absorption, 3D fluorescence, and CD studies indicated that the binding of OME/EPZ to HSA has induced micro environmental changes around the protein which resulted changes in its secondary structure.     

Insights into the binding of paclitaxel to human serum albumin: multispectroscopic studies

The interaction of paclitaxel with human serum albumin (HSA) was studied using fluorescence, resonance light scattering, ultraviolet‐visible, circular dichroism and Fourier transform infrared spectroscopy at pH 7.4. Fluorescence data revealed that the fluorescence quenching of HSA by paclitaxel was a static quenching procedure. Time‐resolved fluorescence data also confirmed the quenching mode, which present a constant decay time of about 5 ns. The binding sites were approximately 1 and the binding constant suggested a weak association (324/M at 298 K), which is helpful for the release of the drug to targeted organs. The thermodynamic parameters, ΔG^○, ΔH° and ΔS° were calculated as – 1.06 × 10⁴ J/mol, 361 J/mol per K and 9.7 × 10⁴ J/mol respectively at 298 K, suggesting that binding was spontaneous and was driven mainly by hydrophobic interactions. The binding distance between HSA and paclitaxel was determined to be 2.23 nm based on the Förster theory. Analysis of circular dichroism, ultraviolet‐visible, three‐dimensional fluorescence, Fourier transform infrared and resonance light scattering spectra demonstrated that HSA conformation was slightly altered in the presence of paclitaxel and dimension of the individual HSA molecules were larger after interacting with paclitaxel. These results were confirmed by a molecular docking study. Copyright © 2013 John Wiley & Sons, Ltd.     

Chiral recognition of metalaxyl enantiomers by human serum albumin: evidence from molecular modeling and photophysical approach

Metalaxyl is an acylamine fungicide, belonging to the most widely known member of the amide group. This task is aimed to scrutinize binding region and spatial structural change of principal vector human serum albumin (HSA) complex with (R)-/(S)-metalaxyl by exploiting molecular modeling, steady-state and time-resolved fluorescence, and circular dichroism (CD) approaches. According to molecular modeling, (R)-metalaxyl is situated within subdomains IIA and IIIA and the affinity of site I with (R)-metalaxyl is greater than site II, whereas (S)-metalaxyl is only located at subdomain IIA and the affinity of (S)-metalaxyl with site I is superior compared with that with (R)-metalaxyl. This coincides with the competitive ligand binding, guanidine hydrochloride-induced unfolding of protein, and hydrophobic 8-anilino-1-naphthalenesulfonic acid experiments; the acting forces between (R)-/(S)-metalaxyl and HSA are hydrophobic, π-π interactions, and hydrogen bonds, as derived from molecular modeling. Fluorescence emission manifested that the complex of (R)-/(S)-metalaxyl to HSA is the formation of adduct with an affinity of 10(4) M(-1), which corroborates the time-resolved fluorescence that the static type was operated. Furthermore, the changes of far-UV CD spectra evidence the polypeptide chain of HSA partially unfolded after conjugation with (R)-/(S)-metalaxyl. Through this work, we envisage that it can offer central clues on the biodistribution, absorption, and bioaccumulation of (R)-/(S)-metalaxyl.     

Ultraviolet-circular dichroism spectroscopy and potentiometric study of the interaction between human serum albumin and sodium perfluorooctanoate

The interaction of a fluorinated surfactant, sodium perfluorooctanoate, with human serum albumin (HSA) has been investigated by a combination of ultraviolet-circular dichroism (UV-CD) spectroscopy and potentiometry (by a home-built ion-selective electrode) techniques to detect and characterize the conformational transitions of HSA. By using difference spectroscopy, the transition was followed as a function of temperature, and the data were analyzed to obtain the parameters characterizing the thermodynamics of unfolding. The results indicate that the presence of surfactant drastically changes the melting unfolding, acting as a structure stabilizer and delaying the unfolding process. Potentiometric measurements were used to determine the binding isotherms and binding capacity for this system. The isotherm shows a high affinity of surfactant molecules for HSA. The average number of surfactant molecules absorbed per protein molecule (at 28 mM of surfactant concentration) was found to be approximately 900, about 6 g of surfactant per gram of protein. The shape of the binding capacity curve and the relation between binding capacity and extend of cooperativity were examined. From these analysis, the values of g (number of ligand-binding sites), KH (Hill binding constant), and nH (Hill coefficient) were determined.