Spectroscopic Analysis of an Antimalarial Drug's (Quinine) Influence on Human Serum Albumin Reduction and Antioxidant Potential

Abstract

Quinine (Qi) is a well-known drug used in malaria therapy; it is also a potential anti-arrhythmic drug used in the treatment of calf cramps, rheumatoid arthritis, colds, and photodermatitis. Moreover, it is used in the food industry for the production of tonics. This study aimed to analyze the interaction between quinine and a transporting protein-human serum albumin (HSA)-as well as the influence of Qi on both protein reduction and antioxidant potential. It was found that Qi (via spectrofluorometric measurements and circular dichroism spectroscopy) binds to HSA with a low affinity and slightly affects the secondary structure of albumin. As demonstrated by the use of ABTS and FRAP assays, HSA has a higher antioxidant and reduction potential than Qi, while their mutual interaction results in a synergistic effect in antioxidant activity and reduction potential.

Keywords: antioxidant and reduction potential; human serum albumin; quinine; spectroscopy.

Figure 1

Absorption spectra of analyzed ligand–protein mixtures: (a) “Qi”, “HSA”, “Qi-HSA_complex” and “Qi_calc = Qi-HSA_complex − HSA” at ligand:protein 1:1 molar ratio; (b) “Qi”, “HSA”, “Qi-HSA_complex” and “Qi_calc = Qi-HSA_complex − HSA” at ligand:protein 1:2 molar ratio; (c) “Qi”, “HSA”, “Qi-HSA_complex” and “Qi_calc = Qi-HSA_complex − HSA” at ligand:protein 1:4 molar ratio; (d) “Qi”, “Qi_calc 1:1 molar ratio”, “Qi_calc 1:2 molar ratio” and “Qi_calc 1:4 molar ratio”; [Qi] 7 × 10⁻⁵ M, [HSA] 7 × 10⁻⁵ M, 1.4 × 10⁻⁴ M, 2.8 × 10⁻⁴ M; all tested solutions were prepared in phosphate buffer (pH 7.4).

Figure 1

Absorption spectra of analyzed ligand–protein mixtures: (a) “Qi”, “HSA”, “Qi-HSA_complex” and “Qi_calc = Qi-HSA_complex − HSA” at ligand:protein 1:1 molar ratio; (b) “Qi”, “HSA”, “Qi-HSA_complex” and “Qi_calc = Qi-HSA_complex − HSA” at ligand:protein 1:2 molar ratio; (c) “Qi”, “HSA”, “Qi-HSA_complex” and “Qi_calc = Qi-HSA_complex − HSA” at ligand:protein 1:4 molar ratio; (d) “Qi”, “Qi_calc 1:1 molar ratio”, “Qi_calc 1:2 molar ratio” and “Qi_calc 1:4 molar ratio”; [Qi] 7 × 10⁻⁵ M, [HSA] 7 × 10⁻⁵ M, 1.4 × 10⁻⁴ M, 2.8 × 10⁻⁴ M; all tested solutions were prepared in phosphate buffer (pH 7.4).

Figure 2

Spectrofluorimetric analysis: (a) the fluorescent emission spectra of HSA ([HSA] 3 × 10 ⁻⁶ M): without or in the presence of Qi ([Qi] 1 × 10⁻³ M); λ_ex 285 [nm]; (b) the Klotz plot for HSA in the presence of Qi at Qi:HSA molar ratio from 0.67:1 to 6.67:1. In the insert: quenching of HSA fluorescence by Qi at Qi:HSA molar ratio from 0.67:1 to 6.67:1; λ_ex 285 nm; [HSA] 3 × 10⁻⁶ M; [Qi] 1 × 10⁻³ M; pH = 7.4.

Figure 2

Spectrofluorimetric analysis: (a) the fluorescent emission spectra of HSA ([HSA] 3 × 10 ⁻⁶ M): without or in the presence of Qi ([Qi] 1 × 10⁻³ M); λ_ex 285 [nm]; (b) the Klotz plot for HSA in the presence of Qi at Qi:HSA molar ratio from 0.67:1 to 6.67:1. In the insert: quenching of HSA fluorescence by Qi at Qi:HSA molar ratio from 0.67:1 to 6.67:1; λ_ex 285 nm; [HSA] 3 × 10⁻⁶ M; [Qi] 1 × 10⁻³ M; pH = 7.4.

Figure 3

The value of % inhibition of Qi ([Qi] = 7 × 10⁻⁵ M), HSA ([HSA] = 2.8 × 10⁻⁴ M) and Qi-HSA_complex (1:4 molar ratio; [Qi] = 7 × 10⁻⁵ M; [HSA] = 2.8 × 10⁻⁴ M); Ex: expected, De: designated (DPPH assay); pH = 7.4.

Figure 4

The structural formula of Qi (ChemSketch 12.1.0.31258).