Insight into the Interaction between the HIV-1 Integrase Inhibitor Elvitegravir and Bovine Serum Albumin : A Spectroscopic Study

The interaction between the anti-HIV drug Elvitegravir (EVG) and bovine serum albumin (BSA) was investigated by fluorescence spectroscopy and UV-visible absorption spectra. The mechanism for quenching the fluorescence of BSA by EVG is discussed. It was found that EVG can quench the intrinsic fluorescence of BSA through a static quenching procedure. The quenching type, association constant, and number of binding sites were investigated. The binding constant of EVG with BSA was calculated at different temperatures based on fluorescence quenching results. The thermodynamic parameters ΔH, ΔG, and ΔS were determined. The positive ΔS and negative ΔH and ΔG values showed that a spontaneous interaction may involve both roles of hydrophobic interaction and hydrogen bonding. The interaction of BSA with EVG was also confirmed by UV absorption spectra. The average distance, r, between donor (BSA) and acceptor (EVG) was obtained according to Förster’s theory of nonradiation energy transfer. Synchronous fluorescence and three-dimensional fluorescence spectra were used to investigate the conformational change of BSA molecules that occur upon addition of EVG and showed, upon binding, a possibility of increasing hydrophobicity around tryptophan residues of BSA.


Introduction
Serum albumins are the major soluble protein constituents of the circulatory systems, and they have many physiological functions [1,2].They act as storage and transport carriers for various endogenous and exogenous compounds [3,4].Therefore, interaction of drugs with and competition for the binding sites on albumins may strongly affect the distribution and elimination rate of the drugs as well as their action mechanism and pharmacodynamics.Hence, investigating the drug binding with albumins will facilitate interpretation of the metabolism and transport process of the drug and will help to elaborate the relationship between structures and functions of proteins.We have selected bovine serum albumin (BSA), for which association constants and binding sites are known for a number of compounds [5,6].BSA and human serum albumin (HSA) display approximately 76% sequence homology [7,8].Similar to other serum albumin, bovine serum albumin (BSA) is a multifunctional protein that possesses various physiological functions including binding, transport, and delivery of fatty acids [9,10], porphyrins [11], bilirubin [12], and several other molecules.BSA is also responsible for certain binding aggregation and conformational dynamics in solution [13].BSA has been extensively studied [14][15][16], and its molecular interactions with several molecules are often determined via spectroscopic techniques due to their high sensitivity, reliability, and ease of use [17][18][19][20][21]. On the other hand, integrase inhibitors are one of the more recently introduced classes of antiretroviral [22,23].Elvitegravir (EVG) (formerly GS-9137, JTK-303) (6-(3-Chloro-2-fluorobenzyl)-1-[(2S)-1-hydroxy-3-methyl-2-butanyl]-7-methoxy-4oxo-1,4-dihydro-3-quinolinecarboxylic acid) (Figure 1) is a potent, boosted, once daily, HIV integrase inhibitor through the inhibition of DNA strand transfer [24,25].EVG has antiviral activity against wild-type and drug resistant strains of HIV [24].EVG inhibits the process of viral DNA strand transfer with the greatest potency 50% maximal inhibitory concentration [IC50] of <0.06 mol⋅L −1 in vitro [26], thereby preventing HIV DNA from being integrated into host chromosomes and blocking provirus formation and infection propagation [27].The inhibition of strand transfer occurs via the binding of Elvitegravir to the integrase-viral DNA complex and the drug's subsequent interaction with the DNA and the essential magnesium ions within the active site of the integrase [22,28].EVG initially became available as part of a fixed-dose tablet (Stribild) in combination with emtricitabine, tenofovir disoproxil fumarate, and the pharmacokinetic boosting agent cobicistat in several countries, including North America and the EU [22].Elvitegravir has also been approved in the EU and more recently (Sep.2014) by the FDA as a single-agent tablet (Vitekta 85 or 150 mg) for use in combination with a ritonavirboosted protease inhibitor plus one or more antiretrovirals for treatment-experienced HIV-positive adults [29,30].With the use of antiretroviral drugs, several potential interactions have to be considered.In the current study, the EVG induced quenching of the intrinsic fluorescence of BSA was investigated.BSA consists of a chain of 580 amino acid residues composing a well-defined single polypeptide sequence, which contains three homologous -helix domains (I-III) [31,32] and two tryptophan residues located at positions 134 and 212 of the chain [33].The fluorescence intensity of BSA could be decreased by quenchers such as many drugs or small molecule organic compounds [34].The changes of fluorescence or absorption spectra of BSA reflect the interaction of drugs and macromolecule protein.Extensive survey of the literature revealed that no reports studying EVG and serum albumin interactions were published.Hence, the current study is designed to study the interaction of EVG and BSA using fluorescence spectroscopy to explain the quenching type, the association constants, number of binding sites, and the basic thermodynamic parameters.

Sample Preparation.
EVG standard solution was prepared in methanol yielding a final concentration of 1.0 mg mL −1 .
A volume of 1.0 mL stock solution was diluted 10 mL with methanol to give a 100 g mL −1 concentration as a working standard solution.The working solution was further diluted with PBS buffer pH 7.4 to produce a working standard solution of 20 g mL −1 .Bovine serum albumin (BSA) solution was prepared in a concentration of 1.0 mg mL −1 by dissolving 0.1 g pure BSA in PBS buffer pH 7.4 and completing the volume to 100 mL and kept in the cool, dark.This solution was further diluted with PBS buffer pH 7.4 to produce a working standard solution of 100 g mL −1 .

EVG-BSA Interactions.
Upon optimizing the concentrations of both analytes, the BSA concentration was fixed at 50 g mL −1 with EVG concentration that ranged from 0.1 to 15 g mL −1 .Fluorescence measurements were performed at three temperatures (288, 298, and 309 K) in the range of 290-500 nm upon excitation at 280 nm.The threedimensional fluorescence spectrum was performed using the initial excitation wavelength set at 240 nm up to 350 nm with an increment of 2 nm.Emission wavelength was set between 220 and 600 nm and an increment of 0.5 nm.Synchronous fluorescence was performed with a Δ values of 15 and 60 scanning between 260 and 500 with a 0.5 nm data interval.

UV Measurements.
The UV-vis spectra of BSA were recorded for the drug-free protein and upon addition of EVG scanning in the wavelength range of 220-500 nm.BSA concentration was kept constant at 0.8 mg mL −1 while the drug concentration was 3.0, 5.0, 7.0, and 10.0 g mL −1 .

Energy Transfer between EVG and BSA.
A concentration of 10.0 g mL −1 of EVG was used for the absorption measurement in the range of 220-500 nm.Meanwhile, BSA (50 g mL −1 ) emission spectrum was determined in the wavelength range of 280-500 nm.Hence, the energy transfer was computed in accordance to Förster's nonradiative energy transfer theory (FRET) [35] utilizing the overlapped UV absorption spectrum of EVG with the fluorescence emission spectrum of BSA.

Results and Discussion
3.1.Fluorescence Quenching of BSA Induced by EVG.Spectroscopic techniques, particularly fluorescence spectroscopy, are able to provide enormous amount of information on the ligand protein binding, such as type, mechanism and strength of the binding, and so forth.It has been previously established that quenching of fluorescence intensity, that is, fluorescence intensity reduction of a certain fluorophore, can occur via several mechanisms including but not limited to formation of a nonfluorescent complex and energy transfer, and so forth [15,36,37].The fluorescence spectra of BSA in presence of different amounts of EVG were measured in the range of 288-500 nm upon excitation at 280 nm.EVG led to a concentration dependent quenching of the intrinsic fluorescence of BSA (Figures 2 and 3) without changing the emission maximum but with a slight change in peak shape at higher EVG concentrations (>10 g mL −1 ).These results indicated that an interaction between EVG and BSA is taking place [38].
Furthermore, the fluorescence quenching data were analyzed by the Stern-Volmer equation (1) [39] and Lineweaver-Burk equation (2) [40].Consider where  0 and  are the steady-state fluorescence intensities of BSA in the absence and presence of EVG, respectively,  SV the Stern-Volmer quenching constant and  is the concentration of quencher (EVG).  is the quenching rate constant,  0 is the average lifetime of the protein without the quencher and  LB is the static quenching constant.Within certain concentration, the curve of  0 / versus  (Stern-Volmer curve) would be linear if the quenching type is single static or dynamic quenching [17]; similarly, the curve of ( 0 − ) −1 versus  −1 (Lineweaver-Burk curve) would be linear [41] for static quenching.If the quenching type is combined quenching (both static and dynamic), the Stern-Volmer plot is an upward curvature [42].One way to distinguish dynamic from static quenching is to examine the temperature effect on the interaction of the drug to BSA.The Stern-Volmer and Lineweaver-Burk curves of EVG-BSA at different temperatures are shown in Figure 5.It can be seen that both Stern-Volmer and Lineweaver-Burk curves were linear, which illustrated that the quenching type was probably a static quenching.Moreover, Table 1 demonstrates the gradual reduction of  SV and  LB values upon temperature increase that supports the idea of a static type quenching of BSA fluorescence [42].This hypothesis was also backed up by the   values (quenching rate constant), which was computed using the following equation Since the fluorescence lifetime of the biopolymer ( 0 ) can be taken as 10 −8 s −1 [43], then the   values summarized in Table 1 were calculated for EVG-BSA system to be in the order of 10 12 L Mol −1 s −1 .Previous reports have shown that the maximum scatter collision quenching constant,   of various quenchers with the biopolymer is 2 × 10 10 L M −1 s −1 [43].
Ultimately, the   values of protein quenching procedure initiated by EVG are higher than the   of the scattered procedure.This indicated that the quenching is not initiated by dynamic collision but from the formation of a complex [41].

Binding Constants and the Number of Binding Sites.
Based upon the probability of a static quenching process is taking place between BSA and EVG and if it is assumed that there are independent binding sites to a set of equivalent sites on the   protein, the apparent binding constant () and the number of binding sites () can be determined from [42,44] log where  is the binding constant, reflecting the reaction degree of BSA and EVG;  is the number of binding sites, specifying the number of EVG bound to a BSA molecule.Thus, a plot of Log( 0 − )/ versus log  could be used to determine  and .The  and  at different temperatures are summarized in Table 2, indicating good assumptions made for deriving (4).Table 2 also demonstrates a decrease in the binding constant with the increase in temperature, yielding a less stable EVG-BSA complex.Additionally,  values were found to be ∼1, which in turn may infer the involvement of one independent class of binding sites on BSA for EVG.

Thermodynamics and Interaction Forces between EVG and BSA.
As the binding constant is dependent on temperature; hence, a thermodynamic process was regarded responsible for the complex formation.Therefore, temperature dependent thermodynamic parameters were determined for further investigating the acting forces between EVG and BSA.Generally, the force between a small molecule and a biological macromolecule includes hydrophobic interaction, hydrogen bonding, van der Waals force, and electrostatic force.The thermodynamic parameters associated with the different types of interactions were previously reported [45].In which, hydrophobic force may increase Δ  and Δ  of a system, while van der Waals power may decrease them with negative Δ  indicative for hydrogen bonding and electrostatic force usually makes Δ  ∼ 0 and Δ  > 0 [46].Thermodynamic parameters, enthalpy (Δ  ), and entropy (Δ  ) of EVG-BSA interaction are significant to confirm binding mode.Binding studies were performed at 288, 298, and 309 K, at which BSA does not undergo any structural degradation.The thermodynamic parameters were evaluated using the Van't Hoff and Gibbs-Helmholtz [47,48] equations: where  and  are the binding constant and gas constant, respectively.Δ  , Δ  , and Δ  are enthalpy change, free energy change, and entropy change, respectively.Therefore, a plot of log  versus 1/ (Figure 4) enables the determination of Δ  , Δ  , and Δ  by (5).Thermodynamic parameters for EVG-BSA binding are summarized in Table 3  Figure 6: Absorbance spectra of EVG 10 g mL −1 (A), BSA 0.8 mg mL −1 (B), and EVG-BSA system at EVG concentration of (C) 3.0 g mL −1 , (D) 5.0 g mL −1 , (E) 7.0 g mL −1 , and (F) 10 g mL −1 .Table 3: Thermodynamic parameters of BSA-ELV binding.and demonstrate that hydrogen bonding and hydrophobic interactions play a role in the binding between EVG and BSA.

Energy
Transfer between EVG and BSA.The aforementioned spectral studies have suggested that EVG forms a nonfluorescent complex with BSA.Three types of intrinsic fluorophores are known to be found in BSA sequence, namely, tryptophan (Trp), tyrosine (Tyr), and phenylalanine (Phe); however the intrinsic fluorescence of BSA is mainly due to tryptophan [49].The efficiency of energy transfer can be used to determine the distance, , between the drug and the tryptophan residues in the protein.The distance  is an average value between the bound EVG and the two tryptophan residues of BSA (Trp-135 and Trp-214) and can be determined using Förster's resonance energy transfer (FRET) [35,44].Generally, energy transfer occurs whenever the emission spectrum of a fluorophore (donor) overlaps with the absorption spectrum of another molecule (acceptor) and the distance between them is less than 8 nm.The overlap of the UV absorption spectrum of EVG with the fluorescence emission spectra of BSA is shown in Figure 5.The efficiency of energy transfer, , is calculated using where  and  0 are the fluorescence intensities of BSA in the presence and absence of EVG,  is the distance between intensity of BSA increased regularly with the variation of EVG concentration, indicating that the peptide strands of BSA molecules extended more upon the addition of EVG [51].

Synchronous Fluorescence.
The synchronous fluorescence spectra were measured, shown in Figure 7.In the synchronous spectra, the sensitivity associated with fluorescence is maintained while offering several advantages: spectral simplification, spectral bandwidth reduction, and avoidance of different perturbing effects.It shows the tyrosine residues (Tyr) and tryptophan residues (Trp) of BSA when the wavelength interval Δ is 15 nm and Δ is 60 nm, respectively [54,55].It is obvious that the emission strengths of both tyrosine and tryptophan decreased.A slight blue shift can be observed in Figure 7(b) which indicates that the polarity around tryptophan residue decreased.That is to say, tryptophan residues were placed in a more hydrophobic environment and less exposed to the solvent.Additionally, it can be seen from Figure 7 that a stronger fluorescence quenching effect of tryptophan residues compared with the tyrosine residues after EVG is added.It indicates that the binding site of EVG may be closer to tryptophan than that of the tyrosine residues.
3.5.3.Three-Dimensional Fluorescence.Moreover, threedimensional (3D) fluorescence spectra were recorded, and some characteristic 3D parameters are reported in Table 4. Two typical fluorescence peaks can easily be observed in the three-dimensional fluorescence spectra Figures 8  and 9.The peak at (224 → 333) (Peak 1) mainly exhibits the fluorescence characteristic of  → * transition of the polypeptide backbone of protein structure, C=O [56].The second peak at (280 → 337) (Peak 2) revealed the spectral characteristic of Trp and Tyr residues [19].The Rayleigh scattering peaks and two typical fluorescence peaks can be easily observed in 3D fluorescence contour map of BSA (Figure 9).It is clear that both BSA's fluorescence peaks were quenched by the addition of EVG, differing in their extents (in the presence of EVG, the intensity ratio of the fluorescence peak to the second order one is 1.776 : 1 whereas it is 1.564 : 1 in the absence of EVG).Additionally, Rayleigh scattering peaks were enhanced that also give clear evidence of complex formation between EVG and BSA [57].

Conclusions
In this work we have employed a set of fluorescence spectroscopic UV-visible spectrophotometric analyses to study the interactions of BSA with EVG.EVG proved to bind to BSA resulting in reduction of its fluorescence intensity via static binding.We have computed the binding parameters, and conformational changes leading to increased stability of EVG-BSA complex.Data analysis showed that only one binding site on BSA is available for EVG with a binding constant in the order of 10 3 L mol −1 .Thermodynamic parameters were also computed to be negativefree energy and enthalpy while positive entropy referring a spontaneous reaction of BSA with EVG with both hydrogen bonding and hydrophobic interactions probably involved in the binding.Conformation study showed that the polarity around tryptophan residue decreased.This demonstrates that tryptophan residues were placed in a more hydrophobic microenvironment and less exposed to the solvent.The binding distance between the donor (BSA) and the acceptor (EVG) was calculated based on the Förster's theory of nonradiation energy transfer.

Figure 5 :
Figure 5: The overlap of the fluorescence spectrum of BSA (a) and the absorbance spectrum of EVG (b).

Table 2 :
Binding constant  and the number of binding sites .

Table 4 :
Three-dimensional fluorescence characteristic parameters of EVG-BSA binding.