Point substitutions in albumin genetic variants from Asia.

Despite their rarity and physiologically neutral character, more inherited structural variants of serum albumin (alloalbumins) are known than for any other human protein except hemoglobin. Including three previously unreported examples described here, we have identified 13 different point substitutions in alloalbumins of Japanese origin. Of these only albumin B and two proalbumins have been reported in other ethnic groups, and these are the most common variants of European origin. Some alloalbumins of Asiatic origin, but not yet identified in Japanese, are present in diverse ethnic groups. An alloalbumin found in indigenes of New Guinea (lysine----asparagine at position 313) is also present in Caucasians of various European descents. Albumin Lambadi, occurring in a tribal group in south India, has a mutation (glutamic acid----lysine at position 501) also found as a rare variant in individuals of diverse ethnic origin resident on four continents. These results suggest that some alloalbumins with the same substitution may have originated by independent mutations in various populations. This, together with the apparent clustering of point substitutions in the protein structure, may reflect hypermutability of the albumin gene.     

Amino acid substitutions in albumin variants found in Brazil.

Conventional horizontal starch-gel electrophoresis in four buffer systems and structural studies were performed on four albumin variants, and the findings were compared with similar previous data. Albumins Coari I and Porto Alegre I have a previously unreported amino acid substitution (glutamic acid replaced by lysine at position 358, denoted 358 Glu----Lys). The alteration in albumin Porto Alegre II (501 Glu----Lys) is the same as that found for three alloalbumins of Asiatic origin, designated Vancouver, Birmingham, and Adana. Albumin Oriximiná I has the same exchange as albumin Maku (541 Lys----Glu). Some of these findings can be explained only by the occurrence of independent mutations at the same site in the albumin gene. They also point to a third cluster of mutations in that gene, indicating hypermutability in some of its segments.     

Identical structural changes in inherited albumin variants from different populations.

Alloalbuminemia is rare and has a cumulative frequency of only approximately 1 in 3,000 in Europeans and Japanese. The worldwide ethnic and geographic distribution of certain albumin genetic variants appears to be nonrandom. Moreover, we have found that structurally identical variants may occur at different frequencies in ethnically distinct populations, presumably owing to independent mutations. In this study, albumin B and two types of proalbumins, which as a group are the most common European albumin variants, have also been found in Asians. We have identified the amino acid substitution characteristic of albumin B (glutamic acid----lysine at position 570) in alloalbumins from six unrelated individuals of five different European descents and also in two Japanese and one Cambodian. The two types of proalbumins most common in Europe (Lille type, arginine----histidine at position -2; Christchurch type, arginine----glutamic acid at position -1) also occur in Japan. These results provide evidence for independent mutations at single sites in the albumin genome. The clustering of these and of several other amino acid exchanges in certain regions of the albumin molecule suggests two possibilities: that certain sites are hypermutable or that mutants involving certain sites are more subject to selection than mutants involving others.     

Point substitutions in Japanese alloalbumins.

We have completed the structural study of five rare types of inherited albumin variants (alloalbumins) discovered in the Biochemical Genetics Study of 15,581 unrelated children in Hiroshima and Nagasaki. We have also identified the structural change in five other alloalbumin specimens detected during clinical electrophoresis of sera from Japanese living near Tokyo. Each of the five albumin variants from Nagasaki and Hiroshima has a single amino acid substitution. All of these substitutions differ, and none has been reported in non-Japanese populations. No instances of proalbumin variants or of albumin B (the most frequent alloalbumins in Caucasians) were detected in the children in Hiroshima and Nagasaki. However, one instance of a variant proalbumin and two examples of albumin B occurred in Japanese from the vicinity of Tokyo. In addition a previously unreported point substitution was found in albumin Tochigi, which is present in two unrelated persons from Tochigi prefecture. Four of the point mutations in the Japanese alloalbumins are in close proximity in a short segment of the polypeptide chain (residues 354-382) in which three additional point substitutions have been reported in diverse populations. These results, combined with earlier data, suggest that point substitutions are grouped in certain segments of the albumin molecule.     

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.     

Amino acid substitutions in genetic variants of human serum albumin and in sequences inferred from molecular cloning.

The structural changes in four genetic variants of human serum albumin were analyzed by tandem high-pressure liquid chromatography (HPLC) of the tryptic peptides, HPLC mapping and isoelectric focusing of the CNBr fragments, and amino acid sequence analysis of the purified peptides. Lysine-372 of normal (common) albumin A was changed to glutamic acid both in albumin Naskapi, a widespread polymorphic variant of North American Indians, and in albumin Mersin found in Eti Turks. The two variants also exhibited anomalous migration in NaDodSO4/PAGE, which is attributed to a conformational change. The identity of albumins Naskapi and Mersin may have originated through descent from a common mid-Asiatic founder of the two migrating ethnic groups, or it may represent identical but independent mutations of the albumin gene. In albumin Adana, from Eti Turks, the substitution site was not identified but was localized to the region from positions 447 through 548. The substitution of aspartic acid-550 by glycine was found in albumin Mexico-2 from four individuals of the Pima tribe. Although only single-point substitutions have been found in these and in certain other genetic variants of human albumin, five differences exist in the amino acid sequences inferred from cDNA sequences by workers in three other laboratories. However, our results on albumin A and on 14 different genetic variants accord with the amino acid sequence of albumin deduced from the genomic sequence. The apparent amino acid substitutions inferred from comparison of individual cDNA sequences probably reflect artifacts in cloning or in cDNA sequence analysis rather than polymorphism of the coding sections of the albumin gene.     

Genetic variants of human serum albumin in Italy: point mutants and a carboxyl-terminal variant.

Of the > 50 different genetic variants of human serum albumin (alloalbumins) that have been characterized by amino acid or DNA sequence analysis, almost half have been identified in Italy through a long-term electrophoretic survey of serum. Previously we have reported structural studies of 11 Italian alloalbumins with point mutations, 2 different carboxyl-terminal variants, and 1 case of analbuminemia in an Italian family. This article describes confirmation by DNA sequencing of mutations previously inferred from protein sequencing of 4 of the above alloalbumins; it also reports the mutations identified by protein and DNA sequence analysis of 4 other Italian alloalbumins not previously recorded: albumin Larino, His3-->Tyr; Tradate-2 (protein sequencing only), Lys225-->Gln; Caserta, Lys276-->Asn; and Bazzano, a carboxyl-terminal variant. The first 3 have point mutations that produce a single amino acid substitution, but a nucleotide deletion causes a frameshift and an altered and truncated carboxyl-terminal sequence in albumin Bazzano. In these 4 instances the expression of the alloalbumin is variable, ranging from 10% to 70% of the total albumiN, in contrast to the usual 50% each for the normal and mutant albumin. The distribution of point mutations in the albumin gene is nonrandom; most of the 47 reported point substitutions involve charged amino acid residues on the surface of the molecule that are not concerned with ligand-binding sites.     

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.     

cDNA and protein sequence of polymorphic macaque albumins that differ in bilirubin binding.

The rhesus monkey, Macaca mulatta, exhibits a geographically restricted polymorphism of serum albumins Mac A and Mac B that is recognized by electrophoresis and is associated with a difference in bilirubin-binding parameters. To identify the basis of the polymorphism, the cDNA and protein sequences of serum albumin from M. mulatta were determined. Screening of a lambda gt11 rhesus liver cDNA library yielded a 1988-bp cDNA sequence that encodes the complete amino acid sequence of mature albumin, the entire propeptide, and part of the prepropeptide. Isoelectric focusing and amino-terminal protein sequencing of CNBr fragments of albumin from A/A and B/B homozygotes were performed, and the structural difference was localized to a CNBr fragment (MCB3) spanning residues 124-264. Sequence analysis of lysyl endopeptidase peptides of MCB3 established that Mac A albumin has a glutamine residue at position 188 while the Mac B albumin has a glutamic residue at the same position. PCR amplification, subcloning, and DNA sequence analysis of clones from A/A and B/B homozygotes confirmed the protein sequence data and the codon difference of CAA versus GAA, respectively. Comparison of macaque and human serum albumin shows a 93.5% identity at the amino acid level. In human serum albumin, Glu188 is located close to the IIA binding pocket for ligands, probably including bilirubin. Derivatives of coumarin compete more efficiently with bilirubin for binding sites on the Mac A albumin than on the Mac B albumin. In regions where coumarin-containing plants are important food resources, Mac B albumin may confer a selective advantage because bilirubin is less readily displaced from it.     

A MOLECULAR TIME SCALE FOR HUMAN EVOLUTION

We discuss published molecular evidence concerning the relationship of man to African apes and Old World monkeys. Quantitative comparisons of their serum albumins, transferrins, hemoglobins, and DNA show that man is genetically much more similar to the African apes than to the Old World monkeys.The amino acid sequences of hemoglobins from humans, chimpanzees, gorillas, and rhesus monkeys are consistent with the hypothesis that the probability of an amino acid substitution occurring in a given interval of time is the same for every hemoglobin lineage. This allows the use of these data as a hemoglobin evolutionary clock, just as we have previously done with the albumins. It is shown that concordance exists between the hemoglobin and albumin results and that both support the suggestion that the human lineage diverged from that leading to the African apes far more recently than is generally supposed. Considering both the albumin and hemoglobin data, we would set the most probable date at 4 to 5 million years.     

Fibrinogens Bern IV, Bern V and Milano XI: three dysfunctional variants with amino acid substitutions in the thrombin cleavage site of the Aalpha-chain.

Thrombin-induced cleavage of fibrinopeptide A is the initial step in the conversion of fibrinogen to fibrin. Three dysfunctional fibrinogen variants are described with an amino acid substitution at position 16 of the Aalpha-chain: the fibrinogen variants Bern IV and Milano XI having an Arg-->His substitution and the variant Bern V having an Arg-->Cys substitution. Routine coagulation studies revealed prolonged plasma thrombin and reptilase clotting times in all patients, and a discrepancy between the plasma levels of fibrinogen determined by the clotting assay and electroimmunoassay. The defect was localized by high-performance liquid chromatography analysis of fibrinopeptide release and confirmed by polymerase chain reaction and sequencing of exon 2 of the Aalpha-chain. Immunoblotting analysis with a rabbit antiserum against human serum albumin indicated that albumin was linked to the additional sulfhydryl group of fibrinogen Bern V. The assay of tissue-plasminogen-activator-induced plasmic degradation revealed that the fibrinolysis of fibrin Bern V was delayed, whereas fibrin Bern IV was digested in the same way as normal fibrin.     

Albumin Naskapi variant in North American Indians and Eti Turks.

Both conventional polyacrylamide gel electrophoresis and a new type of electrophoretic screening procedure indicate that the polymorphic albumin variants Naskapi, found chiefly in the Naskapi Indians of Quebec, and Mersin, found in the Eti Turks of southeastern Turkey, are molecularly identical or very similar and that the amino acid substitution site in these variants is located between residues 330 and 446. This discovery is consistent with a genetic relationship between the Eti Turks and American Indians. We also report a new variant found in the Eti Turks, albumin Adana, which migrates similarly to albumin B on conventional gels but which our new system shows to differ from the common albumin A and albumin B by a substitution between residues 549 and 585.     

Amino acid substitutions in inherited albumin variants from Amerindian and Japanese populations.

We report an effort to determine the basis for the altered migration of seven inherited albumin variants detected by one-dimensional electrophoresis in population surveys involving tribal Amerindians and Japanese children. An amino acid substitution has thus far been determined for four of the variants. The randomness in the albumin polypeptide of these and the other sixteen independently ascertained amino acid substitutions of albumin and proalbumin thus far established was analyzed; the clustering of eight of these at two positions in the six-amino acid propeptide sequence seems noteworthy. By comparison with other proteins studied by electrophoresis, albumin exhibits "average" variability. It is a paradox that individuals who, for genetic reasons, lack albumin exhibit no obvious ill effects; yet, electrophoretic variants of albumin are no more numerous than are variants of proteins, the absence of which results in severe disease.     

Importance of albumin in cross-reactivity among cat, dog and horse allergens.

Different allergenic proteins have been involved in cross-reactivity among animals. Albumins seem to be cross-sensitizing allergenic components. The aim of this study was to assess the importance of albumin as a cross-reactive allergen in patients sensitized to cat, dog and horse. One hundred and seventeen patients sensitized to cat were tested for IgE reactivity using skin prick tests and RAST assays with cat, dog and horse hair/dander extracts and their purified albumin extracts. RAST-inhibition studies were carried out to assess cross-reactivity among cat, dog and horse and among their purified albumins. It was found that 22% of patients exhibited specific IgE to cat albumin; 41% of patients sensitized to cat were also sensitized to dog and horse. Out of these patients, 21% had IgE to three albumins and 17% to two. Reciprocal inhibitions were observed among cat, dog and horse albumins and also among cat, dog and horse hair/dander extracts, using in the latter experiment sera from patients not sensitized to albumins. IgE binding to horse extract was inhibited 30% by its homologous albumin and IgE binding to cat and dog extracts in almost 15% by their respective albumins. It was concluded that albumins from these three animals share some epitopes that account for the cross-reactivity observed in around one-third of patients sensitized to cat, dog and horse. Nevertheless, more than 50% of specific IgE that cross-reacts among these three animals is directed to allergens other than albumin.     

HYPERTHYROXINAEMIA: ABNORMAL BINDING OF T4 BY AN INHERITED ALBUMIN VARIANT

Two euthyroid subjects with high total concentrations of T4 in their sera have been studied (J.D.: T4, 170; T3, 1·90; rT3, 0·54 nmol/1. E.T.: T4, 185; T3, 1·63; rT3, 0·37 nmol/1). Concentrations of all three T4-binding proteins were within normal limits in both cases. However, on reverse-flow electrophoresis an abnormally large amount of thyroxine was found to travel with albumin. The three T4-binding proteins in the sera of both patients were separated from each other by a novel affinity chromatographic method using dye-Sepharose conjugates. Affinity constants for T4 binding to TBG and to prealbumin from both patients were normal. The albumin preparations were further purified and shown by physical and immunochemical techniques to be uncontaminated by other proteins. Scatchard plots of the binding of T4 to each of these pure albumins revealed two components, one having a normal affinity constant (J.D., 1·8 ± 10⁵ lmol^?1 and E.T., 2·3 × 10⁵ lmol^?1), the other having a raised affinity constant (J.D., 5·4 × 10⁶ lmol^?6 and E.T., 5·8 × 10⁶ lmol^?1). Extrapolation of the plots showed that the high affinity components comprised 66% (J.D.) and 54% (E.T.) of the total purified albumin. The raised affinity and high concentrations of the variants thus account for the raised total T4 concentrations in the patients. The presumed amino acid substitution in the albumins may be different in the two patients since the affinities for rT3 differ. Some methods for the estimation of free T4 levels give misleading results in the presence of these albumin variants. In the course of two episodes of illness, patient J.D. manifested large falls in serum T4 levels which could only be accounted for by reduced carriage of T4 by the abnormal and conventional binding proteins. Many cases reported in the literature as partial peripheral resistance to thyroid hormone may be examples of similar albumin variants.     

Fetal and adult albumins are indistinguishable by immunological and physicochemical criteria

The existence of a functionally immature fetal albumin has been postulated to explain the reduced ability of newborn plasma to bind bilirubin and various drugs. In support of this, cord and adult albumin, isolated by a simple salting-out technique, were reported to differ in electrophoretic and chromatographic properties and in their resistance to alkali and proteolytic enzymes. However, the interpretation of these findings has since been questioned. To resolve this controversy, we have purified to homogeneity human serum albumins from pooled umbilical cord and adult donor plasma. The two albumins were compared and found to be indistinguishable by polyacrylamide gel electrophoresis with and without sodium dodecyl sulfate, as well as by immunoelectrophoresis and double immunodiffusion using specific antibodies against both albumins. Furthermore, the amino acid compositions, the aminoterminal sequence (Asp-Ala-His-Lys-Ser-Glu-Val-Ala-), the carboxy terminus (Leu), and the peptide fingerprints were identical in the two albumins. No significant differences were found by circular dichroism in the ultraviolet (200-350 nm). Binding studies with bilirubin showed association constants of 3.7 +/- 0.7 x 10(7) M(-1) for cord and 2.9 +/- 0.3 x 10(7) M(-1) for adult albumin, respectively. The circular dichroic spectra of 1:1 bilirubin.albumin complexes showed considerable variation between the batches but were not significantly different. The only difference was found in the fluorescence spectra of the bilirubin.albumin complexes, in which complexes with adult albumin showed only 75% of the relative fluorescence exhibited with cord albumin. The combined results nevertheless strongly indicate that fetal and adult albumins are very similar, if not identical.     

Identification of the binding domain for NADP+ of human glucose-6-phosphate dehydrogenase by sequence analysis of mutants.

Human erythrocyte glucose-6-phosphate dehydrogenase is normally quite stable in the presence of 10 microM NADP+. Certain glucose-6-phosphate dehydrogenase variants lose virtually all their activity at this concentration of NADP+ but are reactivated by 200 microM NADP+. Such variants presumably have a defect in their NADP+-binding site. We analyzed the sequence of cDNA or genomic DNA from seven unrelated patients with hemolytic anemia due to the inheritance of variants that are reactivated by NADP+. Six patients had substitutions of one of three adjacent amino acids, and the seventh patient had another amino acid substitution 23 residues downstream. These amino acids are highly conserved, all being present in rat and all but one being found also in Drosophila. The anomalous electrophoretic behavior of some of the variants can be explained by their loss of ability to bind NADP+. We conclude that the region in which these mutations occur defines the binding domain for NADP+ and that binding NADP+ that has been designated as "structural" and as "catalytic" probably occurs at the same site.     

Localization of the amino acid substitution site in a new variant of human serum albumin, albumin Mexico-2

Using an electrophoretic screening procedure, we have discovered that two species of human serum albumin Mexico occur that are indistinguishable by conventional electrophoretic methods. We suggest that these species be referred to as albumins Mexico-1 and Mexico-2. Isolation and determination of the partial sequence of the cyanogen bromide fragment of albumin Mexico-2 that differs from the corresponding fragment of the common albumin A revealed this variant to arise from at least a glycine/aspartic acid substitution at position 550. This region of the albumin molecule is involved in the binding of the fatty acid, palmitate.     

Superoxide dismutase in Drosophila melanogaster: biochemical and structural characterization of allozyme variants.

Superoxide dismutase (SOD; superoxide:superoxide oxidoreductase, EC 1.15.1.1) is known to be polymorphic in many organisms; in Drosophila, the degree of polymorphism has a wide range of variation from locality to locality within a given species. We have thoroughly purified from D. melanogaster the two common electromorphs, SODS and SODF. These differ in properties such as isoelectric point, specific activity, rate constant, thermostability, and amino acid composition. The specific activity is three times greater in SODS than in SODF, but the latter is more thermostable. In strains from California, SODS differs from SODF by at least one amino acid substitution: lysine in SODS is replaced by either aspartic acid or asparagine in SODF. This difference is consistent with the electrophoretic mobility and isoelectric points of the two electromorphs. In strains from Africa, SODS and SODF differ by two amino acid substitutions (histidine and proline in SODS vs. serine and either glutamic acid or glutamine in SODF) in addition to the one distinguishing the California strains. Thus the SODF electromorphs from California and from Tunisia, in spite of their identical electrophoretic mobility, differ by at least two amino acid substitutions.