Structure and ligand binding properties of human serum albumin

1. INTRODUCTION: Serum albumin possesses a unique capability to bind, covalently or reversibly, a great number of various endogenous and exogenous compounds. Several different transport proteins exist in blood plasma but albumin only is able to bind a wide diversity of ligands reversibly with high affinity. The subject of the present thesis is these binding properties. In 1981 the author proposed in a review a model for binding of ligands to serum albumin. In the model, binding of ligands to at least 6 distinct regions was considered. The purpose of the experimental work described here was to test the validity of the model. This was done by performing a large number of competition experiments. From these new data a revised model for ligand binding is presented. 2. STRUCTURE AND CONFORMATIONAL CHANGES OF SERUM ALBUMIN: Human serum albumin consists of 585 amino acids forming a single polypeptide of known sequence. A number of well characterized genetic variants have been reported. The physico-chemical characteristics of the protein are well-established. By contrast, the complete secondary and tertiary structures are not known; information about major structural features only has been obtained. The albumin molecule seems to have an overall ellipsoidal shape (about 140 x 40 A) and to be composed of domains. On the basis of the amino acid sequence, Brown (1977a) proposed a 3-domain model for the protein. Each domain is believed to consist of 6 helices forming a hydrophobic channel with basic and hydrophobic amino acid residues placed at the ends. Experimental data, however, indicate that the domains cannot be identical. Long-chain fatty acid ions are proposed to bind with high affinity within the channels. The ability to fluctuate between isomeric forms in aqueous solution could assist in adapting the albumin molecule to bind ligands with a diverse nature with high affinity. This possibility is discussed on the basis of several physico-chemical techniques including hydrogen-deuterium exchanges. Also the importance of the N-B transition for ligand binding is considered. 3. PRELIMINARY BINDING MODEL OF SERUM ALBUMIN: Single binding of ligands to serum albumin is usually described as high-affinity binding to one or two sites and weaker binding to a larger number of sites. In this chapter, the original binding model for high-affinity binding is elaborated. Region 1 seems to be specific for binding of one, or possibly two, ions of long-chain fatty acids. Region 2 is somewhat less specific and includes binding of octanoate, tryptophan, chlorazepate, thyroxine, p-iodobenzoate and possibly also chloride. Region 3 accommodates bilirubin, Phenol Red, Bromophenol Blue and iopanoate. Region 4 is a special site for strong binding of metal ions such as Cu++ and Ni++. Evidence is presented for placing the primary haemin site in a separate region (no. 5). The existence of additional binding regions, well-suited for high-affinity binding of drugs, is discussed...     

Competition between serum amyloid protein and apoprotein E for binding with human serum albumin

Research Center for preventive Medicine, Ministry of Health of Russian Federation, Moscow. (Presented by Academician of the Russian Academy of Medical Sciences V. N. Smirnov.) Translated from Byulleten' Éksperimental'noi Biologii i Meditsiny, Vol. 114, No. 12, pp. 598–600, December, 1992.     

Studies on HBsAg binding with polymerised human serum albumin by ELISA

A simple and sensitive ELISA was developed to characterize the interaction between polymerised human serum albumin (pHSA) and HBsAg, using pHSA-coated polyvinylmicrotitre plates as solid phase and anti-HBs-coupled HRPO as the conjugate. The interaction was found to be specific and dependent on the size of albumin polymer. pHSA-binding activity (pHSA-BA) was studied in both HBsAg-negative and HBsAg-positive sera from various liver diseases including acute viral hepatitis, fulminant hepatitis, cirrhosis of liver, chronic active hepatitis, and healthy HBsAg carriers. pHSA-BA was detected only in HBsAg-positive sera. Analysis of HBsAg-positive sera indicated pHSA-BA in high proportions of patients sera as compared to sera from healthy HBsAg carriers. pHSA-BA was detected both in the presence and absence of HBe markers, though the mean BA was relatively high in presence of HBeAg. The effect of human serum immunoglobulins (IgG, IgA, and IgM) on the BA was investigated and a correlation between pHSA-BA and HBsAg-IgM complex positivity in sera was established. Finally, the probable role of human serum IgM in facilitating the binding process was discussed.     

Chromatographic study of magnesium and calcium binding to immobilized human serum albumin.

The use of immobilized human serum albumin (HSA) as a stationary phase in affinity chromatography has been shown to be useful in resolving optical antipodes or to investigate interactions between drugs and protein. However, to our knowledge, no inorganic ion binding has been studied on this immobilized protein type. To do this, the human serum albumin stationary phase was assimilated to a weak cation-exchanger by working with a mobile phase pH equal to 6.5. A study of the eluent ionic strength effect on ion retention was carried out by varying the buffer concentrations and the column temperatures. The thermodynamic parameters for magnesium and calcium transfer from the mobile to the stationary phase were determined from linear van't Hoff plots. An enthalpy-entropy compensation study revealed that the type of interaction was independent of the mobile phase composition. A simple model based on the Gouy-Chapman theory was considered in order to describe the retention behavior of the test cations with the mobile phase ionic strength. From this theoretical approach, the relative charge densities of the human serum albumin surface implied in the binding process were estimated at different column temperatures.     

Characterisation of the polymerised and monomeric human serum albumin binding sites on hepatitis B surface antigen

Receptors for polymerised human albumin are present on the pre-S sequence of the envelope protein of HBV and on the hepatocyte membrane and are thought to be involved in uptake of the virus by hepatocytes. Using a solid phase radioimmunoassay we demonstrate binding of HBsAg to polymerised human serum albumin (pHSA) in both HBe antigen-positive and -negative patients, and this binding is linearly related to the HBsAg titre in both groups. There are probably several modes of interaction between HBsAg and pHSA. Here we show that pHSA binds to the 22,000-dalton polypeptide of HBsAg, which does not contain the pre-S sequence. This pHSA-HBsAg interaction is inhibited by physiological concentrations of human serum albumin, suggesting that the albumin known to be present in the envelop of HBsAg plays a role in this binding. The inhibition of pHSA/HBsAg interaction by native albumin suggests that this interaction is probably not an important mechanism of virus uptake during infection of hepatocytes.     

Characterisation of the binding of digitoxin and acetyldigitoxin to human serum albumin by high-performance affinity chromatography

Zonal elution and high-performance affinity chromatography were used to examine interactions of the drugs digitoxin and acetyldigitoxin with the protein human serum albumin (HSA). This was done by injecting small amounts of digitoxin and acetyldigitoxin onto an immobilized HSA column in the presence of mobile phases that contained various concentrations of digitoxin, acetyldigitoxin or other solutes as competing agents. A fixed concentration of beta-cyclodextrin was also present in the mobile phase as a solubilising agent. It was found that digitoxin and acetyldigitoxin each had strong interactions at a single common binding site on HSA, but with slightly different equilibrium constants for this region. Neither compound showed any competition with warfarin or L-tryptophan, which were used as probes for binding at the warfarin-azapropazone and indole-benzodiazepine sites of HSA. These results confirmed the presence of a separate binding region on HSA for digitoxin-related compounds.     

Immune recognition of serum albumin—XIII. Autoreactivity with rabbit serum albumin of rabbit antibodies against bovine or human serum albumins and autoimmune recognition of rabbit serum albumin

Recently, this laboratory reported an autoreactivity of myoglobin (Mb)3 antisera with the Mb of the species in which the antisera are raised. Also, animals injected with autologous Mb mounted an autoimmune antibody and T-lymphocyte proliferative response against this protein. This posed the possibility that autoimmune recognition might be a general phenomenon not confined only to sequestered proteins such as Mb. Using RSA, we have demonstrated unambiguously that RSA cross-reacted with rabbit antisera to bovine (BSA) or to human (HSA) albumin. In exchange experiments, ¹²⁵I-labelled RSA was bound by the IgG in the immune complexes isolated from rabbit antisera to BSA or HSA. Also, ¹²⁵I-antibodies were bound by RSA-adsorbents. Both binding activities were inhibited specifically by RSA. RSA was isolated from three rabbits and each rabbit was immunized with its own RSA. In each rabbit autoantibodies were found by exchange between immune complexes and the rabbit's own [¹²⁵I]-RSA. Also, ¹²⁵I-antibodies from each rabbit were bound by adsorbents of the rabbit's own RSA. Inhibition studies, data on preimmune sera and on antisera against other proteins confirmed the specificity of the binding. The findings confirm the universality of autoimmune recognition and lend support to our previous suggestion that antigenic sites are ‘structurally-inherent’ in the protein.     

戊二醛交联对壳聚糖/羟基磷灰石-庆大霉素缓释材料性能的影响

背景：交联是骨组织工程材料改性的一种常用方法,但目前仍缺乏交联剂对载药人工骨材料性能影响的相关研究与报道。 目的：研究戊二醛交联对壳聚糖/羟基磷灰石-庆大霉素载药人工骨材料力学性能、降解性能及体外药物缓释行为的影响。 方法：分别制备壳聚糖质量分数为10%,20%,30%的壳聚糖/羟基磷灰石-庆大霉素载药人工骨材料与戊二醛交联壳聚糖/羟基磷灰石-庆大霉素载药人工骨材料,检测各组材料的机械强度、吸水率、降解率及体外药物释放行为。 结果与结论：壳聚糖含量为10%,20%,30%壳聚糖/羟基磷灰石-庆大霉素的抗压强度分别为(10.16±1.17),(28.40±0.64),(23.28±1.30) MPa,经戊二醛交联后材料的抗压强度分别增大至(36.30±1.20),(51.60±2.08),(36.90±3.22) MPa。壳聚糖含量为10%,20%,30%壳聚糖/羟基磷灰石-庆大霉素交联后的吸水率与降解率均低于交联前。在体外缓释的第1天,30%壳聚糖/羟基磷灰石-庆大霉素的药物释放量为42.2%,材料经戊二醛交联处理后药物释放量降至33.6%,在随后的9 d,交联壳聚糖/羟基磷灰石-庆大霉素的总释放量均低于壳聚糖/羟基磷灰石-庆大霉素。表明戊二醛交联赋予了材料更好的生物稳定性,减缓了材料降解速率,显著改善了药物突释现象。中国组织工程研究杂志出版内容重点：生物材料;骨生物材料; 口腔生物材料; 纳米材料; 缓释材料; 材料相容性;组织工程全文链接：     

Demonstration of specific binding sites for human serum albumin in group C and G streptococci.

A total of 297 bacterial strains belonging to 27 species was tested for quantitative uptake of radiolabeled human serum albumin. Specific binding sites with high affinity for human serum albumin were found exclusively in group C and G streptococci. The albumin binding was found to be a time-dependent, saturable, and displaceable process which obeyed simple kinetic equations. Scatchard analysis revealed that human serum albumin bound to a homogeneous population of receptors with an affinity in the order ot 10(7) liters/mol and that the average bacterial cell carried more than 80,000 binding sites. The albumin receptor is a heat-stable component susceptible to proteolytic digestion. It has a surface localization separate from the receptors for immunolgobulin G, fibrinogen, aggregated beta 2-microglobulin, and haptoglobin. In individual strains, albumin reactivity was also detected independently of these other types of interactions with human proteins.     

Adorption of human fibrinogen and human serum albumin onto polyethylene

The adsorption of human fibrinogen (HFb) onto polyethylene at pH 7.4 and 37°C is irreversible with the saturated value 0.51 μg/cm². The established dependence of the concentration of irreversibly adsorbed HFb on the concentration of HFb in solution does not reflect an equilibrium between adsorption and desorption and cannot be described by an adsorption isotherm. No reversible adsorption of HFb and HSA at 37°C has been observed. At lower temperatures, both reversibly and irreversibly adsorbed HFb have been detected.     

Study of the antigenic structure of human serum albumin with monoclonal antibodies

Analysis of the antigenic structure of human serum albumin was undertaken using monoclonal antibodies. Nineteen antibodies were prepared and their specificities were studied using fragments which encompass the whole sequence of the albumin molecule. These antibodies recognized 13 different epitopes which are different from the one previously identified with two other monoclonal antibodies [Doyen et al., Immun. Lett. 3, 365-370 (1981)]. Among those 13 different epitopes, six were overlapping. Four epitopes were located on the N-terminal half of the albumin molecule. One of these required integrity of methionine 87 and the other three were overlapping and located around methionine 123. Eight epitopes were located on the C-terminal half of the albumin. Two of them were within the sequence, 330-422 and 299-496 respectively; the other six appeared to be topographic determinants which were altered or lost in the albumin fragments. A last epitope could not be located on any region of albumin. Four monoclonal antibodies directed against a given portion of the albumin molecule reacted slightly with another part of albumin, thus confirming the existence of an intramolecular cross-reactivity between the different domains of human albumin.     

Superhydrophobic effect on the adsorption of human serum albumin

Analytical protocol greatly influences the measurement of human serum albumin (HSA) adsorption to commercial expanded polytetrafluororethylene (ePTFE) exhibiting superhydrophobic wetting properties. Degassing of buffer solutions and evacuation of ePTFE adsorbent to remove trapped air immediately prior to contact with protein solutions are shown to be essential. Results obtained with ePTFE as a prototypical superhydrophobic test material suggest that vacuum degassing should be applied in the measurement of protein adsorption to any surface exhibiting superhydrophobicity. Solution depletion quantified using radiometry (¹²⁵I-labeled HSA) or electrophoresis yield different measures of adsorption, with nearly 4-fold higher surface concentrations of unlabeled HSA measured by the electrophoresis method. This outcome is attributed to the influence of the radiolabel on HSA hydrophilicity which decreases radiolabeled-HSA affinity for a hydrophobic adsorbent in comparison to unlabeled HSA. These results indicate that radiometry underestimates the actual amount of protein adsorbed to a particular material. Removal of radiolabeled HSA adsorbed to ePTFE by 3× serial buffer rinses also shows that the remaining “bound fraction” was about 35% lower than the amount measured by radiometric depletion. This observation implies that measurement of protein bound after surface rinsing significantly underestimates the actual amount of protein concentrated by adsorption into the surface region of a protein-contacting material.     

Aspirin blocks binding of photosensitizer SnET2 into human serum albumin: implications for photodynamic therapy

Tin etiopurpurin dichloride (SnET2) is one of the photosensitizers under investigation to be used in photodynamic therapy of prostate cancer. The drug is delivered intravenously, transported in vivo by liposomes and plasma proteins and localized within the prostate. SnET2 exists in two tautomeric forms (I - closed ring, II - open ring) with I converting spontaneously into the more energetically stable form II at physiological pH. Up to approximately 50% of the drug can be carried by serum albumin, although this association can increase photo-bleaching and diminish the drug efficiency. Molecular modeling and force field calculations indicate that Sudlow Site I in human serum albumin (HSA) is the most probable binding site for both forms of SnET2, with the porphyrin moiety nestling between domains IIA and IB, and the esterolytic side group oriented toward domain IIIA of HSA. Other drugs, including aspirin, bind to the same part of HSA. SnET2 does not bind to HSA when pre-incubated with aspirin, which confirms that its place of binding to this protein must be located near Lys199. This observation could be exploited to improve photo-efficiency of SnET2 by finding drugs that could compete with the photosensitizer for binding into Sudlow Site I of HSA.