A nucleotide insertion and frameshift cause albumin Kénitra, an extended and O-glycosylated mutant of human serum albumin with two additional disulfide bridges

Variations in the Human Serum Albumin Gene: Molecular and Functional Aspects

The human albumin gene, the most abundant serum protein, is located in the long arm of chromosome 4, near the centromere, position 4q11–3. It is divided by 14 intervening introns into 15 exons, the last of which is untranslated. To date, 74 nucleotide substitutions (mainly missense) have been reported, determining the circulating variants of albumin or pre-albumin. In a heterozygous state, this condition is known as alloalbuminaemia or bisalbuminaemia (OMIM # 103600). The genetic variants are not associated with disease, neither in the heterozygous nor in the homozygous form. Only the variants resulting in familial dysalbuminaemic hyperthyroxinaemia and hypertriiodothyroninaemia are of clinical relevance because affected individuals are at risk of inappropriate treatment or may have adverse drug effects. In 28 other cases, the pathogenic variants (mainly affecting splicing, nonsense, and deletions), mostly in the homozygous form, cause a premature stop in the synthesis of the protein and lead to the condition known as congenital analbuminaemia. In this review, we will summarize the current knowledge of genetic and molecular aspects, functional consequences and potential therapeutic uses of the variants. We will also discuss the molecular defects resulting in congenital analbuminaemia, as well as the biochemical and clinical features of this rare condition

Unraveling the Interaction between FcRn and Albumin: Opportunities for Design of Albumin-Based Therapeutics

The neonatal Fc receptor (FcRn) was first found to be responsible for transporting antibodies of the immunoglobulin G (IgG) class from the mother to the fetus or neonate as well as for protecting IgG from intracellular catabolism. However, it has now become apparent that the same receptor also binds albumin and plays a fundamental role in homeostatic regulation of both IgG and albumin, as FcRn is expressed in many different cell types and organs at diverse body sites. Thus, to gain a complete understanding of the biological function of each ligand, and also their distribution in the body, an in-depth characterization of how FcRn binds and regulates the transport of both ligands is necessary. Importantly, such knowledge is also relevant when developing new drugs, as IgG and albumin are increasingly utilized in therapy. This review discusses our current structural and biological understanding of the relationship between FcRn and its ligands, with a particular focus on albumin and design of albumin-based therapeutics.

Polythiol-containing, recombinant mannosylated-albumin is a superior CD68+/CD206+ Kupffer cell-targeted nanoantioxidant for treatment of two acute hepatitis models

Since reactive oxygen species (ROS) derived from Kupffer cells (KC), especially CD68(+) KC, play a key role in the induction of hepatic oxidative stress and injuries, we developed a polythiolated- and mannosylated human serum albumin (SH-Man-HSA), which functions as a novel nanoantioxidant for delivering thiol to CD68(+) KC. In vitro electron paramagnetic resonance coupled with pharmacokinetics and immunohistochemical studies showed that SH-Man-HSA possessed powerful radical-scavenging activity and rapidly and selectively delivered thiols to the liver via mannose receptor (CD206) on CD68(+) cells. SH-Man-HSA significantly improved the survival rate of concanavalin-A (Con-A)-treated mice. Moreover, SH-Man-HSA exhibited excellent hepatoprotective functions, not by decreasing tumor necrosis factor or interferon-γ production that is closely associated with Con-A-induced hepatitis, but by suppressing ROS production. Interestingly, the protective effect of SH-Man-HSA was superior to N-acetyl cysteine (NAC). This could be attributed to the difference in the inhibition of hepatic oxidative stress between the two antioxidants depending on their potential for thiol delivery to the liver. Similar results were also observed for acetaminophen (APAP)-induced hepatopathy models. Flow cytometric data further confirmed that an increase in F4/80(+)/ROS(+) cells was dramatically decreased by SH-Man-HSA. The administration of SH-Man-HSA at 4 hours following a Con-A or APAP injection also exhibited a profound hepatoprotective action against these hepatitis models, whereas this was not observed for NAC. It can be concluded therefore that SH-Man-HSA has great potential for use in a rescue therapy for hepatopathy as a nanoantioxidant because of its ability to efficiently and rapidly deliver thiols to CD68(+)/CD206(+) KC.

Human serum albumin isoforms: genetic and molecular aspects and functional consequences

BACKGROUND

At present, 67 different genetic variants of human serum albumin and proalbumin have been molecularly characterized at the protein and/or gene level.

SCOPE OF REVIEW

This review summarizes present knowledge about genetic and molecular aspects, functional consequences and potential uses of the variants.

MAJOR CONCLUSIONS

The frequency of bisalbuminemia in the general population is probably about 1:1000, but it can be much higher in isolated populations. Mutations are often due to hypermutable CpG dinucleotides, and in addition to single-amino acid substitutions, glycosylated variants and C-terminally modified alloalbumins have been found. Some mutants show altered stability in vivo and/or in vitro. High-affinity binding of Ni(++) and Cu(++) is blocked, or almost so, by amino acid changes at the N-terminus. In contrast, substitution of Leu90 and Arg242 leads to strong binding of triiodothyronine and l-thyroxine, respectively, resulting in two clinically important syndromes. Variants often have modified plasma half-lives and organ uptakes when studied in mice.

GENERAL SIGNIFICANCE

Because alloalbumins do not seem to be associated with disease, they can be used as markers of migration and provide a model for study of neutral molecular evolution. They can also give valuable molecular information about albumins binding sites, antioxidant and enzymatic properties, as well as stability. Mutants with increased affinity for endogenous or exogenous ligands could be therapeutically relevant as antidotes, both for in vivo and extracorporeal treatment. Variants with modified biodistribution could be used for drug targeting. In most cases, the desired function can be further elaborated by producing site-directed, recombinant mutants. This article is part of a Special Issue entitled Serum Albumin.

Human serum albumin: from bench to bedside

Human serum albumin (HSA), the most abundant protein in plasma, is a monomeric multi-domain macromolecule, representing the main determinant of plasma oncotic pressure and the main modulator of fluid distribution between body compartments. HSA displays an extraordinary ligand binding capacity, providing a depot and carrier for many endogenous and exogenous compounds. Indeed, HSA represents the main carrier for fatty acids, affects pharmacokinetics of many drugs, provides the metabolic modification of some ligands, renders potential toxins harmless, accounts for most of the anti-oxidant capacity of human plasma, and displays (pseudo-)enzymatic properties. HSA is a valuable biomarker of many diseases, including cancer, rheumatoid arthritis, ischemia, post-menopausal obesity, severe acute graft-versus-host disease, and diseases that need monitoring of the glycemic control. Moreover, HSA is widely used clinically to treat several diseases, including hypovolemia, shock, burns, surgical blood loss, trauma, hemorrhage, cardiopulmonary bypass, acute respiratory distress syndrome, hemodialysis, acute liver failure, chronic liver disease, nutrition support, resuscitation, and hypoalbuminemia. Recently, biotechnological applications of HSA, including implantable biomaterials, surgical adhesives and sealants, biochromatography, ligand trapping, and fusion proteins, have been reported. Here, genetic, biochemical, biomedical, and biotechnological aspects of HSA are reviewed.

Mutations and polymorphisms of the gene of the major human blood protein, serum albumin

We have tabulated the 77 currently known mutations of the familiar human blood protein, serum albumin (ALB). A total of 65 mutations result in bisalbuminemia. Physiological and structural effects of these mutations are included where observed. Most of the changes are benign. The majority of them were detected upon clinical electrophoretic studies, as a result of a point mutation of a charged amino acid residue. Three were discovered by their strong binding of thyroxine or triiodothyronine. A total of 12 of the tabulated mutations result in analbuminemia, defined as a serum albumin concentration of <1 g/L. These were generally detected upon finding a low albumin concentration in patients with mild edema, and involve either splicing errors negating translation or premature stop codons producing truncated albumin molecules. A total of nine mutations, five of those with analbuminemia and four resulting in variants modified near the C-terminal end, cause frameshifts. Allotypes from three of the point mutations become N-glycosylated and one C-terminal frameshift mutation shows O-glycosylation.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: an update covering the period 2001-2002

This review is the second update of the original review on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates that was published in 1999. It covers fundamental aspects of the technique as applied to carbohydrates, fragmentation of carbohydrates, studies of specific carbohydrate types such as those from plant cell walls and those attached to proteins and lipids, studies of glycosyl-transferases and glycosidases, and studies where MALDI has been used to monitor products of chemical synthesis. Use of the technique shows a steady annual increase at the expense of older techniques such as FAB. There is an increasing emphasis on its use for examination of biological systems rather than on studies of fundamental aspects and method development and this is reflected by much of the work on applications appearing in tabular form.

Analbuminemia Produced by a Novel Splicing Mutation

Analbuminemia is a rare autosomal recessive disorder manifested by the absence or severe reduction of circulating human serum albumin in homozygous or compound heterozygous individuals. It is an allelic heterogeneous defect, caused by a variety of mutations within the albumin gene. The analbuminemic condition was diagnosed in a Turkish female infant on the basis of low albumin concentration (∼9.0 g/L). The albumin gene was screened by single-strand conformation polymorphism and heteroduplex analysis and submitted to direct sequencing. The proband was found to be homozygous for a T→C transition at nucleotide 13381, the 2nd base of intron 11. The effect of this previously unreported mutation, which inactivates the strongly conserved GT dinucleotide at the 5′ splice site consensus sequence of intron 11, was evaluated by examining the cDNA obtained by reverse transcription-PCR from the albumin mRNA extracted from the proband leukocytes. This analysis revealed that the mutation, named Bartin for the geographical origin of the patient’s family, results in the skipping of exon 11. The subsequent frameshift within exon 12 originates a premature stop codon located 5 codons downstream at position 411. The predicted translation product would consist of 410 amino acids. This novel extensive cDNA alteration is responsible for the analbuminemic trait.

Optimization of a trypsin-bioreactor coupled with high-performance liquid chromatography–electrospray ionization tandem mass spectrometry for quality control of biotechnological drugs

The optimization of a silica-based trypsin bioreactor and its use in the quality control of biotechnological drugs like peptides and proteins is described. Five bioreactors based on monolithic material have been prepared, with different amount of bound trypsin. The performances of these bioreactors were compared to the proteolytic activity of a bioreactor based on silica material. The trypsin-based chromatographic columns were coupled on-line with an LC/ESI/MS/MS system for digestion and identification of proteins. First, human serum albumin has been used as test protein to compare the ability of the bioreactors to hydrolyse high-molecular-weight proteins. The best chromatographic material (epoxy monolithic silica) and the optimum amount of enzyme bound (7.13 mg) have been identified to obtain the highest protein recovery and an analytical reproducibility of the whole digestion, separation and identification process. The optimized enzyme-reactor has been used for the on-line digestion of some biotechnological drugs such as somatotropin. Somatotropin for parentheral use has been analyzed, without sample pre-treatment, with both an on-line procedure and the traditional off-line procedure described in the European Pharmacopoeia. It was found that the cleavage efficiency (aminoacidic recovery, %AA) achieved within minutes by the developed protocol is at least comparable or even better than the conventional 4h consuming method.

Trypsin-based monolithic bioreactor coupled on-line with LC/MS/MS system for protein digestion and variant identification in standard solutions and serum samples

The applicability of a trypsin-based monolithic bioreactor coupled on-line with LC/MS/MS for rapid proteolytic digestion and protein identification is here described. Dilute samples are passed through the bioreactor for generation of proteolytic fragments in less than 10 min. After digestion and peptide separation, electrospray ionization tandem mass spectrometry is used to generate a peptide map and to identify proteolytic peptides by correlating their fragmentation spectra with amino acid sequences from a protein database. By digesting picomoles of proteins sufficient data from ESI and MS/MS were obtained to unambiguously identify proteins alone and in serum samples. This approach was also extended to locate mutation sites in beta-lactoglobulin A and B variants.

Effect of genetic variation on the thermal stability of human serum albumin

Reversible thermal denaturation of 33 genetic variants of human serum albumin (HSA) appeared to be a two-state process when studied by circular dichroism (CD). Fourteen single-residue variants have Tm values (midpoint of denaturation) higher than, and nine have Tm values lower than, their endogenous, wild-type counterpart. Nine single-residue variants have DeltaHv values (van't Hoff enthalpy) higher than, and 14 have DeltaHv values lower than, normal albumin. All types of combinations of positive and negative DeltaTm values and Delta(DeltaHv) values were found. Good linear correlations between mutation-induced changes of alpha-helical content and Delta(DeltaHv) values, but not DeltaTm values, were found especially for the variants mutated in domains I and III. The effect of altered chain length and glycosylation on Tm and DeltaHv was also studied. For all variants, no clear relationship was found between the changes in the thermodynamic parameters and the type of substitution, changes in protein charge or hydrophobicity. However, the protein changes taking place in domain I have a rather uniform effect (almost all of the nine variants have positive DeltaTm values and negative Delta(DeltaHv) values, i.e., they denature more easily than normal albumin but they do so at a higher temperature). The present results can be of both protein chemical relevance and of clinical interest, because they could be useful when designing stable, recombinant HSAs for clinical applications.

Structural analysis and fatty acid-binding properties of two Croatian variants of human serum albumin

BACKGROUND

The aim of the present work was to characterize the molecular defects of a slow-migrating (albumin Zagreb) and a fast-migrating (albumin Krapina) genetic variant of human serum albumin detected in heterozygous persons living in Croatia and to elucidate the fatty acid-binding properties of the two alloalbumins.

METHODS

Purification and structural identification of the variants were performed by conventional protein chemistry methods, whereas types and amounts of albumin-bound, endogenous fatty acids were determined by gas chromatography.

RESULTS

Protein sequencing established that albumin Zagreb is a proalbumin variant (-1Arg-->Gln), and that albumin Krapina is due to a mutation within the mature polypeptide chain (573Lys-->Glu). The gas chromatographic results showed that the fatty acid-binding properties of the proalbumin variant are normal, while the amino acid substitution in position 573 resulted in a general decrease of fatty acid binding.

CONCLUSIONS

The structural defects of the first alloalbumins, detected by routine clinical electrophoresis among the Croatian population, were characterized. Albumin Zagreb is caused by a hot-spot mutation occurring in a CpG sequence in the albumin gene. It is commonly assumed that bisalbuminaemia has no direct clinical relevance. However, the present study suggests that naturally occurring mutations can affect the ligand-binding properties of human serum albumin.

Structural analysis, fatty acid and thyroxine binding properties of Vancouver and Naskapi variants of human serum albumin

OBJECTIVES

To purify and structurally identify two albumin variants found in the Canadian population of native Amerindian origin. To assess the ability of variant albumins to bind lauric acid and L-thyroxine.

METHODS

The structural characterization of the alloalbumins was performed by conventional protein chemistry methods and by mass spectrometric analysis. Lauric acid and L-thyroxine affinities to variant albumins were assessed by kinetic dialysis and equilibrium dialysis techniques, respectively.

RESULTS

The sequence investigations proved the two variants to be albumin Naskapi [372Lys --> Glu] and albumin Vancouver [501Glu --> Lys], respectively. Among the carriers of albumin Naskapi, we found a rare case of homozygosity. Furthermore, this is the first reported case of the 501Glu-->Lys mutation in the native North American population. Scatchard plot analysis revealed that the association constants for lauric acid and L-thyroxine to the two variants were indistinguishable from the endogenous form of albumin.

CONCLUSION

We show that albumin variants Vancouver and Naskapi have normal fatty acid and L-thyroxine binding capabilities. These findings support the assumption that bisalbuminemias associated with these albumin variants are benign conditions.

The glycan structure of albumin Redhill, a glycosylated variant of human serum albumin

Although human serum albumin is synthesized without carbohydrate, glycosylated variants of the protein can be found. We have determined the structure of the glycan bound to the double-mutant albumin Redhill (-1 Arg, 320 Ala-->Thr). The oligosaccharide was released from the protein using anhydrous hydrazine, and its structure was investigated using neuraminidase and a reagent array analysis method, which is based on the use of specific exoglycosidases. The glycan was shown to be a disialylated biantennary complex type oligosaccharide N-linked to 318 Asn. However, a minor part (11 mol%) of the glycan was without sialic acid. The structure is principally the same as that of glycans bound to two other types of glycosylated albumin variants. Glycosylation can affect, for example, the fatty acid binding properties of albumin. Taking the present information into account, it is apparent that different effects on binding are caused not by different glycan structures but by different locations of attachment, with the possible addition of local conformational changes in the protein molecule.