Abstract
The study on the interaction of human serum albumin (HSA) with three widely used drugs (diclofenac sodium (DIC), furosemide (FUR) and dexamethasone phosphate (DEX)) was investigated by fluorescence method. Fluorescence emission spectra of HSA in presence of the studied drugs was recorded at excitation wavelength 278 nm and showed that the studied drugs act as quenchers. A decrease in fluorescence emission at 340 nm was attributed to changes in environment of the protein fluorophore caused due to presence of the ligand. The modified Stern–Volmer equation was used as a mathematical model to calculate the binding constants between the drug and HSA. The binding constants for the studied drugs with HSA were found inversely related with temperature. The thermodynamic parameters, the changes of standard Gibbs free energy (ΔG°), enthalpy change (DH°) and entropy change (DS°) for the drug-HSA interaction were calculated according to van't Hoff equation. Among the thermodynamic parameters, the values of ΔG° were: -25.83, -25.29 and -25.09 kJ/mol for the drugs DIC, FUR and DEX, respectively, and the values of ΔH° and ΔS° were negative suggested that the hydrogen bonding and van der Waals forces were the predominant intermolecular forces in stabilizing the drug-HSA complex formed. The effect of pH on the binding of the studied drugs to human serum albumin in phosphate buffer solutions (pH 6.0-8.0) has been investigated in this study. The results showed that the binding of each studied drug was decreased with pH studied and the results were attributed according to the ionic forms of both protein and drug in the pH range studied. Finally, the distances between the donor (HSA) and the acceptor (drug) were estimated to be 2.98, 3.52 and 5.30 nm for the drugs DIC, FUR and DEX, respectively, based on Förster’s resonance energy transfer theory (FRET).
Highlights
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 altering the drug’s pharmacokinetic and pharmacodynamic properties
It is obvious that HSA fluorescence emission that is peaked at 340 nm was quenched after being excited with a wavelength of 278 nm as the drug concentrations increased. and there was a slight red shift at the maximum wavelength from 340 to 344 nm, which suggested that there was interaction between the drug used and HSA forming drug-HSA complex, making a slight change in the environment around the chromophore of HSA and indicates that a slight conformational change may take place during the binding of the drugs with the HSA
It was found that the interaction between the studied drugs (DIC, FUR and DEX) with HSA occur with variable affinities and the binding constants (Kass) for the studied drugs were decreased with increasing temperature as shown in Figure 4, confirming occurance of static quenching mechanisms in these interactions
Summary
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 altering the drug’s pharmacokinetic and pharmacodynamic properties. HSA being the most abundant protein of blood plasma, and has a limited number of high-affinity binding sites. HSA comprises three homologous domains that assemble to form a heart-shaped molecule. Each domain is a product of two subdomains that possess common structural motifs. The principal regions of ligand binding to human serum albumin are located in hydrophobic cavities in subdomains IIA and IIIA, which exhibit similar chemistry. The structure explains numerous physical phenomena and should provide insight into future pharmacokinetic and genetically engineered therapeutic applications of serum albumin [1]
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