Equilibrium dialysis of metal-serum albumin. I. Successive stability constants of Zn(II)-serum albumin and the Zn(2+)-induced cross-linking self-association

Processing of the spectrofluorimetric data using the graphical methods and the maximum likelihood approach

The present work describes the calculation of the binding constants from spectrofluorimetric data using simple graphical methods and specialized software implementing the maximum likelihood approach. The following popular cases are analyzed: 1) protein-small molecule; 2) protein-metal complex; 3) DNA-small molecule; 4) DNA-metal complex interactions. The inability of graphical plots to return the correct results except for the simplest situation (single reaction with a non-fluorescent product) is demonstrated. The possibility of determining the most probable stoichiometric model using the maximum likelihood estimation (LSQ as its special case) is discussed as well as the limitations.

Effect of Cu2+ and Zn2+ ions on human serum albumin interaction with plasma unsaturated fatty acids

Human serum albumin (HSA) serves as a depot and carrier of multiple unrelated ligands including several participants of the pathogenesis of Alzheimer's disease (AD), such as amyloid β peptide (Aβ), Zn²⁺/Cu²⁺ ions, docosahexaenoic (DHA), linoleic (LA), and oleic (OA) acids. To explore the interplay between HSA interaction with Zn²⁺/Cu²⁺ and the plasma unsaturated fatty acids (DHA, LA, OA, and arachidonic acid (ArA)), we have studied the metal dependence of the fatty acid (FA) binding capacity of HSA (n_max) and structural consequences of the HSA-FA interactions. HSA loading with Zn²⁺ decreases n_max value by 0.3-1.5, while its saturation with Cu²⁺ causes the FA-dependent n_max changes by up to 0.9. The Cu²⁺-induced decline in n_max value for DHA is due to conformational rearrangements in HSA molecule. In other cases, the changes in n_max are attributed to steric hindarance/facilitation of the HSA-FA interaction because of the protein multimerization/monomerization, as confirmed by chemical crosslinking. The surface hydrophobicity of HSA is Cu²⁺-, Zn²⁺-, and FA-dependent and decreases upon the FA binding, according to bis-ANS fluorescence data. Overall, Zn²⁺ or Cu²⁺ binding selectively affect HSA interaction with the FAs studied, in part due to changes in quaternary structure of the protein.

Ischemia-modified albumin: Crosstalk between fatty acid and cobalt binding

Myocardial ischemia is difficult to diagnose effectively with still few well-defined biochemical markers for identification in advance, or in the absence of myocardial necrosis. "Ischemia-modified albumin" (IMA), a form of albumin displaying reduced cobalt-binding affinity, is significantly elevated in ischemic patients, and the albumin cobalt-binding (ACB) assay can measure its level indirectly. Elucidating the molecular mechanism underlying the identity of IMA and the ACB assay hinges on understanding metal-binding properties of albumin. Albumin binds most metal ions and harbours four primary metal binding sites: site A, site B, the N-terminal site (NTS), and the free thiol at Cys34. Previous efforts to clarify the identity of IMA and the causes for its reduced cobalt-binding capacity were focused on the NTS site, but the degree of N-terminal modification could not be correlated to the presence of ischemia. More recent work suggested that Co2+ ions as used in the ACB assay bind preferentially to site B, then to site A, and finally to the NTS. This insight paved the way for a new consistent molecular basis of the ACB assay: albumin is also the main plasma carrier for free fatty acids (FFAs), and binding of a fatty acid to the high-affinity site FA2 results in conformational changes in albumin which prevent metal binding at site A and partially at site B. Thus, this review advances the hypothesis that high IMA levels in myocardial ischemia and many other conditions originate from high plasma FFA levels hampering the binding of Co2+ to sites A and/or B. This is supported by biophysical studies and the co-association of a range of pathological conditions with positive ACB assays and high plasma FFA levels.

Modulation of linoleic acid-binding properties of human serum albumin by divalent metal cations

Human serum albumin (HSA) is an abundant multiligand carrier protein, linked to progression of Alzheimer's disease (AD). Blood HSA serves as a depot of amyloid β (Aβ) peptide. Aβ peptide-buffering properties of HSA depend on interaction with its ligands. Some of the ligands, namely, linoleic acid (LA), zinc and copper ions are involved into AD progression. To clarify the interplay between LA and metal ion binding to HSA, the dependence of LA binding to HSA on Zn²⁺, Cu²⁺, Mg²⁺ and Ca²⁺ levels and structural consequences of these interactions have been explored. Seven LA molecules are bound per HSA molecule in the absence of the metal ions. Zn²⁺ binding to HSA causes a loss of one bound LA molecule, while the other metals studied exert an opposite effect (1-2 extra LA molecules are bound). In most cases, the observed effects are not related to the metal-induced changes in HSA quaternary structure. However, the Zn²⁺-induced decline in LA capacity of HSA could be due to accumulation of multimeric HSA forms. Opposite to Ca²⁺/Mg²⁺-binding, Zn²⁺ or Cu²⁺ association with HSA induces marked changes in its hydrophobic surface. Overall, the divalent metal ions modulate LA capacity and affinity of HSA to a different extent. LA- and Ca²⁺-binding to HSA synergistically support each other. Zn²⁺ and Cu²⁺ induce more pronounced changes in hydrophobic surface and quaternary structure of HSA and its LA capacity. A misbalanced metabolism of these ions in AD could modify interactions of HSA with LA, other fatty acids and hydrophobic substances, associated with AD.

Tyrosine fluorescence probing of the surfactant-induced conformational changes of albumin

Tyrosine fluorescence in native proteins is known to be effectively quenched, whereas its emission increases upon proteins’ unfolding.

Pb2+, Cu2+, Zn2+, Mg2+ and Mn2+ reduce the affinities of flavone, genistein and kaempferol for human serum albumin in vitro

Flavone (Fl), genistein (Gen) and kaempferol (Kol) were studied for their affinities towards human serum albumin (HSA) in the presence and absence of Pb2+,Cu2+,Zn2+,Mg2+ and Mn2+. The fluorescence intensities of HSA decreased with increasing concentration of the three flavonoids. Kaempferol resulted in a blue-shift of the ?em of HSA from 336 to 330 nm; flavone showed an obvious red-shift of the ?em of HSA from 336 to 342 nm; genistein did not cause an obvious blue-shift or red-shift of the ?em of HSA. However, the extents of ?em-shifts induced by the flavonoids in the presence of metal ions were much bigger than that in the absence of mental ions. Pb2+,Cu2+,Zn2+,Mg2+ and Mn2+ reduced the quenching constants of the flavonoids for HSA by 14.6% to 60.7% , 28% to 67.9%,3.5% to 59.4%, 23.2% to 63.7% and 14% to 65%, respectively. The affinities of flavone, genistein and kaempferol for HSA decreased about 10.84%, 10.05%and 3.56% in the presence of Pb2+, respectively. Cu2+ decreased the affinities of flavone, genistein and kaempferol for HSA about 14.04%, 5.14%and 8.89%, respectively. Zn2+ decreased the affinities of flavone, genistein and kaempferol for HSA about 3.79%, 0.55% and 3.58%, respectively. Mg2+ decreased the affinities of flavone, genistein and kaempferol for HSA about 16.94%, 2.94%and 7.04%, respectively. Mn2+ decreased the affinities of flavone, genistein and kaempferol for HSA about 14.24%, 3.66% and 4.78%, respectively.

In vitro studies on dissolved substance of cinnabar: chemical species and biological properties

Cinnabar is one of traditional Chinese medicines widely used in many Asian countries. It is also a medicine with potential toxicity especially when taking overdose. Up to date, studies on the mechanism of the biological activity of cinnabar were still insufficient.

AIM OF THE STUDY

To investigate the possible bioactive species from cinnabar after oral administration, which is the fundamental of biological effects of cinnabar.

MATERIALS AND METHODS

Under mimetic intestinal and gastric conditions, the chemical components dissolved from cinnabar were analyzed by infrared spectroscopy (IR) and Raman spectroscopy. Furthermore, binding of mercuric species of cinnabar extractions to human serum protein (HSA) was characterized and their intestinal permeability was determined using the Caco-2 cell monolayer. The cytotoxicity of cinnabar extractions was assessed on human kidney-2 (HK-2) cell.

RESULTS

Major dissolved species included mercuric polysulfide (i.e. HgS(2)(OH)(-) and Hg(3)S(2)Cl(2)). The apparent permeability coefficient (P(app)) of mercuric polysulfides was (1.6+/-0.3)x10(-6)cm/s, which is slightly lower than that of mercuric chloride (HgCl(2)). Unlike HgCl(2), mercuric polysulfides exhibited two tightly binding sites to HSA and had little effect on viability of HK-2 cells.

CONCLUSION

Mercuric polysulfides, as the major dissolved components, may serve as the active species of cinnabar exhibiting pharmacological and/or toxicological effects.

Effect of metal ions on the interaction between bovine serum albumin and berberine chloride extracted from a traditional Chinese Herb coptis chinensis franch

The effect of four metal ions Cu(2+), Ni(2+), Zn(2+) and Co(2+) on the interaction between bovine serum albumin (BSA) and berberine chloride (BC) extracted from a traditional Chinese Herb coptis chinensis franch, was investigated mainly by means of UV and fluorescence spectroscopy in this paper. The four metal ions make the quenching efficacy of BC to BSA higher than that in the absence of these metal ions because of the possible transition of BSA molecular conformation caused by metal ions. It was found that the quenching mechanism is a combination of static quenching with nonradiative energy transfer. In the presence of metal ions, the apparent association constant K(A) and the number of binding sites of BC on BSA are both decreased in a range of 8-19% and 25-28%, respectively, which indicates that the metal ions decrease the binding efficacy of BC on BSA and increase the concentration of free BC simultaneously. The scheme of interaction between BC and BSA in the presence of metal ions is a strong quenching but a weak binding.

Studies on the interaction between Ag(+) and human serum albumin

The interaction between Ag(+) and human serum albumin (HSA) has been intensively studied by means of equilibrium dialysis, ligand-to-metal charge transition (LMCT) bands, circular dichroism (CD) and Raman spectroscopy. Scatchard analysis of the results of equilibrium dialysis indicates the presence of two types of binding sites for Ag(+) on HSA, and the orders of magnitude of binding stability constants are found to be 10(5) and 10(4), respectively. During the binding process, a gradual increase in absorbance values of LMCT bands is observed with time-scanning UV absorption spectra, implying the Ag(I) centers are continually formed in HSA. The time-scanning CD spectra provide evidence that the binding of Ag(+) induces HSA to undergo a slow rearrangement of tertiary structure, and to change from the original conformation in the absence of Ag(+) (B-state) to conformation binding with Ag(+) (A-state). The rate constants and activation free energy of A-B transition are calculated. The Raman spectrum of Ag(I)-HSA system shows distinct vibration bands at 224 and 246 cm(-1) in the low-frequency region, which significantly reveal the formation of Ag-S and Ag-N bonds. In addition, the electrostatic interaction between Ag(+) and negatively charged oxygen is also detected with Raman spectroscopy.

The novel behaviour of interactions between Ni2+ ion and human or bovine serum albumin

We discovered a series of novel behaviours of interactions between Ni2+ ion and human or bovine serum albumin. Our results indicated that there exist two closely neighbouring identical prior binding sites in the binding of human or bovine serum albumin with Ni2+ ions, not only one. It is very likely that, after the binding of the first Ni2+ ion, an induced slow conformational transition happens, which leads to the binding of the second Ni2+ ion and shows itself as a hysteretic effect for a process of non-enzymic protein binding with metal ions. As the concentrations of the 1:1 (molar ratio of Ni2+ ion to protein) system increase, an increasing hypochromic effect is observed. Such a hypochromic effect has not been reported previously; however, it is in accord with the mechanism of dipole-dipole interactions between the electric dipole transition moments of chromophores.