The effects of drug complexation on the stability and conformation of human serum albumin: protein unfolding.

We report different analytical methods used to study the effects of 3\'-azido-3\'-deoxythymidine, aspirin, taxol, cisplatin, atrazine, 2,4-dichlorophenoxyacetic, biogenic polyamines, chlorophyll, chlorophyllin, poly(ethylene glycol), vanadyl cation, vanadate anion, cobalt-hexamine cation, and As2O3, on the stability and secondary structure of human serum albumin (HSA) in aqueous solution, using capillary electrophoresis, Fourier transform infrared, ultraviolet visible, and circular dichroism (CD) spectroscopic methods. The concentrations of HSA used were 4% to 2% or 0.6 to 0.3 mM, while different ligand concentrations were 1 microM to 1 mM. Structural data showed drugs are mostly located along the polypeptide chains with both specific and nonspecific interactions. The stability of drug-protein complexes were in the order K(VO(2+)) 1.2 x 10(8) M(-1) > K(AZT) 1.9 x 10(6) M(-)1 > K(PEG) 4.1 x 10(5) M(-1) > K(atrazine) 3.5 x 10(4) M(-1) > K(chlorophyll) 2.9 x 10(4) M(-1) > K2,4-D 2.5 x 10(4) M-1 > K(spermine) 1.7 x 10(4) M(-1) > K(taxol) 1.43 x 10(4) M(-1) > K(Co(3+)) > 1.1 x 10(4) M(-1) > K(aspirin) 1.04 x 10(4)i(-1) > K(chlorophyllin) 7.0 x 10(3) M(-1) > K(VO(3)(-)) 6.0 x 103 M(-1) > K(spermidine) 5.4 x 10(3) M(-1) > K(putrescine) 3.9 x 10(3) M(-1) > K(As(2)O(3)) 2.2 x 10(3) M(-1)> K(cisplatin) 1.2 x 10(2) M(-1). The protein conformation was altered (infrared and CD results) with major reduction of alpha-helix from 60 to 55% (free HSA) to 49 to 40% and increase of beta-structure from 22 to 15% (free HSA) to 33 to 23% in the drug-protein complexes. The alterations of protein secondary structure are attributed to a partial unfolding of HSA on drug complexation.

The effects of poly(ethylene glycol) on the solution structure of human serum albumin.

Protein physical and chemical properties can be altered by polymer interaction. The presence of several high affinity binding sites on human serum albumin (HSA) makes it a possible target for many organic and polymer molecules. This study was designed to examine the interaction of HSA with poly(ethylene glycol) (PEG) in aqueous solution at physiological conditions. Fourier transform infrared, ultraviolet-visible, and CD spectroscopic methods were used to determine the polymer binding mode, the binding constant, and the effects of polymer complexation on protein secondary structure. The spectroscopic results showed that PEG is located along the polypeptide chains through H-bonding interactions with an overall affinity constant of K = 4.12 x 10(5) M(-1). The protein secondary structure showed no alterations at low PEG concentration (0.1 mM), whereas at high polymer content (1 mM), a reduction of alpha-helix from 59 (free HSA) to 53% and an increase of beta-turn from 11 (free HSA) to 22% occurred in the PEG-HSA complexes (infrared data). The CDSSTR program (CD data) also showed no major alterations of the protein secondary structure at low PEG concentrations (0.1 and 0.5 mM), while at high polymer content (1 mM), a major reduction of alpha-helix from 69 (free HSA) to 58% and an increase of beta-turn from 7 (free HSA) to 18% was observed.