Using bound fatty acids to disclose the functional structure of serum albumin

Background

Serum albumin is a major transport protein in mammals and is known to have at least seven binding sites for long-chain fatty acids (FAs).

Scope of review

We have devised a new electron paramagnetic resonance (EPR) spectroscopic approach to gain information on the functional structure of serum albumin in solution in a “coarse-grained” manner from the ligands' point of view. Our approach is based on using spin labeled (paramagnetic) stearic acids self-assembled with albumin and subsequent nanoscale distance measurements between the FAs using double electron–electron resonance spectroscopy (DEER). Simple continuous wave (CW) EPR spectroscopy, which allows for quantification of bound ligands, complements our studies.

Major conclusions

Based on DEER nanoscale distance measurements, the functional solution structure of human serum albumin (HSA) has remarkably been found to have a much more symmetric distribution of entry points to the FA binding sites than expected from the crystal structure, indicating increased surface flexibility and plasticity for HSA in solution.In contrast, for bovine serum albumin (BSA), the entry point topology is in good agreement with that expected from the crystal structure of HSA. Changes in the solution structures between albumins can hence be revealed and extended to more albumins to detect functional differences at the nanoscale.Going beyond fundamental structural studies, our research platform is also excellently suited for general studies of protein–solvent interactions, temperature effects and ligand binding.

General significance

We discuss how our research platform helps illuminate protein dynamics and function and can be used to characterize albumin-based hybrid materials. This article is part of a Special Issue entitled Serum Albumin.     

New insights into deleterious impacts of in vivo glycation on albumin antioxidant activities

Background

Albumin constitutes the most abundant circulating antioxidant and prevents oxidative damages. However, in diabetes, this plasmatic protein is exposed to several oxidative modifications, which impact on albumin antioxidant properties.

Methods

Most studies dealing on albumin antioxidant activities were conducted on in vitro modified protein. Here we tried to decipher whether reduced antioxidant properties of albumin could be evidenced in vivo. For this, we compared the antioxidant properties of albumin purified from diabetic patients to in vitro models of glycated albumin.

Results

Both in vivo and in vitro glycated albumins displayed impaired antioxidant activities in the free radical-induced hemolysis test. Surprisingly, the ORAC method (Oxygen Radical Antioxidant Capacity) showed an enhanced antioxidant activity for glycated albumin. Faced with this paradox, we investigated antioxidant and anti-inflammatory activities of our albumin preparations on cultured cells (macrophages and adipocytes). Reduced cellular metabolism and enhanced intracellular oxidative stress were measured in cells treated with albumin from diabetics. NF-kB –mediated gene induction was higher in macrophages treated with both type of glycated albumin compared with cells treated with native albumin. Anti inflammatory activity of native albumin is significantly impaired after in vitro glycation and albumin purified from diabetics significantly enhanced IL6 secretion by adipocytes. Expression of receptor for advanced glycation products is significantly enhanced in glycated albumin-treated cells.

Conclusions and general significance

Our results bring new evidences on the deleterious impairments of albumin important functions after glycation and emphasize the importance of in vivo model of glycation in studies relied to diabetes pathology.     

Deciphering metal-induced oxidative damages on glycated albumin structure and function

Background

Metal ions such as copper or zinc are involved in the development of neurodegenerative pathologies and metabolic diseases such as diabetes mellitus. Albumin structure and functions are impaired following metal- and glucose-mediated oxidative alterations. The aim of this study was to elucidate effects of Cu(II) and Zn(II) ions on glucose-induced modifications in albumin by focusing on glycation, aggregation, oxidation and functional aspects.

Methods

Aggregation and conformational changes in albumin were monitored by spectroscopy, fluorescence and microscopy techniques. Biochemical assays such as carbonyl, thiol groups, albumin-bound Cu, fructosamine and amine group measurements were used. Cellular assays were used to gain functional information concerning antioxidant activity of oxidized albumins.

Results

Both metals promoted inhibition of albumin glycation associated with an enhanced aggregation and oxidation process. Metal ions gave rise to the formation of β-amyloid type aggregates in albumin exhibiting impaired antioxidant properties and toxic activity to murine microglia cells (BV2). The differential efficiency of both metal ions to inhibit albumin glycation, to promote aggregation and to affect cellular physiology is compared.

Conclusions and general significance

Considering the key role of oxidized protein in pathology complications, glycation-mediated and metal ion-induced impairment of albumin properties might be important parameters to be followed and fought.     

Binding,unfolding and refolding dynamics of serum albumins

Background

The serum albumins (human and bovine serum albumin) occupy a seminal position among all proteins investigated until today as they are the most abundant circulatory proteins. They play the major role of a transporter of many bio-active substances which include various fatty acids, drug molecules, and amino acids to the target cells. Hence, studying the interaction of these serum albumins with different binding agents has attracted enormous research interests from decades. The nature and magnitude of these bindings have direct consequence on drug delivery, pharmacokinetics, therapeutic efficacy and drug design and control.

Scope of the review

In the present review, we summarize the binding characteristics of both the serum albumins with surfactants, lipids and vesicles, polymers and dendrimers, nanomaterials and drugs. Finally we have reviewed the effect of various chemical and physical denaturants on these albumins with a special emphasis on protein unfolding and refolding dynamics.

Major conclusions

The topic of binding and dynamics of protein unfolding and refolding spans across all areas of inter-disciplinary sciences and will benefit clinical toxicology and medicines. The extensive data from several contemporary research based on albumins will help us to understand protein dynamics in a more illustrious manner.

General significance

These data have immense significance in understanding and unravelling the mechanisms of protein unfolding/refolding and thus can pave the way to prevent protein mis-folding/aggregation which sometimes leads to severe consequences like Parkinson's and Alzheimer's diseases. This article is a part of a Special Issue entitled Serum Albumin. This article is part of a Special Issue entitled Serum Albumin.     

Serum thioredoxin reductase levels increase in response to chemically induced acute liver injury

Background

Mammalian thioredoxin reductases (TrxR) are selenoproteins with important roles in antioxidant defense and redox regulation, principally linked to functions of their main substrates thioredoxins (Trx). All major forms of TrxR are intracellular while levels in serum are typically very low.

Methods

Serum TrxR levels were determined with immunoblotting using antibodies against mouse TrxR1 and total enzyme activity measurements were performed, with serum and tissue samples from mouse models of liver injury, as triggered by either thioacetamide (TAA) or carbon tetrachloride (CCl₄).

Results

TrxR levels in serum increased upon treatment and correlated closely with those of alanine aminotransferase (ALT), an often used serum biomarker for liver damage. In contrast, Trx1, glutathione reductase, superoxide dismutase or selenium-containing glutathione peroxidase levels in serum displayed much lower increases than TrxR or ALT.

Conclusions

Serum TrxR levels are robustly elevated in mouse models of chemically induced liver injury.

General significance

The exaggerated TrxR release to serum upon liver injury may reflect more complex events than a mere passive release of hepatic enzymes to the extracellular milieu. It can also not be disregarded that enzymatically active TrxR in serum could have yet unidentified physiological functions.     

Novel insights into the pleiotropic effects of human serum albumin in health and disease

Background

Human serum albumin is the principal protein in human serum. It participates in regulation of plasma oncotic pressure and transports endogenous and exogenous ligands such as thyroxine, free fatty acids, bilirubin, and various drugs. Therefore, studying its ligand binding mechanism is important in understanding many functions of the protein.

Scope of review

This review discusses the pleiotropic biochemical effects and their relevance to physiologic functions of albumin.

Major conclusions

Although HSA is traditionally recognized for its ligand transport and oncotic effects in human circulation, our studies have revealed its participation in several other important physiological functions. In some instances, it may function as a catalyst. Pleiotropic properties of HSA have been exploited by development of recombinant HSA and its mutants, and the use of these recombinant proteins in studies with various biochemical and biophysical techniques. These studies allowed us to obtain new insights on the diverse roles of HSA in human physiology. The following aspects of HSA were discussed in this review: 1) HSA and its mutants' role in thyroxine transport, 2) structural details of the ligand binding functions of HSA to ligands such as warfarin, digoxin, halothane anesthetics, nitric oxide, bilirubin, free fatty acids, etc, and 3) the formation of modified albumin during myocardial ischemia, its diagnostic significance, and HSA's role in cardiovascular disease.

General significance

The appreciation and understanding of structural details and new physiological roles has provided a renewed interest in HSA research. Specific structural information gained on various mechanisms of HSA–ligand interaction can be used to develop a model to better understand protein–drug interactions, aid in the development of new drugs with improved pharmacokinetic effects, and ultimately be used to improve the quality of healthcare. This article is part of a Special Issue entitled Serum Albumin.     

Allosteric modulation of zinc speciation by fatty acids

Background

Serum albumin is the major protein component of blood plasma and is responsible for the circulatory transport of a range of small molecules that include fatty acids, hormones, metal ions and drugs. Studies examining the ligand-binding properties of albumin make up a large proportion of the literature. However, many of these studies do not address the fact that albumin carries multiple ligands (including metal ions) simultaneously in vivo. Thus the binding of a particular ligand may influence both the affinity and dynamics of albumin interactions with another.

Scope of review

Here we review the Zn^2 + and fatty acid transport properties of albumin and highlight an important interplay that exists between them. Also the impact of this dynamic interaction upon the distribution of plasma Zn^2 +, its effect upon cellular Zn^2 + uptake and its importance in the diagnosis of myocardial ischemia are considered.

Major conclusions

We previously identified the major binding site for Zn^2 + on albumin. Furthermore, we revealed that Zn^2 +-binding at this site and fatty acid-binding at the FA2 site are interdependent. This suggests that the binding of fatty acids to albumin may serve as an allosteric switch to modulate Zn^2 +-binding to albumin in blood plasma.

General significance

Fatty acid levels in the blood are dynamic and chronic elevation of plasma fatty acid levels is associated with some metabolic disorders such as cardiovascular disease and diabetes. Since the binding of Zn^2 + to albumin is important for the control of circulatory/cellular Zn^2 + dynamics, this relationship is likely to have important physiological and pathological implications. This article is part of a Special Issue entitled Serum Albumin.     

Tyrosine residues play an important role in heme detoxification by serum albumin

Background

Serum albumin binds avidly to heme to form heme–serum albumin complex, also called methemalbumin, and this binding is thought to protect against the potentially toxic effects of heme. However, the mechanism of detoxification has not been fully elucidated.

Methods

SDS-PAGE and Western blot were used to determine the efficiency of methemalbumin on catalyzing protein carbonylation and nitration. HPLC was used to test the formation of heme to protein cross-linked methemalbumin.

Results

The peroxidase activity of heme increased upon human serum albumin (HSA) binding. Methemalbumin showed higher efficiency in catalyzing tyrosine oxidation than free heme in the presence of H₂O₂. Methemalbumin catalyzed self-nitration and significantly promoted the nitration of tyrosine in coexistent protein, but decreased the carbonylation of coexistent protein compared with heme. The heme to protein cross-linked form of methemalbumin suggested that HSA trapped the free radical accompanied by the formation of ferryl heme. When tyrosine residues in HSA were modified by iodination, HSA lost of protection effect on protein carbonylation. The low concentration of glutathione could effectively inhibit tyrosine nitration, but had no effect on protein carbonylation.

Conclusion

HSA protects against the toxic effect of heme by transferring the free radical to tyrosine residues in HSA, therefore protecting surrounding proteins from irreversible oxidation, rather than by direct inhibiting the peroxidase activity. The increased tyrosine radicals can be reduced by endogenic antioxidants such as GSH.

General significance

This investigation indicated the important role of tyrosine residues in heme detoxification by HSA and suggested a possible novel mechanism.     

Albumin–drug interaction and its clinical implication

Background

Human serum albumin acts as a reservoir and transport protein for endogenous (e.g. fatty acids or bilirubin) and exogenous compounds (e.g. drugs or nutrients) in the blood. The binding of a drug to albumin is a major determinant of its pharmacokinetic and pharmacodynamic profile.

Scope of review

The present review discusses recent findings regarding the nature of drug binding sites, drug-albumin binding in certain diseased states or in the presence of coadministered drugs, and the potential of utilizing albumin–drug interactions in clinical applications.

Major conclusions

Drug–albumin interactions appear to predominantly occur at one or two specific binding sites. The nature of these drug binding sites has been fundamentally investigated as to location, size, charge, hydrophobicity or changes that can occur under conditions such as the content of the endogenous substances in question. Such findings can be useful tools for the analysis of drug–drug interactions or protein binding in diseased states. A change in protein binding is not always a problem in terms of drug therapy, but it can be used to enhance the efficacy of therapeutic agents or to enhance the accumulation of radiopharmaceuticals to targets for diagnostic purposes. Furthermore, several extracorporeal dialysis procedures using albumin-containing dialysates have proven to be an effective tool for removing endogenous toxins or overdosed drugs from patients.

General significance

Recent findings related to albumin–drug interactions as described in this review are useful for providing safer and efficient therapies and diagnoses in clinical settings. This article is part of a Special Issue entitled Serum Albumin.     

Redox properties of serum albumin

Background

Oxidative damage results in protein modification, and is observed in numerous diseases. Human serum albumin (HSA), the most abundant circulating protein in the plasma, exerts important antioxidant activities against oxidative damage.

Scope of review

The present review focuses on the characterization of chemical changes in HSA that are induced by oxidative damage, their relevance to human pathology and the most recent advances in clinical applications.

Major conclusions

The antioxidant properties of HSA are largely dependent on Cys34 and its contribution to the maintenance of intravascular homeostasis, including protecting the vascular endothelium under disease conditions related to oxidative stress. Recent studies also evaluated the susceptibility of other important amino acid residues to free radicals. The findings suggest that a redox change in HSA is related to the oxidation of several amino acid residues by different oxidants. Further, Cys34 adducts, such as S-nitrosylated and S-guanylated forms also play an important role in clinical applications. On the other hand, the ratio of the oxidized form to the normal form of albumin (HMA/HNA), which is a function of the redox states of Cys34, could serve as a useful marker for evaluating systemic redox states, which would be useful for the evaluation of disease progression and therapeutic efficacy.

General significance

This review provides new insights into our current understanding of the mechanism of HSA oxidation, based on in vitro and in vivo studies.This article is part of a Special Issue entitled Serum Albumin.     

The Global Sequence Signature algorithm unveils a structural network surrounding heavy chain CDR3 loop in Camelidae variable domains

Background

A large fraction of camelid (camels and llamas) antibodies is composed of heavy chain-only homodimers, able to recognise antigens with their variable domain. Events in somatic assembly and maturation of antibodies such as hypermutations and rearrangement of variable loops (CDRs — complementary determining regions) and selection among a wide range of framework variants are generally considered to be random processes.

Methods

An original algorithmic approach (Global Sequence Signature—GSS) was developed, able to take into account multiple functional and/or local sequence properties to detect scattered evolutionary constraints into sequences.

Results

Using the GSS approach, we show that the length of the main hypervariable loop (CDR3) is linked to the nature of 19 surrounding residues on the scaffold. Surprisingly, the relation between CDR3 size and scaffold residues strongly depends on the considered species, illustrating either significant differences in selection mechanisms or functional constraints during antibody maturation.

Conclusions

Combined with the statistical coupling analysis (SCA) approach at the level of scaffold residues, this study has unravelled a robust interaction network on antibody structure surrounding the CDR3 loop.

General significance

In addition to the general applicability of the GSS algorithm, which can bring together functional and sequence data to locate hot spots of constrained evolution, the relationship between CDR3 and scaffold discussed here should be taken into account in protein engineering when designing antibody libraries.     

Human serum albumin from recombinant DNA technology: Challenges and strategies

Background

As the most abundant protein in the blood, human serum albumin (HSA) plays an important role in maintaining plasma oncotic pressure and fluid balance between the body's compartments. HSA is thus widely used in the clinic to treat diseases. However, the shortage of and safety issues arising from using plasma HSA (pHSA) underscore the importance of recombinant HSA (rHSA) as a promising substitute for pHSA.

Scope of review

Here, we review the production of rHSA, from expression to downstream processing, and highlight the scalability and cost-effectiveness of the two main expression platforms. We also discuss the biosafety of commercially available pharmaceutical rHSA with respect to impurities and contaminants, followed by an analysis of recent progress in preclinical and clinical trials. We emphasise the challenges of producing pharmaceutical-grade rHSA.

Major conclusions

rHSA can be highly expressed in various hosts and seems to be identical to pHSA. rHSA generated from yeast appears to be as efficient and safe as pHSA in a series of preclinical and clinical trials, whereas rHSA from rice seeds exhibits great potential for more cost-effective production. Cost-effective products with no adverse effects will likely play a vital role in future human therapeutics.

General significance

Our understanding of pharmaceutical-grade rHSA production has improved with respect to expression hosts, biochemical properties, downstream processing, and the detection and removal of impurities. However, due to the large dosages required for clinical applications, the production of sufficient quantities of rHSA still presents challenges. This article is part of a Special Issue entitled Serum Albumin.     

N-acetyl-l-methionine is a superior protectant of human serum albumin against photo-oxidation and reactive oxygen species compared to N-acetyl-l-tryptophan

Background

Sodium octanoate (Oct) and N-acetyl-l-tryptophan (N-AcTrp) are widely used as stabilizers during pasteurization and storage of albumin products. However, exposure to light photo-degrades N-AcTrp with the formation of potentially toxic compounds. Therefore, we have examined the usefulness of N-acetyl-l-methionine (N-AcMet) in comparison with N-AcTrp for long-term stability, including photo stability, of albumin products.

Methods

Recombinant human serum albumin (rHSA) with and without additives was photo-irradiated for 4 weeks. The capability of the different stabilizers to scavenge reactive oxygen species (ROS) was examined by ESR spectrometry. Carbonyl contents were assessed by a spectrophotometric method using fluoresceinamine and Western blotting, whereas the structure of rHSA was examined by SDS-PAGE, far-UV circular dichroism and differential scanning calorimetry. Binding was determined by ultrafiltration.

Results

N-AcMet was found to be a superior ROS scavenger both before and after photo-irradiation. The number of carbonyl groups formed was lowest in the presence of N-AcMet. According to SDS-PAGE, N-AcMet stabilizes the monomeric form of rHSA, whereas N-AcTrp induces degradation of rHSA during photo-irradiation. The decrease in α-helical content of rHSA was the smallest in the presence of Oct, without or with N-AcMet. Photo-irradiation did not affect the denaturation temperature or calorimetric enthalpy of rHSA, when N-AcMet was present.

Conclusion

The weakly bound N-AcMet is a superior protectant of albumin, because it is a better ROS-protector and structural stabilizer than N-AcTrp, and it is probable and also useful for other protein preparations.

General significance

N-AcMet is an effective stabilizer of albumin during photo-irradiation, while N-Ac-Trp promotes photo-oxidative damage to albumin.     

Fatty acid binding to serum albumin: Molecular simulation approaches

Background

Binding affinity for human serum albumin (HSA) is one of the most important factors affecting the distribution and free blood concentration of many ligands. The effect of fatty acids (FAs) on HSA-ligand binding has long been studied. Since the elucidation of the 3-dimensional structure of HSA, molecular simulation approaches have been applied to studies of the structure–function relationship of HSA–FA binding.

Scope of review

We review current insights into the effects of FA binding on HSA, focusing on the biophysical insights obtained using molecular simulation approaches such as docking, molecular dynamics (MD), and binding free energy calculations.

Major conclusions

Possible conformational changes on binding of FA molecules to HSA have been observed through MD simulations. High- and low-affinity FA-binding sites on HSA have been identified based on binding free energy calculations. The relationship between the warfarin binding affinity of HSA and FA molecules has been clarified based on the results of simulations of multi-site FA binding that cannot be experimentally observed.

General significance

Molecular simulation approaches have great potentials to provide detailed biophysical insights into HSA as well as the effects of the binding of FAs or other ligands to HSA. This article is part of a Special Issue entitled Serum Albumin.     

Glycated albumin and diabetes mellitus

Background

Diabetes is a growing worldwide problem that is strongly associated with atherosclerosis. Screening and intervention for diabetes in the earliest stages are advocated for the prevention of diabetic complications and cardiovascular disease.

Scope of review

This review gives a background of and discusses the potential clinical utility of glycated albumin (GA) in diabetes.

Major conclusions

GA is a ketoamine formed via a non-enzymatic glycation reaction of serum albumin and it reflects mean glycemia over two to three weeks. GA can be used for patients with anemia or hemoglobinopathies for whom the clinically measured hemoglobin A1c level may be inaccurate. Because both serum and plasma samples can be used, GA can be analyzed from the same samples as common biological markers. GA is a useful marker for the screening of diabetes in a medical evaluation. It can be also used to determine the effectiveness of treatment before initiating or changing medications for diabetic patients. GA is potentially an atherogenic protein in the development of diabetic atherosclerosis.

General significance

GA measurement is useful as part of a routine examination to screen for both diabetes and atherosclerosis. This article is part of a Special Issue entitled Serum Albumin.     

Albumin domain II mutant with high bilirubin binding affinity has a great potential as serum bilirubin excretion enhancer for hyperbilirubinemia treatment

Background

4Z,15Z-bilirubin-IXα (BR), an endogenous toxic compound that is sparingly soluble in water, binds human serum albumin (HSA) with high affinity in a flexible manner. Our previous findings suggest that both Lys195 and Lys199 in subdomain IIA are important for the high-affinity binding of BR, and especially Lys199 in stand-alone domain II plays a prominent role in the renal elimination of BR. Our hypothesis is that HSA-domain II with high BR binding would be a useful therapeutic agent to treat hyperbilirubinemia in patients with impaired liver function.

Methods

Unbound BR concentrations were determined using a modified HRP assay. To evaluate the effect of pan3_3-13 domain II mutant in promoting urinary BR excretion, the serum concentration and urinary excretion amount of BR were determined using bile duct ligation mice.

Results

After three or six rounds of panning, pan3_3-13 and pan6_4 were found to have a significantly higher affinity for BR than wild-type domain II. Administration of pan3_3-13 significantly reduced serum BR level and increased its urinary excretion in the disease model mice as compared to wild-type domain II treatment.

Conclusions

These results suggest that pan3_3-13 has great potential as a therapeutic agent that promotes urinary BR excretion in hyperbilirubinemia.

General significance

This is the first study to be applied to other HSA bound toxic compounds that are responsible for the progression of disease, thereby paving the way for the development of non-invasive and cost effective blood purification treatment methods.     

Structural studies of several clinically important oncology drugs in complex with human serum albumin

Background

Serum albumin is a major pharmacokinetic effector of drugs. To gain further insight into albumin binding chemistry, the crystal structures of six oncology agents were determined in complex with human serum albumin at resolutions of 2.8 to 2.0 Å: camptothecin, 9-amino-camptothecin, etoposide, teniposide, bicalutamide and idarubicin.

Methods

Protein crystal growth and low temperature X-ray crystallography

Results

These large, complex drugs are all bound within the subdomain IB binding region which can be described as a hydrophobic groove formed by α-helices h7, h8 and h9 covered by the extended polypeptide L1. L1 creates a binding cavity with two access sites, one between loop L1 and α-helices h7 and h8 (distal site: IB_d) and the other between L1 and α-helix h9 (proximal site: IB_p). Camptothecin (2.4 Å) and 9 amino camptothecin (2.0 Å) are clearly bound as the open lactone form (IB_p). Idarubicin (2.8 Å) binds in a DNA like dimer complex via an intermolecular π stacking arrangement in IB_d. Bicalutamide (2.4 Å) is bound in a folded intramolecular π stacking arrangement between two aromatic rings in IB_d similar to idarubicin. Teniposide (2.7 Å) and etoposide (2.7 Å), despite small chemical differences, are bound in two distinctly different sites at or near IB. Teniposide is internalized via primarily hydrophobic interactions and spans through both openings (IB_p-d). Etoposide is bound between the exterior of IB and IIA and exhibits an extensive hydrogen bonding network.

Conclusions

Subdomain IB is a major binding site for complex heterocyclic molecules.

General significance

The structures have important implications for drug design and development. This article is part of a Special Issue entitled Serum Albumin.     

Serum ferritin: Past,present and future

Background

Serum ferritin was discovered in the 1930s, and was developed as a clinical test in the 1970s. Many diseases are associated with iron overload or iron deficiency. Serum ferritin is widely used in diagnosing and monitoring these diseases.

Scope of review

In this chapter, we discuss the role of serum ferritin in physiological and pathological processes and its use as a clinical tool.

Major conclusions

Although many aspects of the fundamental biology of serum ferritin remain surprisingly unclear, a growing number of roles have been attributed to extracellular ferritin, including newly described roles in iron delivery, angiogenesis, inflammation, immunity, signaling and cancer.

General significance

Serum ferritin remains a clinically useful tool. Further studies on the biology of this protein may provide new biological insights.     

Albumin heterogeneity in low-abundance fluids. The case of urine and cerebro-spinal fluid

Background

Serum albumin is a micro-heterogeneous protein composed of at least 40 isoforms. Its heterogeneity is even more pronounced in biological fluids other than serum, the major being urine and cerebrospinal fluid. Modification ‘in situ’ and/or selectivity of biological barriers, such as in the kidney, determines the final composition of albumin and may help in definition of inflammatory states.

Scope of review

This review focuses on various aspects of albumin heterogeneity in low ‘abundance fluids’ and highlights the potential source of information in diseases.

Major conclusions

The electrical charge of the protein in urine and CSF is modified but with an opposite change and depending on clinical conditions.In normal urine, the bulk of albumin is more anionic than in serum for the presence of ten times more fatty acids that introduce equivalent anionic charges and modify hydrophobicity of the protein. At the same time, urinary albumin is more glycosylated compared to the serum homolog. Finally, albumin fragments can be detected in urine in patients with proteinuria.For albumin in CSF, we lack information relative to normal conditions since ethical problems do not allow normal CSF to be studied. In multiple sclerosis, the albumin charge in CSF is more cationic than in serum, this change possibly involving structural anomalies or small molecules bindings.

General significance

Massively fatty albumin could be toxic for tubular cells and be eliminated on this basis. Renal handling of glycosylated albumin can alter the normal equilibrium of filtration/reabsorption and trigger mechanisms leading to glomerulosclerosis and tubulo-interstitial fibrosis. This article is part of a Special Issue entitled Serum Albumin.     

Molecular and practical aspects of the enzymatic properties of human serum albumin and of albumin–ligand complexes

Background

Human serum albumin and some of its ligand complexes possess enzymatic properties which are useful both in vivo and in vitro.

Scope of review

This review summarizes present knowledge about molecular aspects, practical applications and potentials of these properties.

Major conclusions

The most pronounced activities of the protein are different types of hydrolysis. Key examples are esterase-like activities involving Tyr411 or Lys199 and the thioesterase activity of Cys34. In the first case, hydrolysis involves water and both products are released, whereas in the latter cases one of the products is set free, and the other stays covalently bound to the protein. However, the modified Cys34 can be converted back to its reduced form by another compound/enzymatic system. Among the other activities are glucuronidase, phosphatase and amidase as well as isomerase and dehydration properties. The protein has great impact on the metabolism of, for example, eicosanoids and xenobiotics. Albumin with a metal ion-containing complex is capable of facilitating reactions involving reactive oxygen and nitrogen species.

General significance

Albumin is useful in detoxification reactions, for activating prodrugs, and for binding and activating drug conjugates. The protein can be used to construct smart nanotubes with enzymatic properties useful for biomedical applications. Binding of organic compounds with a metal ion often results in metalloenzymes or can be used for nanoparticle formation. Because any compound acting as cofactor and/or the protein can be modified, enzymes can be constructed which are not naturally found and therefore can increase, often stereospecifically, the number of catalytic reactions. This article is part of a Special Issue entitled Serum Albumin.