Interaction of aspirin and vitamin C with bovine serum albumin

Vitamin C (L-ascorbic acid) has a major biological role as a natural antioxidant. Aspirin belongs to the nonsteroidal anti-inflammatory drugs and functions as an antioxidant via its ability to scavenge-OH radicals. Bovine serum albumin (BSA) is the major soluble protein constituent of the circulatory system and has many physiological functions including transport of a variety of compounds. In this report, the competitive binding of vitamin C and aspirin to bovine serum albumin has been studied using constant protein concentration and various drug concentrations at pH 7.2. FTIR and UV-Vis spectroscopic methods were used to analyze vitamin C and aspirin binding modes, the binding constants and the effects of drug complexation on BSA stability and conformation. Spectroscopic evidence showed that vitamin C and aspirin bind BSA via hydrophilic interactions (polypeptide and amine polar groups) with overall binding constants of K(vitamin C-BSA)=1.57×10(4)M(-1) and K(aspirin-BSA)=1.15×10(4)M(-1); assuming that there is one drug molecule per protein. The BSA secondary structure was altered with major decrease of α-helix from 64% (free protein) to 57% (BSA-vitamin C) and 54% (BSA-aspirin) and β-sheet from 15% (free protein) to 6-7% upon drug complexation, inducing a partial protein destabilization.     

Conformational analysis of Na,K-ATPase in drug-protein complexes

This review reports the effects of several drugs such as AZT (anti-AIDS), cis-Pt (antitumor), aspirin (anti-inflammatory) and vitamin C (antioxidant) on the stability and conformation of Na,K-ATPase in vitro. Drug-enzyme binding was found to be via H-bonding to the polypeptide CO and C-N groups with two binding constants K(1(AZT))=5.30 (+/-2.1)x10(5)M(-1) and K(2(AZT))=9.80 (+/-2.9)x10(3)M(-1) for AZT and one binding constant K(cis)(-Pt)=1.93 (+/-1.2)x10(4)M(-1) for cis-Pt, K(aspirin)=6.45 (+/-2.5)x10(3)M(-1) and K(ascorbate)=1.04 (+/-0.5)x10(4)M(-1) for aspirin and ascorbic acid. The enzyme secondary structure was altered with major increase of alpha-helix from 19.9% (free protein) to 22-26% and reduction of beta-sheet from 25.6% (free protein) to 17-23% upon drug complexation indicating a partial stabilization of protein conformation. The order of induced stability is AZT>cis-Pt>ascorbate>aspirin.     

Betulinic acid binding to human serum albumin: a study of protein conformation and binding affinity

Betulinic acid (BA) has anti cancer and anti-HIV activity and has been proved to be therapeutically effective against cancerous and HIV-infected cells. Human serum albumin (HSA) is the predominant protein in the blood. Most drugs that bind to HSA will be transported to other parts of the body. Using micro TOF-Q mass spectrometry, we have shown, for the first time that BA isolated from a plant (Tephrosia calophylla) binds to HSA. The binding constant of BA to HSA was calculated from fluorescence data and found to be K(BA)=1.685+/-0.01 x 10(6) M(-1), indicating a strong binding affinity. The secondary structure of the HSA-BA complex was determined by circular dichroism. The results indicate that the HSA in this complex is partially unfolded. Further, binding of BA at nanomolar concentrations of BA to free HSA was detected using micro TOF-Q mass spectrometry. The study revealed a mass increase from 65199 Da (free HSA) to 65643 Da (HSA+drug), where the additional mass of 444 Da was due to bound BA. Based on the results of this study, it is suggested that micro TOF-Q mass spectrometry is useful technique for drug binding studies.     

Investigation of the behavior of HSA upon binding to amlodipine and propranolol: Spectroscopic and molecular modeling approaches

The interaction between human serum albumin (HSA) and two drugs - amlodipine and propranolol - was investigated using fluorescence, UV absorption and circular dichroism (CD) spectroscopy. In addition, the binding site was established by applying molecular modeling technique. Fluorescence data suggest that amlodipine will quench the intrinsic fluorescence of HSA; whereas propranolol enhances the fluorescence of HSA. The binding constants for the interaction of amlodipine and propranolol with HSA were found to be 3.63×10(5)M(-1) and 2.29×10(4)M(-1), respectively. The percentage of secondary structure feature of each one of the HSA-bound drugs, i.e. the α-helix content, was estimated empirically by circular dichroism. The results indicated that amlodipine causes an increase, and that propranolol leads to a decrease in α-helix content of HSA. The spectroscopic analysis indicates that the binding mechanisms of the two drugs are different from each other. The data obtained by the molecular modeling study indicated that these drugs bind, with different affinity, to different sites located in subdomain IIA and IIIA.     

Interaction Between Ginkgolic Acid and Human Serum Albumin by Spectroscopy and Molecular Modeling Methods

The interaction of ginkgolic acid (15:1, GA) with human serum albumin (HSA) was investigated by FT–IR, CD and fluorescence spectroscopic methods as well as molecular modeling. FT–IR and CD spectroscopic showed that complexation with the drug alters the protein’s conformation by a major reduction of α-helix from 54 % (free HSA) to 46–31 % (drug–complex), inducing a partial protein destabilization. Fluorescence emission spectra demonstrated that the fluorescence quenching of HSA by GA was by a static quenching process with binding constants on the order of 10⁵ L·mol^?1. The thermodynamic parameters (ΔH = ?28.26 kJ·mol^?1, ΔS = 11.55 J·mol^?1·K^?1) indicate that hydrophobic forces play a leading role in the formation of the GA–HSA complex. The ratio of GA and HSA in the complex is 1:1 and the binding distance between them was calculated as 2.2 nm based on the Förster theory, which indicates that the energy transfer from the tryptophan residue in HSA to GA occurs with high probability. On the other hand, molecular docking studies reveal that GA binds to Site II of HSA (sub-domain IIIA), and it also shows that several amino acids participate in drug–protein complexation, which is stabilized by H-bonding.     

Spectroscopic investigation of the interaction between human serum albumin and three organic acids

The interactions of human serum albumin (HSA) with sinapic acid (SA), gallic acid (GA) and shikimic acid (SI) were investigated by fluorescence and Fourier transformed infrared spectrometry. Fluorescence results showed that one molecule of protein combined with one molecule of GA at the molar ratio of drug to HSA ranging from 0.1 to 30, and their binding constant (K(A)) is 1.1x10(4) M(-1). While one HSA molecule combined with one or two molecule of SA at the molar ratio of drug to HSA ranging from 0.1 to 4.26 or 4.26 to 30, and their binding affinities (K(A)) are 1.92x10(3) M(-1) and 6.87x10(8) M(-1), respectively. There is no specific interaction between HSA and SI. Combining the curve-fitting results of infrared amide I and amide III bands, the alterations of protein secondary structures induced by drugs were estimated. The drug-protein combination brought gradual reductions of the protein alpha-helix structure with increasing the concentrations of SA and GA, but SI did not change the protein secondary structure. From the fluorescence and FT-IR results, the binding mode was discussed in relation to the structures of the organic acids.     

Femtosecond to second studies of a water-soluble porphyrin derivative in chemical and biological nanocavities

The interactions of 5,10,15,20-tetrakis(4-sulfonatophenyl)-porphyrin (TSPP) with a quaternary ammonium modified β-cyclodextrin (QA-β-CD) and human serum albumin (HSA) protein in aqueous solutions at pH 7 were studied using steady-state, stopped-flow, and femtosecond to millisecond spectroscopy. TSPP forms 1:1 and 1:2 complexes with QA-β-CD (K(1) = 1.9 × 10(5) M(-1) and K(2) = 7 × 10(3) M(-1)) at 293 K, whereas with the HSA protein only 1:1 complex (K(1) = 1.7 × 10(6) M(-1)) has been found. The chemical and biological nanocavities have notable effects on the fluorescence lifetimes of the Q(x) state (from 9.3 to 11.1 ns in QA-β-CD and 11.6 ns in HSA). Furthermore, the rotational times (400 ps for the free TSPP, 1.6 and 19 ns for QA-β-CD and HSA protein complexes, respectively) clearly indicate the robustness of the formed entities. The confined environment does not affect much the fs dynamics (0.1-0.2 ps) of the encapsulated molecule. However, it clearly affect the ps one (1-2 ps (H(2)O) and 5-10 ps (QA-β-CD and HSA)). The effect of O(2) on the relaxation of the triplet state of the free and encapsulated TSPP is also studied and the obtained results are discussed in light of the shielding effect provided by the chemical and biological cavities. The observed difference, longer triplet lifetime upon encapsulation, might be relevant to the efficiency of this porphyrin in photodynamic therapy. The presteady-state kinetics of the TSPP:HSA has been studied by the stopped-flow spectrometer, and a two-step model was proposed for the complexation processes. The results show the importance of the initial association step for the overall ligand recognition process. This first step occurs with rate constant of ~4 × 10(5) M(-1) s(-1), which is about 5 orders of magnitude larger than the rate constant of the consecutive relaxation processes. We believe that our observations of molecular interaction between TSPP, QA-β-CD, and HSA protein from femtosecond to second at both ground and electronically first excited state give detailed information to improve our understanding of this kind of system and thus for a better design of drug delivery nanocarriers.     

RNA binding to antioxidant flavonoids

Flavonoids are an interesting group of natural polyphenolic compounds that exhibit extensive bioactivities such as scavenging free radical, antitumor and antiproliferative effects. The anticancer and antiviral effects of these natural products are attributed to their potential biomedical applications. While flavonoids complexation with DNA is known, their bindings to RNA are not fully investigated. This study was designed to examine the interactions of three flavonoids; morin (Mor), apigenin (Api) and naringin (Nar) with yeast RNA in aqueous solution at physiological conditions, using constant RNA concentration (6.25 mM) and various pigment/RNA (phosphate) ratios of 1/120 to 1/1. FTIR, UV-visible spectroscopic methods were used to determine the ligand binding modes, the binding constant and the stability of RNA in flavonoid-RNA complexes in aqueous solution. Spectroscopic evidence showed major binding of flavonoids to RNA with overall binding constants of K(morin) = 9.150 x 10(3) M(-1), K(apigenin)=4.967 x 10(4) M(-1), and K(naringin)=1.144 x 10(4) M(-1). The affinity of flavonoid-RNA binding is in the order of apigenin>naringin>morin. No biopolymer secondary structural changes were observed upon flavonoid interaction and RNA remains in the A-family structure in these pigment complexes.     

Interaction between phillygenin and human serum albumin based on spectroscopic and molecular docking

In this paper, the interaction of human serum albumin (HSA) with phillygenin was investigated by fluorescence, circular dichroism (CD), UV-vis spectroscopic and molecular docking methods under physiological conditions. The Stern-Volmer analysis indicated that the fluorescence quenching of HSA by phillygenin resulted from static mechanism, and the binding constants were 1.71×10(5), 1.61×10(5) and 1.47×10(4) at 300, 305 and 310K, respectively. The results of UV-vis spectra show that the secondary structure of the protein has been changed in the presence of phillygenin. The CD spectra showed that HSA conformation was altered by phillygenin with a major reduction of α-helix and an increase in β-sheet and random coil structures, indicating a partial protein unfolding. The distance between donor (HSA) and acceptor (phillygenin) was calculated to be 3.52nm and the results of synchronous fluorescence spectra showed that binding of phillygenin to HSA can induce conformational changes in HSA. Molecular docking experiments found that phillygenin binds with HSA at IIIA domain of hydrophobic pocket with hydrogen bond interactions. The ionic bonds were formed with the O (4), O (5) and O (6) of phillygenin with nitrogen of ASN109, ARG186 and LEU115, respectively. The hydrogen bonds are formed between O (2) of phillygenin and SER419. In the presence of copper (II), iron (III) and alcohol, the apparent association constant K(A) and the number of binding sites of phillygenin on HSA were both decreased in the range of 88.84-91.97% and 16.09-18.85%, respectively. In view of the evidence presented, it is expected to enrich our knowledge of the interaction dynamics of phillygenin to the important plasma protein HSA, and it is also expected to provide important information of designs of new inspired drugs.     

Human serum albumin unfolding pathway upon drug binding: A thermodynamic and spectroscopic description

The interest on phenothiazine drugs has been increased during last years due to their proved utility in the treatment of several diseases and biomolecular processes. In the present work, the binding of the amphiphilic phenothiazines promazine and thioridazine hydrochlorides to the carrier protein human serum albumin (HSA) has been examined by ζ-potential, isothermal titration calorimetry (ITC), fluorescence and circular dichorism (CD) spectroscopies, and dynamic light scattering (DLS) at physiological pH with the aim of analyzing the role of the different interactions in the drug complexation process with this protein. The ζ-potential results were used to check the existence of complexation. This is confirmed by a progressive screening of the protein charge up to a reversal point as a consequence of drug binding. On the other hand, binding causes alterations on the tertiary and secondary structures of the protein, which were observed by fluorescence and CD spectroscopies, involving a two-step, three-state transition. The thermodynamics of the binding process was derived from ITC results. The binding enthalpies were negative, which reveal the existence of electrostatic interactions between protein and drug molecules. In addition, increases in entropy are consistent with the predominance of hydrophobic interactions. Two different classes of binding sites were detected, viz. Binding to the first class of binding sites is dominated by an enthalpic contribution due to electrostatic interactions whereas binding to a second class of binding sites is dominated by hydrophobic bonding. In the light of these results, protein conformational change resembles the acid-induced denaturation of HSA with accumulation of an intermediate state. Binding isotherms were derived from microcalorimetric results by using a theoretical model based on the Langmuir isotherm. On the other hand, the population distribution of the different species in solution and their sizes were determined through dynamic light scattering (DLS). Aggregation of drug/protein complexes was found as a result of a possible expansion of protein structure induced at high drug concentrations. In addition, the presence of free drug aggregates at concentrations below the drug critical micelle concentration was also detected.     

Study of interaction between drug enantiomers and human serum albumin by flow injection-capillary electrophoresis frontal analysis

Flow injection (FI)-CE coupled with frontal analysis (FA) was applied to the study of stereoselectivity binding of amlodipine (AL) to HSA. Under protein-drug binding equilibrium, the unbound concentrations of drug enantiomers were measured by plateau height. The stereoselectivity of AL binding to HSA was proved by the different free fractions of two enantiomers. In physiological phosphate solution (pH 7.4, ionic strength 0.17) when 200 microM (+/-)AL was equilibrated with 300 microM HSA, the concentration of unbound R-AL was about 1.5 times higher than that of its antipode. The binding constants of two enantiomers, KR-AL and KS-AL, were 9910-11200 and 90200-104000 M(-1), respectively. The results obtained by the method were compared with those determined by conventional equilibrium dialysis (ED)-CE and fluorescence spectra. Hydroxypropyl-beta-CD (HP-beta-CD) (10 mM) was used as a chiral selector in pH 3.7 phosphate buffer. L-tryptophan (L-try) and ketoprofen (Ket) were used as displacement reagents to investigate the binding sites of AL to HSA. A binding synergism effect between hydrochlorothiazide (QL) and AL was observed and the results suggested that QL can destroy binding equilibrium of R-AL and S-AL toward HSA and they can occupy the same binding site of HSA (site I). The reproducibility was confirmed by RSD (RSD<1.5%) of the plateau height determined by FI-CE frontal analysis (FI-CE-FA). The FI-CE-FA was a good method to study protein-drug interaction.     

Characterization of interaction between doxycycline and human serum albumin by capillary electrophoresis‐frontal analysis

The binding of doxycycline to HSA under simulated physiological conditions (pH 7.4, 67 mM phosphate, I=0.17, drug concentration 100 μM, HSA concentration up to 475 μM, 36.5°C) was studied by CE‐frontal analysis. The number of primary binding sites, binding constant and physiological protein‐binding percentage were 1.9, 1.51×10³ M^?1 and 59.80%, respectively. In addition, the thermodynamic parameters including enthalpy change (ΔH), entropy change (ΔS) and free energy change (ΔG) of the reaction were obtained in order to characterize the acting forces between doxycycline and HSA. Furthermore, to better understand the nature of doxycycline–HSA binding and to get information about potential interaction with other drugs, displacement experiments were performed. The results showed that doxycycline binds at site II of HSA.     

人血清白蛋白与手性药物相巨作用的毛细管电泳研究Ⅰ.液相预柱毛细管电泳技术定量可靠性的考察

以药物Verapamil（VER）与人血清白蛋白（HSA）相互作用体系中游离的药物对映体浓度定量测定为目标,建立了一项适用于相互作用研究的液相预柱毛细管电泳（LPC－CE）技术。通过对该技术的考察,确定了这项技术的定量可靠性。在生理pH值条件下（pH7.4,离子强度I=0．17）,使药物与人血清白蛋白达到结合平衡。在毛细管电泳手性拆分[pH2.5缓冲浪;三甲基-β-环糊精（TM－β-CD）浓度为45mmol／L]柱（32cm×50μm）内预先注入一段生理pH缓冲液,形成一段液相预柱（2．8cm）。     

Evaluation of enantioselective binding of antihistamines to human serum albumin by ACE

The drug binding to plasma and tissue proteins is a fundamental factor in determining the overall pharmacological activity of a drug. HSA, together with alpha(1)-acid glycoprotein, are the most important plasma proteins, which act as drug carriers, with implications on the pharmacokinetic of drugs. Among plasma proteins, HSA possesses the highest enantioselectivity. In this paper, a new methodology for the study of enantiodifferentiation of chiral drugs with HSA is developed and applied to evaluate the possible enantioselective binding of four antihistamines: brompheniramine, chlorpheniramine, hydroxyzine and orphenadrine to HSA. This study includes the determination of affinity constants of drug enantiomers to HSA and the evaluation of the binding sites of antihistamines on the HSA molecule. The developed methodology includes the ultrafiltration of samples containing HSA and racemic antihistaminic drugs and the analysis of the free or bound drug fraction using the affinity EKC-partial filling technique and HSA as chiral selector. The results shown in this paper represent the first evidence of the enantioselective binding of antihistamines to HSA, the major plasmatic protein.     

Study on the protein binding of ketoprofen using capillary electrophoresis frontal analysis compared with liquid chromatography frontal analysis

A method of capillary electrophoresis frontal analysis (CEFA) is developed for the first time to study the binding of ketoprofen to human serum albumin (HSA) and compared with high-performance liquid chromatography frontal analysis (LCFA). The separation is performed in an uncoated fused-silica capillary (60-cm x 75- micro m i.d., 50-cm effective length) with a phosphate buffer (pH 7.4, ionic strength of 0.17M) as the running buffer. The applied voltage is 13 kV and the detection is set at 254 nm. A trapezoidal peak of the unbound ketoprofen appears after HSA elution in the electropherogram. The plateau height of the peak is employed to determine the unbound concentration of ketoprofen in the HSA equilibrated sample solution. The CEFA method provides the advantages of small sample injection volume and rapidity and the disadvantage of low sensitivity compared with LCFA. CEFA is applicable to the binding parameter estimation of ketoprofen to the secondary binding site; an association constant (K(2)) of 0.24 x 10(6)M(-1) and the number for the binding site per molecule HSA of 2.54 is estimated. In contrast, LCFA measures parameters for both primary and secondary sites, which are 1.05 x 10(6)M(-1) and 0.94 for K(1) and n(1), respectively, and 0.12 x 10(6)M(-1) and 3.16 for K(2) and n(2), respectively. It is found that ketoprofen binds mainly at the primary site at a molecular ratio of ketoprofen versus HSA lower than 0.75, and the binding at the secondary site occurs at a higher ratio.     

Kinetic studies of drug-protein interactions by using peak profiling and high-performance affinity chromatography: examination of multi-site interactions of drugs with human serum albumin columns

Carbamazepine and imipramine are drugs that have significant binding to human serum albumin (HSA), the most abundant serum protein in blood and a common transport protein for many drugs in the body. Information on the kinetics of these drug interactions with HSA would be valuable in understanding the pharmacokinetic behavior of these drugs and could provide data that might lead to the creation of improved assays for these analytes in biological samples. In this report, an approach based on peak profiling was used with high-performance affinity chromatography to measure the dissociation rate constants for carbamazepine and imipramine with HSA. This approach compared the elution profiles for each drug and a non-retained species on an HSA column and control column over a board range of flow rates. Various approaches for the corrections of non-specific binding between these drugs and the support were considered and compared in this process. Dissociation rate constants of 1.7 (±0.2) s(-1) and 0.67 (±0.04) s(-1) at pH 7.4 and 37°C were estimated by this approach for HSA in its interactions with carbamazepine and imipramine, respectively. These results gave good agreement with rate constants that have determined by other methods or for similar solute interactions with HSA. The approach described in this report for kinetic studies is not limited to these particular drugs or HSA but can also be extended to other drugs and proteins.     

Application of liquid pre-column capillary electrophoresis technique to the study of interaction between drug enantiomers and human serum albumin

Based on the chiral separation of several basie drugs, dimetindene, tetryzoline, theodrenaline and verapamil, the liquid pre-colunm capillary electrophoresis (LPC-CE) technique was established. It was used to determine free concentrations of drug enantiomers in mixed solutions with human serum albumin (HSA). To prevent HSA entering the CE chiral separation zone, the mobility differences between HSA and drugs under a specific pH condition were employed in the LPC. Thus, the detection confusion caused by protein was totally avoided. Further study of binding constants determination and protein binding competitions was carried out. The study proves that the LPC technique could be used for complex media, particularly the matrix of protein coexisting with a variety of drugs.     

Sequential injection affinity chromatography utilizing an albumin immobilized monolithic column to study drug-protein interactions

In this study, sequential injection affinity chromatography was used for drug-protein interactions studies. The analytical system used consisted of a sequential injection analysis (SIA) manifold directly connected with convective interaction media (CIM) monolithic epoxy disks modified by ligand-immobilization of protein. A non-steroidal, anti-inflammatory drug, naproxen (NAP) and bovine serum albumin (BSA) were selected as model drug and protein, respectively. The SIA system was used for sampling, introduction and propulsion of drug towards to the monolithic column. Association equilibrium constants, binding capacity at various temperatures and thermodynamic parameters (free energy DeltaG, enthalpy DeltaH) of the binding reaction of naproxen are calculated by using frontal analysis mathematics. The variation of incubation time and its effect in on-line binding mode was also studied. The results indicated that naproxen had an association equilibrium constant of 2.90 x 10(6)M(-1) at pH 7.4 and 39 degrees C for a single binding site. The associated change in enthalpy (DeltaH) was -27.36 kcal mol(-1) and the change in entropy (DeltaS) was -73 cal mol(-1)K(-1) for a single type of binding sites. The location of the binding region was examined by competitive binding experiments using a biphosphonate drug, alendronate (ALD), as a competitor agent. It was found that the two drugs occupy the same class of binding sites on BSA. All measurements were performed with fluorescence (lambda(ext)=230 nm, lambda(em)=350 nm) and spectrophotometric detection (lambda=280 nm).     

光谱法与分子模拟对左旋紫草素和人血白蛋白键合作用的研究

采用荧光光谱法、紫外吸收光谱法和圆二色性光谱(CD)研究了模拟生理条件下左旋紫草素和人血清白蛋白(HSA)的相互作用,计算了反应的结合常数、结合位点数和热力学参数,并探讨了左旋紫草素对人血清白蛋白二级结构的影响.在温度为292、303、310和318 K时,根据Scatchard方程测得左旋紫草素和HSA的结合常数分别为3.118×10~6、0.249×10~6、0.112×10~6 和0.102×10~6 L·mol~(-1),结合位点数分别为1.308、1.094、1.026和1.018;焓变(ΔH)和熵变(ΔS)分别为-104.82 kJ·mol~(-1)、-238.18 J·mol~(-1)·K~(-1),左旋紫草素在人血清白蛋白上的结合位置与色氨酸残基间的距离为2.66 nm.分子模型研究表明,左旋紫草素与HSA在亚结构域ⅡA结合,二者间的作用力主要为疏水和氢键作用力.CD结果表明,左旋紫草素与HSA的键合使HSA中α-螺旋结构含量从55.80%降到52.31%.     

Specific determination of unbound oxacillin in protein solution with cefoperazone by high-performance frontal analysis with chemiluminescence detection

Unbound oxacillin concentrations in human serum albumin (HSA) solutions in the presence or absence of cefoperazone were determined using high-performance frontal analysis coupled with chemiluminescence detection (HPFA-CL). The HPFA was performed on an ISRP column with 67 mM potassium phosphate buffer of pH 7.4 and ionic strength of 0.17 as the mobile phase. The luminol-H(2)O(2)-Co(2+) system was employed in the chemiluminescence detection. The detection was highly specific for oxacillin in the presence of cefoperazone. Although both drugs in HSA solutions co-eluted in the same region in HPFA, cefoperazone did not interfere with the determination of unbound concentration of oxacillin. In the solution of 100 microM HSA and 11.33 micro M oxacillin the bound percentage of oxacillin to HSA was estimated as 80.5%. Addition of 30.98 micro M cefoperazone into the HSA-equilibrated solution produced little effect on the protein binding of oxacillin. In the presence of 154.9 micro M cefoperazone, however, the bound percentage of oxacillin was significantly reduced. This specific method could be applied to the investigation of drug-drug interaction in protein binding.