Effect of genetic variation on the fatty acid-binding properties of human serum albumin and proalbumin

Detection of hereditary bisalbuminemia in bottlenose dolphins (Tursiops truncatus, Montagu 1821): comparison between capillary zone and agarose gel electrophoresis

Background

Hereditary bisalbuminemia is a relatively rare anomaly characterized by the occurrence of two albumin fractions on serum protein separation by electrophoresis. In human medicine, it is usually revealed by chance, is not been clearly associated with a specific disease and the causative genetic alteration is a point mutation of human serum albumin gene inherited in an autosomal codominant pattern. This type of alteration is well recognizable by capillary zone electrophoresis (CZE), whilst agarose gel electrophoresis (AGE) not always produces a clear separation of albumin fractions. The aims of this study is to report the presence of this abnormality in two separate groups of related bottlenose dolphins and to compare the results obtained with capillary zone and agarose gel electrophoresis.

Results

Serum samples from 40 bottlenose dolphins kept under human care were analyzed. In 9 samples a double albumin peak was evident in CZE electrophoresis while no double peak was noted in AGE profile. Since only an apparently wider albumin peaks were noted in some AGE electrophoretic profiles, the ratio between base and height (b/h) of the albumin peak was calculated and each point-value recorded in the whole set of data was used to calculate a receiver operating characteristic curve: when the b/h ratio of albumin peak was equal or higher than 0.25, the sensitivity and specificity of AGE to detect bisalbuminemic samples were 87 and 63 %, respectively. The bisalbuminemic dolphins belong to two distinct families: in the first family, all the siblings derived from the same normal sire were bisalbuminemic, whereas in the second family bisalbuminemia was present in a sire and in two out of three siblings.

Conclusions

We report for the first time the presence of hereditary bisalbuminemia in two groups of related bottlenose dolphins identified by means of CZE and we confirm that AGE could fail in the identification of this alteration.

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.

Pharmaceutically important pre- and posttranslational modifications on human serum albumin

Recombinant technology allows engineering and production of proteins with desirable properties. Human serum albumin has been developed with recombinant technology, and thus plays an increasing role as a drug carrier in the clinical setting. Genetic variations usually occur on the surface of the protein, and do not impose significant effects on the conformation of albumin. However, binding of fatty acids by genetic variants is affected according to the location of the mutation. Albumin undergoes three major posttranslational modifications, namely, oxidation, glycation, and S-nitrosylation. This review gives an account of the different posttranslational modifications that should be taken into consideration when designing albumin mutant analogues with desirable pharmaceutical properties.

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.

Albumin binding as a potential biomarker of exposure to moderately low levels of organophosphorus pesticides

We have evaluated the potential of plasma albumin to provide a sensitive biomarker of exposure to commonly used organophosphorus pesticides in order to complement the widely used measure of acetylcholinesterase (AChE) inhibition. Rat or human plasma albumin binding by tritiated-diisopropylfluorophosphate (³H-DFP) was quantified by retention of albumin on glass microfibre filters. Preincubation with unlabelled pesticide in vitro or dosing of F344 rats with pesticide in vivo resulted in a reduction in subsequent albumin radiolabelling with ³H-DFP, the decrease in which was used to quantify pesticide binding. At pesticide exposures producing approximately 30% inhibition of AChE, rat plasma albumin binding in vitro by azamethiphos (oxon), chlorfenvinphos (oxon), chlorpyrifos-oxon, diazinon-oxon and malaoxon was reduced from controls by 9±1%, 67±2%, 56±2%, 54±2% and 8±1%, respectively. After 1 h of incubation with 19 µM ³H-DFP alone, the level of binding to rat or human plasma albumins reached 0.011 or 0.039 moles of DFP per mole of albumin, respectively. This level of binding could be further increased by raising the concentration of ³H-DFP, increasing the ³H-DFP incubation time, or by substitution of commercial albumins for native albumin. Pesticide binding to albumin was presumed covalent since it survived 24 h dialysis. After dosing rats with pirimiphos-methyl (dimethoxy) or chlorfenvinphos (oxon) (diethoxy) pesticides, the resultant albumin binding were still significant 7 days after dosing. As in vitro, dosing of rats with malathion did not result in significant albumin binding in vivo. Our results suggest albumin may be a useful additional biomonitor for moderately low-level exposures to several widely used pesticides, and that this binding differs markedly between pesticides.

Physiological and pathological changes in the redox state of human serum albumin critically influence its binding properties

Binding and transport of a number of endogenous and exogenous compounds is an important function of the main plasma protein, albumin. In vivo and in vitro, albumin may be oxidatively modified in different ways with different agents at different sites. These modifications have various consequences on the physiological functions of albumin. Diabetes mellitus, liver diseases and nephropathy are just a few examples of disorders in which oxidative stress is involved and altered albumin functions have been described. This review is focussed on the consequences of oxidative modification on the binding properties of albumin. These range from no effect to decreased or increased binding affinities depending on the ligand under investigation and the type of modification. Indicators for modification include glycosylation, disulphide formation or the content of carbonyl groups. The redox state of albumin can affect the binding properties in several ways, including altered conformation and consequently altered affinities at binding sites and altered binding when the binding reaction itself is redox sensitive. The physiological or pathophysiological concentrations of different oxidatively modified albumin molecules vary over a wide range and are crucial in assessing the clinical relevance of altered ligand binding properties of a particularly modified albumin species in various disease conditions.

The extraordinary ligand binding properties of human serum albumin

Human serum albumin (HSA), the most prominent protein in plasma, binds different classes of ligands at multiple sites. HSA provides a depot for many compounds, affects pharmacokinetics of many drugs, holds some ligands in a strained orientation providing their metabolic modification, renders potential toxins harmless transporting them to disposal sites, accounts for most of the antioxidant capacity of human serum, and acts as a NO-carrier. The globular domain structural organization of monomeric HSA is at the root of its allosteric properties which are reminiscent of those of multimeric proteins. Here, structural, functional, biotechnological, and biomedical aspects of ligand binding to HSA are summarized.

FT-IR spectrometry analysis of plasma fatty acyl moieties selective mobilization during endurance exercise

This study was designed to describe changes in plasma fatty acyl moieties during a 2-h endurance exercise. Sixteen endurance-trained subjects cycled 2 h at 55% of maximal power output and capillary blood was sampled every 15 min. Fourier-transform infrared (FT-IR) spectrometry was used to determine correlated changes between plasma fatty acyl moieties (FAM) structural characteristics and metabolic parameters (oxygen consumption, respiratory exchange ratio, glucose, lactate, TG, glycerol, and albumin). Opposite changes were found between carbohydrate and fatty acid metabolism during the second hour of exercise, i.e., a decrease of glucose and lactate concentrations while albumin, FAM, and TG increased. For fatty acid metabolism, FAM and TG did not showed the same pattern of changes at the end of exercise, i.e., TG remained constant after 90 min while FAM continued to increase. This late FAM concentration increase was correlated to the changes in albumin concentration and the nu C=C-H/nu(as) CH3 and nu(as) CH2/nu(as) CH3 ratios. These ratios clearly showed that FAM unsaturation increased while chain length decreased. It was hypothesized that PUFA from TG adipose lipolysis ketone bodies (beta-hydroxybutyric acid) from liver may have been released in higher amounts as glycogen stores became depleted after 90 min of exercise.

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.

Albumin genetic variability in South America: Population distribution and molecular studies

A total of 5,020 individuals living in two southern Brazilian states were screened in relation to albumin types; two variants were found, in Passo Fundo (Nagasaki 2) and Vera Cruz (Tradate 2). Another variant, detected in the northeast, was identified as Porto Alegre 2, which also occurs in other places in Brazil, as well as in India, Pakistan, and Turkey. The results were integrated with those obtained in other studies in South America, yielding a total of 16,941 Amerindians and 23,839 non-Indian subjects. Molecular and physiological studies performed in some of the variants suggested clues to explain the restricted distribution of albumin Yanomama 2 and the widespread occurrence of albumin Maku. Am. J. Hum. Biol. 11:359-366, 1999. Copyright 1999 Wiley-Liss, Inc.

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

Albumin Kenitra is a new type of genetic variant of human serum albumin that has been found in two members of a family of Sephardic Jews from Kenitra (Morocco). The slow-migrating variant and the normal protein were isolated by anion-exchange chromatography and, after treatment with CNBr, the digests were analyzed by two-dimensional electrophoresis in a polyacrylamide gel. The CNBr peptides of the variant were purified by reverse-phase high performance liquid chromatography and submitted to sequence analysis. Albumin Kenitra is peculiar because it has an elongated polypeptide chain, 601 residues instead of 585, and its sequence is modified beginning from residue 575. DNA structural studies showed that the variant is caused by a single-base insertion, an adenine at nucleotide position 15 970 in the genomic sequence, which leads to a frameshift with the subsequent translation to the first termination codon of exon 15. Mass spectrometric analyses revealed that the four additional cysteine residues of the variant form two new S-S bridges and showed that albumin Kenitra is partially O-glycosylated by a monosialylated HexHexNAc structure. This oligosaccharide chain has been located to Thr596 by amino-acid sequence analysis of the tryptic fragment 592-597.

Involvement of membrane surface charge in thermal stability of the rat ovarian LH/hCG receptor

Analysis of fluorescence of membrane-bound 8-anilino-1-naphthalene sulfonate and monodansylcadaverine probes revealed that a negative membrane surface charge derived from free fatty acids (FFA) resulted in destabilization of structure-functional properties of the rat ovarian LH/hCG receptor. Removal of FFA from rat luteal and porcine ovarian granulosa cells by BSA increased gonadotropin responsiveness of cells in cAMP formation.

Genetic variants showing apparent hot-spots in the human serum albumin gene

The molecular defects of three different slow-migrating genetic variants of human serum albumin, albumins Kamloops (formerly RIH), Stirling and Amsterdam, previously characterized only by electrophoretic and dye-binding studies, are now reported. Two of them are proalbumin variants: sequential analysis of the purified whole proteins has established the mutation responsible for albumin Kamloops as -1Arg-->Gln, and for albumin Stirling as -2Arg-->His. A Glu-->Lys substitution in position 570 of the mature albumin molecule was determined in albumin Amsterdam by sequential analysis of two abnormal tryptic fragments. The three alloalbumins are caused by single-base changes all of which seem to represent hot-spots in the albumin gene. The possible functional consequences of the presence of a circulating alloalbumin are discussed.

Structural characterization, stability and fatty acid-binding properties of two French genetic variants of human serum albumin

Four bisalbuminemic, unrelated persons were found in Bretagne, France, and their variant and normal albumins were isolated by DEAE ion exchange chromatography, reduced, carboxymethylated and treated with CNBr. Comparative two-dimensional electrophoresis of the CNBr digests showed that three of the variants were modified in fragment CB4, whereas the fourth had an abnormal fragment CB1. These fragments were isolated, digested with trypsin and mapped by reverse-phase HPLC. Sequencing of altered tryptic peptides showed that the three variants modified in CB4 were caused by the same, previously unreported, amino acid substitution: Asp314-->Val (albumin Brest). The fourth, however, was a proalbumin variant with the change Arg-2-->Cys (albumin Ildut). Both amino acid substitutions can be explained by point mutations in the structural gene: GAT-->GTT (albumin Brest) and CGT-->TGT (albumin Ildut). The proalbumin Ildut is very unstable and already in vivo it is to a large extent cleaved posttranslationally to Arg-Albumin and normal albumin. Furthermore, we observed that during a lengthy isolation procedure the remaining proalbumin was changed to Arg-Albumin or proalbumin lacking Arg-6. In addition, part of normal albumin had lost Asp1. Gas chromatographic investigations using isolated proteins indicated that albumin Brest has improved in vivo fatty acid-binding properties, whereas the structural modification(s) of albumin Ildut does not affect fatty acid binding.

Albumin Church Bay: 560 Lys-->Glu a new mutation detected by electrospray ionisation mass spectrometry

Albumin Church Bay is a fast migrating genetic variant of human serum albumin which, in a heterozygous subject, formed about 50% of the circulating albumin. Reversed phase peptide mapping and electrospray ionisation mass spectrometry (ESI-MS) indicated that the C-terminal CNBr peptide had decreased polarity associated with a 1 Da increase in mass. Subdigestion of this peptide with trypsin and chymotrypsin revealed that the increased mass was associated with the chymotrypsin fragment VEKCCKADDKETCF (555-568) which had a mass of 1791.1 compared to 1790.2 for its normal counterpart. Sequence analysis of PCR-amplified DNA indicated an A-->G mutation at position 98 of exon 13, which causes a point mutation of 560 Lys-->Glu and results in a 1 Da mass increase.