Probing of Interaction between Alogliptin Benzoate and Human Serum Albumin Using Multi-Spectral Methods and Molecule Docking Technique

Nan Zhang, Jianbo Chen^*, Zhuowei Han

College of Life Science, Shanghai Normal University, Shanghai, China.

DOI: 10.4236/oalib.1107283                

Abstract

The interaction of alogliptin benzoate with human serum albumin had been characterized under physiological conditions using multi-spectral methods and molecular docking technique. The work presented in this paper focused on the interaction mechanism, the conformational changes of HSA and the binding sites of alogliptin benzoate with human serum albumin. The binding distance, binding constants, the number of binding sites and the binding forces had been investigated through fluorescence and spectral overlaps. Results indicated the presence of static quenching between alogliptin benzoate and human serum albumin. Moreover, the van der Waals forces and hydrogen bonding drove the binding process. The analysis results of UV–vis absorption spectroscopy, synchronous fluorescence spectrometry, circular dichroism spectroscopy and three-dimensional fluorescence spectroscopy revealed that alogliptin benzoate changed the conformation of human serum albumin. In addition, molecule docking and competitive experimental results suggested the binding sites located at IIA subdomain of human serum albumin. This research is vital to providing reference for studying the pharmacodynamics and pharmacokinetics mechanisms of alogliptin benzoate.

Keywords

Alogliptin Benzoate, Human Serum Albumin, Interaction, Spectroscopic Techniques, Molecular Docking

1. Introduction

Diabetes mellitus is a multifactorial metabolic disease with insulin resistance and hyperglycemia. Hitherto, diabetes and its complications have seriously threatened human health [1] [2]. There are numerous antidiabetic drugs such as antidiabetic sulfonylureas, thiazolidinediones, biguanides, and dipeptidyl-peptidase IV (DPP-4) inhibitors [3]. By comparison, DPP-4 inhibitors increase the active level of incretin hormones, thus better regulating blood glucose levels and alleviating hypoglycemia. Based on the excellent effect, the action mechanism of DPP-4 inhibitors has been revealed. Glucagon-like peptide-1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are incretin hormones. They potentiate glucose-induces insulin secretion [4]. DPP-4, a kind of serine protease on the cell surface, can disintegrate GLP-1 and GIP to disrupt glycometabolism. DPP-4 inhibitors delay the degradation of GLP-1 and GIP, so as to regulate blood glucose level of patients with diabetes mellitus type 2 [5].

Human serum albumin (HSA) is a single-chain protein composed of 585 amino acids. It accounts for around 60% in plasma with a molecular weight of about 67 kDa. As the major delivery protein in vivo, it transports endogenous and exogenous substances to target organs or tissues. In addition, it has three identical domains (I, II and III) with two subdomains (A and B), which are able to combine with drugs and other small molecules [6] [7] [8].

Recently, more and more studies focused on the interaction of drugs and HSA. Eraj revealed the presence of hydrophobic and hydrogen bonds between synthesis of copper (II) complexes and HAS [9]. Hekmat confirmed that paclitaxel combined with nano-diamond particles can form a complex with HAS [10]. The interaction between drugs with HSA is quite attention-getting because of its clinical and pharmaceutical importance. It affects the transportation and elimination of drugs in vivo, as well as the pharmacokinetics and pharmacodynamics properties of drugs. As a specific sort of DPP-4 inhibitors, alogliptin benzoate (ALTB) is well known for its better regulation of blood glucose levels and alleviation of hypoglycemia with glucose dependence mechanism, while its interaction mechanism with HSA has not been reported yet. In this study, the interaction of ALTB with HSA was investigated by fluorescent spectrometry, spectral overlaps, UV-vis absorption spectroscopy, synchronous fluorescence spectrometry, circular dichroism (CD) spectrometry, three-dimensional fluorescence spectroscopy, molecule docking and competitive experiments. The interaction mechanism and the effects of ALTB on the conformational changes of HSA were demonstrated in this paper.

2. Materials and Methods

2.1. Materials

HSA and ALTB were purchased from Shanghai yuanye Bio-Technology Co., Ltd (with 98% purity). Ultrapure water was used throughout the experiments. All other reagents were of analytical grade and used without further purification.

ALTB was dissolved in methyl alcohol (guarantee reagent) with a concentration of 1.0 × 10⁻³ mol/L. HSA stock solution was prepared in Tris-HCl buffer solution with a concentration of 1.0 × 10⁻⁴ mol/L. All solutions were stored at below 4˚C before experiments.

2.2. Methods

The UV?vis absorption spectra and the CD spectra measurements had been carried out from 200 nm to 350 nm. The fluorescence spectra were scanned from 300 nm to 500 nm at excitation wavelength of 280 nm. Synchronous fluorescence spectra were scanned from 200 nm to 350 nm with the Δλ values of 15 nm for tyrosine (Tyr) and 60 nm for tryptophan (Trp). Through setting the firing interval as 5 nm, three-dimensional fluorescence spectra were scanned with excitation wavelength from 200 nm to 350 nm, emission wavelength from 200 nm to 500 nm. All the emission and excitation slits were 5 nm. Autodock 2.5 was used for molecule docking. The crystal structure of HSA (PDB ID: 1H9Z) was obtained from protein database (PDB).

3. Results and Discussion

3.1. Interaction Mechanism Analysis

3.1.1. Fluorescence Quenching Experiments

It is reported that the fluorescence spectroscopy provides information on the binding mechanism of ligand to fluorophore [11]. The fluorescence spectra of HSA scanned in the presence of increasing concentrations of ALTB were shown in Figure 1. Results reflected that the addition of ALTB decreased the intrinsic fluorescence of HSA. It indicated that ALTB could interact with HSA by quenching its intrinsic fluorescence [12].

Figure 1. Effects of ALTB on fluorescence emission spectra of HSA. (1) C_(HSA) = 1 × 10⁻⁶ mol/L, (2-6) C_(ALTB)/C_(HSA) = 5, 10, 15, 20, 25. T = 298K, pH = 7.4.

The quenching mechanism is usually classified as static quenching and dynamic quenching [13]. The dynamic quenching depends on the diffusion of quencher to fluorophore, while the static quenching arises from the forming of the ground-state complex between fluorophore and quencher. They can be distinguished by the values of biomolecular quenching rate constant (K_q) and Stern-Volmer quenching constant (K_SV) at different temperatures. Higher temperature leads to larger values of K_q and K_SV for dynamic quenching, while it tends to result in smaller values of K_q and K_SV for static quenching. In addition, the maximal value of K_q had been reported approximately 2.0 × 10¹⁰ L・mol⁻¹・s⁻¹ for dynamic mechanism [14]. In order to investigate the quenching mechanism of ALTB with HSA, the quenching constants calculated through the following equation (Equation) had been displayed in Table 1

$F_0/F=1+K_q{\tau }_0\left[Q\right]=1+K_{sv}\left[Q\right]$ (1)

F₀ and F represent the fluorescence intensity of HSA in the absence and presence of ALTB at concentration [Q], respectively. [Q] is the concentration of ALTB. “τ₀” is the average lifetime of fluorophore without quencher, which is usually 1.0 × 10⁻⁸ s for HSA [15].

K_SV and K_q were obtained from the slopes of linear regressions exhibited in Figure 2. The values of K_SV and K_q decreased with the increase of temperatures in Table 1. Furthermore, the values of K_q were much larger than 2.0 × 10¹⁰ L・mol⁻¹・s⁻¹. These results revealed that ALTB statically quenched the fluorescence of HSA.

Table 1. Quenching constants and binding parameters of ALTB-HSA system.

Figure 2. Stern-volmer plots of ALTB-HSA system.

3.1.2. Binding Constants and the Number of Binding Sites

The binding constants had been calculated by Equation (2) and results are displayed in Table 1.

$\log \left[\left(F_0-F\right)/F\right]=\log K_a+n\log \left[Q\right]$ (2)

K_a is the binding constants. The n is the number of binding sites. The slopes and intercepts exhibited in Figure 3 determined the values of n and K_a. The binding constants decreased as the temperature goes up. It suggested the reduction in the stability of the ground-state complex and hence the existence of static quenching mechanism. In addition, the values of n are approximately 1, which revealed the presence of the single binding site between ALTB and HSA [16].

3.1.3. Thermodynamic Analysis for Binding Forces

The thermodynamic parameters had been calculated by Equations (3)-(5) and results are shown in Table 2 [17] [18].

$\ln K=-\Delta H/RT+\Delta S/R$ (3)

$\Delta G=\Delta H-T\Delta S$ (4)

$\Delta G=-RT\ln K$ (5)

K is the binding constant. R is the gas constant. T is the corresponding thermodynamic temperature. ΔG, ΔH and ΔS are the changes of free energy, enthalpy and entropy, respectively. The negative values of ΔG indicated the binding process of ALTB with HSA was spontaneously. As for negative values of ΔH

Figure 3. Double logarithm plots of ALTB-HSA system.

Table 2. Thermodynamic parameters of ALTB-HSA system at different temperatures.

and ΔS, it implied van der Waals forces and hydrogen bonding drove the binding process [19].

3.1.4. Energy Transfer and Binding Distance

According to Forster non-radiative energy transfer theory, the energy transfer efficiency requires not only enough spectral overlaps, but also a binding distance shorter than 8 nm [20].

Figure 4 is the overlaps of the emission spectrum of HSA with the absorption spectrum of ALTB. The binding distance (r) can be calculated by Equation (6).

$E=1-\frac{F}{F_0}=\frac{R_0^6}{R_0^6+r^6}$ (6)

E is the transfer efficiency. F₀ and F are the fluorescence intensity of HSA in the absence and presence of ALTB, respectively. R₀ is the critical distance when the value of E is 50% [21], the calculation of which is achieved by Equation (7).

$R_0^6=8.8\times {10}^{-25}{K}^2{n}^{-4}\Phi J$ (7)

K² is the spatial orientation factor of the dipole. The n is the refractive index of the medium. The Ф is the fluorescence quantum yield. J represents the overlap integral of the absorption spectrum of ALTB and the fluorescence emission spectrum of HSA. The J can be calculated through Equation (8).

$J=\frac{{\displaystyle \sum F\left(\lambda \right)\epsilon \left(\lambda \right){\lambda }^4\Delta \lambda }}{{\displaystyle \sum F\left(\lambda \right)\Delta \lambda }}$ (8)

F(λ) and ε(λ) reflect the fluorescence intensity of HSA and the absorption coefficient of ALTB at the wavelength λ, respectively. According to Equations (6)-(8), it can be calculated that J = 1.69 × 10⁻¹⁴ cm³・L・mol⁻¹, E = 0.126, R₀ = 2.72 nm and r = 3.76 nm. The values of r indicated that HSA and ALTB were close to each other. A strong bind exists between ALTB and HSA. Moreover, the presence

Figure 4. Overlaps of the fluorescence spectrum of HSA (A) with the absorbance spectrum of ALTB (B), C_(HSA) = C_(ALTB) = 1 × 10⁻⁶ mol/L.

of energy transformation from HSA to ALTB gets proved [19]. And the larger value of r compared to R₀ also revealed the presence of static quenching mechanism of ALTB with HSA [22] [23].

3.2. Conformational Investigations

3.2.1. UV?Vis Absorption Spectra

For static fluorescence quenching, changes in UV?vis absorption spectroscopy arise from the formation of the complex. In this section, UV?vis absorption spectroscopy is applied to explore changes of conformation of HSA and the formation of ALTB-HSA complex [24] [25]. HSA embraces two characteristic absorption peaks due to the transition of valence electrons. The strong absorption peak at 210 nm (Peak 1) exhibits the characteristic of peptide bond. The weak absorption peak at 280 nm (Peak 2) is related to conjugated double bonds in Tyr, Trp [26] [27]. Figure 5 is the UV?vis absorption spectra of ALTB-HSA system. With the addition of ALTB, obvious increments in absorbance peaks show up, thus indicating the formation of ALTB-HSA complex and the changing conformation of HSA [28] [29].

3.2.2. Synchronous Fluorescence Spectra

The endogenous fluorescence of HSA comes from Tyr, Trp and phenylalanine (Phe) residues [30]. The synchronous fluorescence spectroscopy is widely used to explore the changes of the microenvironment and the solvent polarity of the fluorophore molecules (HSA) [31]. Δλ is the wavelength intervals between excitation and emission wavelengths. When Δλ are stabilized at 15 or 60 nm, the synchronous fluorescence provides the characteristic information of Tyr or Trp residues, separately [32]. The shifts of maximum emission wavelength affect the hydrophobicity and the polarity of HSA. The red shift reflects a higher polarity and a lower hydrophobicity of HSA [23] [20]. The synchronous fluorescence

Figure 5. UV?vis absorption spectra of ALTB-HSA system. (1) C_(HSA) = 1 × 10⁻⁶ mol/L, (2-6) C_(ALTB)/C_(HSA) = 5, 10, 15, 20, 25. T = 298K, pH = 7.4.

spectra of HSA-ALTB system were shown in Figure 6. As the concentration of ALTB increased, the fluorescence intensities of HSA decreased accompanied by the red shifts. It implied that ALTB increased the polarity and decreased the hydrophobicity around Tyr and Trp.

3.2.3. CD Spectra

CD signal is sensitive to investigate the secondary structure of HSA. HSA has two negative bands at 208 nm and 222 nm, respectively [33] [34]. These two negative bands are the characteristic peaks of α-helix of HSA. The CD spectra of HSA-ALTB system were shown in Figure 7. The content of а-helix increased from 47.63% to 54.33% with the addition of ALTB. It revealed that ALTB changed the secondary structure of HSA.

Figure 6. Synchronous fluorescence spectra of ALTB-HSA system. (1) C_(HSA) = 1 × 10⁻⁶ mol/L, (2-6) C_(ALTB)/C_(HSA) = 5, 10, 15, 20, 25. T = 298K, pH = 7.4. (A) Δλ = 15 nm, (B) Δλ = 60 nm.

Figure 7. Circular dichroism spectra of ALTB-HSA system. C_(HSA) = 1 × 10⁻⁶ mol/L, C_(ALTB) = 5 × 10⁻⁶ mol/L.

3.2.4. Three-Dimensional Fluorescence Spectra

The three-dimensional fluorescence spectroscopy in Figure 8 was also used to illustrate conformational changes of HSA [35]. The three-dimensional fluorescence spectroscopy of HSA and ALTB-HSA were shown in Figure 8(a) and Figure 8(b), respectively. Peak 1 was the characteristic peak of polypeptide skeleton, and peak 2 was that of Tyr and Trp residues [36] [37]. The values of maximum emission wavelength (E_m) and excitation wavelength (E_x) were shown in Table 3.

The stocks shifts were calculated by subtracting excitation wavelength from emission wavelength. The maximum fluorescence intensities of peak 1 and peak 2 both decreased and exhibited red shifts with the addition of ALTB, which can be observed in Table 3. The decrease of peak 1 indicated the changes of polypeptide skeleton in the presence of ALTB-HSA complex, which conformed to the increase of α-helix in CD spectra [38]. The red shift of peak 2 suggested that ALTB increased the polarity and decreased the hydrophobicity around the Tyr and Trp of HSA [39]. It was consistent with the results of synchronous fluorescence.

In summary, ALTB helps to change the conformation of HSA according to the results presented in this section.

3.3. Binding Sites Investigation

3.3.1. Molecular Docking

HSA contains three homologous α-helical domains (I, II and III), with two subdomains (A and B) of each. As is known to all, Trp 214 locates at the IIA subdomain of HSA [40] [41]. The molecule docking had been used to forecast the binding site of ALTB with HSA, and results of which were shown in Figure 9(a) and Figure 9(b). Based on Figure 9(a), the interaction site was mainly in the vicinity of IIA subdomain. And according to Figure 9(b), ALTB mainly interacted with Lys 351, Arg209, and Glu354 residues of HSA.

Figure 8. Three-dimensional fluorescence spectra of ALTB-HSA system. (A) C_(HSA) = 1 × 10⁻⁶ mol/L, (B) C_(ALTB)/C_(HSA) = 5. T = 298K, pH = 7.4.

3.3.2. Site-Selective Binding of ALTB on HSA

The principal regions of interaction sites on HSA are located in the hydrophobic subdomains IIA and IIIA, which are denominated as Sudlow site I and site II Phenylbutazone and ibuprofen serve as the site markers to monitor Sudlow sites I and site II, respectively [42] [43]. In the following experiments, several sets of competitive experiments were carried out to identify the binding sites.

By keeping equimolar concentrations of site markers and HSA at 1.0 × 10⁻⁶ mol/L, ALTB was gradually added into the system. The binding constants had been calculated and listed in Table 4 by scanning the fluorescence spectra. Results showed that the binding constant was significantly changed in the presence of phenylbutazone. However, the influence was indistinctive in the presence of ibuprofen. In this case, the binding site was mainly located within Sudlow sites I or IIA subdomain of HSA.

Figure 9. Interaction mode between ALTB and HAS.

Table 3. Peak values of three-dimensional fluorescence spectra of ALTB-HSA system.

Table 4. Binding constants of competitive experiments of ALTB-HSA system.

Figure 10. Effect of site markers on HSA-ALTB system.

In order to ensure a better minimal nonspecific binding of the site markers, the molar ratio of ALTB and HSA was fixed at 10:1. Site markers were gradually added to the reaction system. As shown in Figure 10, F₂ and F₁ are the fluorescence intensity in the presence and absence of probes, respectively. Compared with ibuprofen, the phenylbutazone affected the fluorescence intensity of reaction system more obviously. In the meantime, the binding site was mainly located within IIA subdomain of HSA. Based on Table 4 and Figure 10, it is concluded that the binding area is mainly located at Sudlow sites I, the IIA subdomain of HSA [44].

4. Conclusion

In this work, the interaction mechanism, the binding site and the conformational changes of HSA with ALTB were characterized by multi-spectral methods and molecule docking technique under physiological conditions. The fluorescent analysis results indicated that ALTB statically quenched the intrinsic fluorescence of HSA. The negative values of ΔH and ΔS revealed the presence of van der Waals forces and hydrogen bonding in the binding process. There was only one binding site of ALTB with HSA. Besides, the binding distance was obtained by spectral overlaps which demonstrated the presence of static quenching as well. The multi-spectral experimental results suggested that ALTB changed the conformation of HSA by forming the complex. It reduced the hydrophobicity and increased the polarity of HSA. The molecule docking and site marker competitive experiment indicated that binding site was bound at IIA subdomain of HSA. This work provided a reference for the distribution and transportation of ALTB in vivo, and was conducive to improving the pharmacokinetics and pharmacodynamics of ALTB.

Acknowledgements

This work was supported by the national natural science foundation of China [grant number 21303105].

Conflicts of Interest

The authors declare no conflicts of interest regarding the publication of this paper.

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