A new proalbumin variant: albumin Jaffna (-1 Arg----Leu)

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.

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.

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

In the circulation, non-esterified fatty acids are transported by albumin which also facilitates their removal from donor cells and uptake into receptor cells. We have studied whether genetic variations in the albumin molecule can affect its in vivo fatty acid-binding properties. The fatty acids bound to 25 structurally different variants and to their wildtype counterparts, isolated from heterozygous carriers, were determined gas chromatographically. The variants were proalbumins, albumins with single amino acid substitutions and glycosylated or truncated albumins. In eight cases the total amount bound to the variants was diminished (0.4-0.8-fold), and in seven cases the load was increased to 1.3 or more of normal. Twenty-one fatty acids were quantitated, and for 19 alloalbumins significant deviations from normal were found. Usually, changes in total and individual fatty acid binding were of the same type, but several exceptions to this rule was found. The glycosylated albumin Casebrook showed the largest changes, the total load and the amount of bound palmitate was 8.6 and 14 times, respectively, the normal. The most pronounced changes and the majority of cases of increased binding were caused by molecular changes in domain III. Mutations in domain I, II and the propeptide resulted in smaller effects, if any, and these were often reductions in binding.

Circulating proalbumin associated with a second case of antitrypsin Pittsburgh

We report here the conclusive identification of circulating proalbumin of normal N-terminal sequence (Arg Gly Val Phe Arg Arg Asp Ala) in a second child with the alpha 1-antitrypsin Pittsburgh 358 Met-->Arg mutation. As in the first case, the proalbumin made up 3-5% of the total serum albumin. The finding of proalbumin associated with a second de novo mutation at the inhibitory site bait of antitrypsin confirms our earlier hypothesis; that antitrypsin Pittsburgh was acting as a specific intracellular inhibitor of the hepatic proalbumin convertase and that antitrypsin Pittsburgh could be used as a probe to identify the proprotein convertase.

Two alloalbumins with identical electrophoretic mobility are produced by differently charged amino acid substitutions

We describe the amino acid substitutions of albumins Sondrio and Paris 2, two slow moving variants of human serum albumin, which show an identical electrophoretic mobility on cellulose acetate at three different pH values. These variants have been found in several instances in a wide geographic area including Northern Italy and France. Both alloalbumins were isolated from the sera of heterozygous subjects. Isoelectric focusing analysis of CNBr fragments from the purified variants allowed us to localize the mutation of albumin Sondrio in fragment CNBr V (residues 330-446) and that of albumin Paris 2 in CNBr VII (residues 549-585). Sequential analysis of the variant CNBr VII established the molecular defect of albumin Paris 2 as 563 Asp----Asn. Fragments CNBr V from normal and Sondrio albumins were isolated on a preparative scale and subjected to tryptic and V8 proteinase digestion. Sequence determination of the abnormal tryptic and V8 peptides revealed that the variant arises from the substitution of glutamic acid 333 by lysine. Thus, a +1 change in the C-terminal region of the albumin molecule produces a variant with the same electrophoretic mobility as an alloalbumin with a +2 substitution in the central domain, suggesting a higher degree of exposure to the solvent of the C-terminal tailpiece. Both amino acid substitutions are consistent with a G----A transition in the first position of the corresponding codon in the structural gene.

Genetic variants of serum albumin in Americans and Japanese

A collaborative search for albumin genetic variants (alloalbumins) was undertaken by cellulose acetate and agarose electrophoresis at pH 8.6 of the sera of patients at two major medical centers in the United States and of nearly 20,000 blood donors in Japan. Seventeen instances of alloalbuminemia were ascertained, and seven different alloalbumin types were characterized by structural study. Two previously unreported alloalbumin types were identified. In one type, which was present in a Caucasian family and designated Iowa City-1, aspartic acid at position 365 was replaced by valine (365 Asp----Val); this is the second reported mutation at this position. The other type present in a Japanese blood donor had the mutation 128 His----Arg. An unexpected finding was the presence in a single Japanese of a Naskapi-type alloalbumin (372 Lys----Glu), a variant that had previously been described only for certain Amerindian tribes in whom it occurs with a polymorphic frequency (greater than 1%) and in Eti Turks. An arginyl-albumin (-1 Arg, 1 Asp----Val) occurred in an American family. The other alloalbumin types identified were proalbumins Lille and Christchurch and albumin B that have a cumulative frequency of about 1:3500 in Caucasians probably because of the hypermutability of CpG dinucleotides at the mutated sites. All of the variants characterized in this study are point mutants, and the sites are spread throughout the albumin gene. However, about one-fourth of all known albumin mutations are clustered in the sequence segment from position 354 through 382.

Mutations in genetic variants of human serum albumin found in Italy

A long-term electrophoretic survey of genetic variants of serum albumin has identified an alloalbumin in 589 unrelated individuals in Italy. The alloalbumins were classified electrophoretically into 17 types. The number of unrelated carriers for each type varied from 1 for several variants reported here to 103 for albumin B. The structural change in 8 of these types has previously been determined, and the amino acid substitutions in 3 additional types are reported here. Albumin Varese has a substitution, -2 arginine to histidine (-2 Arg----His), the same as that reported for proalbumin Lille; albumin Torino has the substitution 60 Glu----Lys; and albumin Vibo Valentia has the substitution 82 Glu----Lys. The ability to distinguish so many alloalbumin types by electrophoresis at several pH values indicates that similar substitutions at different sites produce variants with different electrophoretic mobilities. Except for chain terminations in two Italian variants, all the mutations thus far determined for alloalbumins are attributable to a single-base change in the structural gene, and there is a preponderance of transitions and purine mutations. Seven alloalbumins for which the structural change has been established have been ascertained only in Italy. Several of these are clustered in specific geographic regions of Italy, which suggests an origin through a founder individual. Other variants that occur worldwide are nonetheless clustered in geographic regions within Italy. In these cases an independent mutation probably occurred at a hypermutable site such as a CpG dinucleotide.

The molecular defect of albumin Castel di Sangro: 536 Lys----Glu

Albumin Castel di Sangro is a rare fast-moving variant of human serum albumin which has been discovered in heterozygous form in the serum of an 85-year-old woman living in Castel di Sangro (Abruzzo, Italy). Isoelectric focusing analysis of CNBr fragments from the purified variant allowed us to localize the mutation in fragment CNBr VI (residues 447-548). This fragment was isolated on a preparative scale and subjected to tryptic digestion. Sequential analysis of the abnormal tryptic peptide, purified by reverse-phase and cation-exchange HPLC, revealed that the variant arises from the substitution of lysine 536 by glutamic acid. This amino acid replacement, probably due to a single-base substitution in the structural gene, causes a change in the net charge of -2 units, which is in keeping with both the increased electrophoretic mobility of the native protein and the isoelectric point of the modified CNBr fragment.

Hypermutability of CpG dinucleotides in the propeptide-encoding sequence of the human albumin gene

An electrophoretically slow albumin variant was detected with a phenotype frequency of about 1:1000 in Sweden and was also found in a family of Scottish descent from Kaikoura, New Zealand, and in five families in Tradate, Italy. Structural study established that the major variant component was arginyl-albumin, in which arginine at the -1 position of the propeptide is still attached to the processed albumin. A minor component with the amino-terminal sequence of proalbumin was also present as 3-6% of the total albumin. After amplification of the gene segment encoding the prepro sequence of albumin, specific hybridization of DNA to an oligonucleotide probe encoding cysteine at position -2 indicated the mutation of arginine at the -2 position to cysteine (-2 Arg----Cys). This produced the propeptide sequence Arg-Gly-Val-Phe-Cys-Arg. This was confirmed by sequence analysis after pyridylethylation of the cysteine. This mutation produces an alternate signal peptidase cleavage site in the variant proalbumin precursor of arginyl-albumin giving rise to two possible products, arginyl-albumin and the variant proalbumin. Another plasma from Bremen had an alloalbumin with a previously described substitution (1 Asp----Val), which also affects propeptide cleavage. Hypermutability of two CpG dinucleotides in the codons for the diarginyl sequence may account for the frequency of mutations in the propeptide. Mutation at these two sites results in a series of recurrent proalbumin variants that have arisen independently in diverse populations.