Albumins

Albumins are operationally defined as proteins that (1) remain soluble in pure water after dialysis of protein samples such as egg white or blood serum against distilled water and (2) are not precipitated in 50% saturated ammonium sulfate. This contrasts with another group of proteins, called globulins, which are precipitated in both distilled water and 50% saturated ammonium sulfate. This operational classification is rather obsolete today, but names like serum albumin, ovalbumin, and lactalbumin have remained. Other albumins include muscle albumin and plant albumins (1).

1. Blood Albumins

Serum albumin and other plasma proteins have been most closely studied, mainly for their medical interest. After the cells have been removed from blood by light centrifugation in the presence of chelating agents such as EDTA, a yellowish fluid called plasma remains. By the addition of Ca⁺⁺ , fibrinogen is converted to fibrin and the clotting process produces a soft gel. Removal of this gel by centrifugation or other methods will leave a clear fluid called serum. Serum thus obtained contains 70 to 80 g/L of total protein, consisting of more than 150 different kinds of proteins. Dialysis of serum against distilled water will precipitate the globulins, while albumins stay in solution. The albumin fraction contains serum albumin as the major protein (2, 3). (see Serum Albumin).

2. Egg Albumins

Egg proteins are first classified into yolk proteins and egg white proteins. Yolk contains proteins called vitellogenin, phosvitin, and lipovitellin, all in association with yolk lipid. Egg white is a reservoir of several kinds of proteins as a protective and nutritious environment for the fetus. Egg white proteins are further divided into albumins and globulins, as for serum proteins. The following is a list of egg white proteins that are not globulins. Descriptions of egg white globulins are found under Globulins.

2.1. Ovalbumin 

This is the major glycoprotein in the white of eggs, comprising 65% of the total egg white protein, with a molecular weight of 43,000 and isoelectric point (pI) of 4.7. The N-terminal glycine residue is acetylated. Two species containing one or two sites of phosphorylation can be separated by electrophoresis. Oligosaccharide containing three moles of N-acetylglucosamine and five moles of mannose is linked to an asparagine residue through an N-glycosidic linkage. Treatment of ovalbumin with subtilisin yields a readily crystallizable plakalbumin, named after the platelike appearance of the resulting crystals. Ovalbumin is often used as an effective antigen in immunological studies. The protein has recently been found to have a similar amino acid sequence and three-dimensional structure to a1-antitrypsin of the serpin family (4) (see Ovalbumin).

2.2. Ovotransferrin (or conalbumin (

This protein binds ferric ions and is the same as apo-transferrin, but differing in the carbohydrate moiety. In egg white, the protein is present in an almost iron-free form. This protein constitutes about 10% of the egg protein.

2.3. Ovomucoid 

Ovomucoid is a glycoprotein (carbohydrate content about 25% by weight) with a molecular weight of 28,000 and constitutes 1.5% of the total egg white protein. It inhibits trypsin and chymotrypsin but not plasmin, thrombin, elastase, or collagenase. Its pI falls in the range 3.9 to 4.5. It remains active in the supernatant after egg white has been coagulated by heat. It has three homologous domains connected by linking peptides, each domain being homologous to bovine pancreatic trypsin inhibitor (BPTI). Thus, it is speculated that ovomucoid has evolved by two tandem repeats of the BPTI gene. Carbohydrates are linked to asparagine residues at residue numbers 10, 53, 69, 75, and 175. Similar inhibitors can be purified from the egg white of the quail, goose, and turkey.

2.4. Ovoinhibitor 

This is a 48-kDa multiheaded proteinase inhibitor that can simultaneously inhibit trypsin, chymotrypsin, and elastase. Unlike ovomucoid, it also inhibits the proteinases of bacterial origins. It is a glycoprotein with 5 to 10% content of carbohydrate.

2.5. Avidin 

This protein occupies a special position in biochemistry in that it is widely used as a probe based on its strong affinity for biotin. Their binding constant reaches 1014 M–1 under optimal conditions. Avidin-biotin systems are widely used for labeling macromolecular interacting systems, not only protein–protein interactions, but also systems based on DNA and other macromolecules. Avidin can be substituted by streptavidin, which is of bacterial origin but with a similar activity. Avidin in the egg white is largely free of biotin, and feeding rats with purified avidin as the sole protein source will cause various symptoms known to accompany biotin (vitamin H) deficiency.

2.6. Ovomucin 

This is a glycoprotein of high molecular weight responsible for the mucous character of egg white. The treatment of ovomucin with reagents that react with thiol groups reduces the viscous nature of egg white.

3. Plant Albumin

Ricin is an example of plant albumins (from Ricinus communis) and has an interesting toxic function. It consists of an A subunit, of 32 kDa, pI of 7.5, and 2.4% carbohydrate, and a B subunit, of 34 kDa, pI of 4.8, and 6.5% carbohydrate. It inactivates the 60S subunit of ribosomes after being internalized into the cell and thus inhibits a peptide elongation step (see Translation). The A subunit carries out the inactivation reaction, while the B subunit, which binds to cell-surface galactose residues, helps the A subunit to be internalized into the cell. Plant albumins are more readily precipitable in 50% ammonium sulfate than animal albumins.

4. Milk Albumins

4.1. a-Lactalbumin 

This is a single-chain 14-kDa protein that is the same as the B chain of lactose synthetase in lactating granules. The A chain of lactose synthetase alone catalyzes the synthesis of N-acetylgalactosamine from N-acetylglucosamine and UDP-galactose, but when in association with a-lactalbumin, it uses glucose as a substrate and synthesizes lactose. The amino acid sequence of a-lactalbumin with 143 amino acids (human, bovine, guinea pig) is homologous to that of lysozyme, and its three-dimensional structure is also similar. A recent review of the relationship between lysozyme and lactalbumin is recommended (5) (see also Alpha-Lactalbumin and Lysozymes).

4.2.  Lactoferrin 

This is an iron-binding protein of 88,000 in humans and 86,000 in cows. It is similar in functional properties to serum transferrin, but immunologically and structurally distinguishable (6).

References

1. B. Blombäck and L. A. Hanson (1979) Plasma Proteins, John Wiley & Sons, New York. 

2. M. Perutz (1992) Protein Structure: New Approach to Disease and Therapy, W. H. Freeman and Co., New York. 

3. T. Peters Jr. (1985) Adv. Prot. Chem. 37, 161–245. 

4. P. E. Stein, G. W. Leslie, J. T. Finch, and R. W. Carrell (1991) Crystal structure of uncleaved ovalbumin at 1.95 Å resolution, J. Mol. Biol. 221, 942–959. 

5.M. A. McKenzie and F. H. White Jr. (1991) Lysozyme and -Lactalbumin: Structure, function, and interrelationships, Adv. Prot. Chem. 41, 173–315.

6. B. F. Anderson, H. M. Baker, E. J. Dodson, G. E. Harris, S. V. Rumball, J. M. Waters, and E. M.  Baker (1987) Structure of lactoferrin at 3.2 Å resolution, Proc. Nat. Acad. Sci. USA 84, 1769–1773.