Egg White, Eggs, Endotoxins, Lipopolysaccharide, Storage Protein
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Ovalbumin

Ovalbumin (abbreviated OVA) is the main protein found in egg white, making up approximately 55% of the total protein. Ovalbumin displays sequence and three-dimensional homology to the serpin superfamily, but unlike most serpins it is not a serine protease inhibitor. The function of ovalbumin is unknown, although it is presumed to be a storage protein.

Storage proteins

Storage proteins serve as biological reserves of metal ions and amino acids, used by organisms. They are found in plant seeds, egg whites, and milk.

Ferritin is an example of a storage protein that stores iron. Iron is a component of heme, which is contained in the transport protein, hemoglobin and in cytochromes.

Some storage proteins store amino acids. Storage proteins’ amino acids are used in embryonic development of animals or plants. Two amino acid storage proteins in animals are casein and ovalbumin.

Seeds, particularly of leguminous plants, contain high concentrations of storage proteins. Up to 25 percent of the dry weight of the seed can be composed of storage proteins. The best known storage protein in wheat is the prolamin gliadin, a component of gluten.

See also

Research

Ovalbumin is an important protein in several different areas of research, including:

  • general studies of protein structure and properties (because it is available in large quantities).
  • studies of serpin structure and function (the fact that ovalbumin does not inhibit proteases means that by comparing its structure with that of inhibitory serpins, the structural characteristics required for inhibition can be determined).
  • proteomics (chicken egg ovalbumin is commonly used as a molecular weight marker for calibrating electrophoresis gels).
  • immunology (commonly used to stimulate an allergic reaction in test subjects; e.g., established model allergen for airway hyper-responsiveness, AHR).

(For in vivo and in vitro studies based on ovalbumin it is important that the endotoxin content is less than 1 EU/mg.)[citation needed][needs context]

Lipopolysaccharides (LPS) are frequent contaminants in plasmid DNA prepared from bacteria or proteins expressed from bacteria, and must be removed from the DNA or protein to avoid contaminating experiments and to avoid toxicity of products manufactured using industrial fermentation.

  • Wicks IP, Howell ML, Hancock T, Kohsaka H, Olee T, Carson DA (March 1995). “Bacterial lipopolysaccharide copurifies with plasmid DNA: implications for animal models and human gene therapy”. Human Gene Therapy. 6 (3): 317–323. doi:10.1089/hum.1995.6.3-317. PMID 7779915.

Ovalbumin is frequently contaminated with endotoxins. Ovalbumin is one of the extensively studied proteins in animal models and also an established model allergen for airway hyper-responsiveness (AHR). Commercially available ovalbumin that is contaminated with LPS can falsify research results, as it does not accurately reflect the effect of the protein antigen on animal physiology.

In pharmaceutical production, it is necessary to remove all traces of endotoxin from drug product containers, as even small amounts of endotoxin will cause illness in humans. A depyrogenation oven is used for this purpose. Temperatures in excess of 300 °C are required to fully break down LPS.

The standard assay for detecting presence of endotoxin is the Limulus Amebocyte Lysate (LAL) assay, utilizing blood from the Horseshoe crab (Limulus polyphemus). Very low levels of LPS can cause coagulation of the limulus lysate due to a powerful amplification through an enzymatic cascade. However, due to the dwindling population of horseshoe crabs, and the fact that there are factors that interfere with the LAL assay, efforts have been made to develop alternative assays, with the most promising ones being ELISA tests using a recombinant version of a protein in the LAL assay, Factor C.

Structure

The ovalbumin protein of chickens consists of 385 amino acids, its relative molecular mass is 42.7 kDa, and it adopts a serpin-like structure. Ovalbumin also has several modifications, including N-terminal acetylation (G1), phosphorylation (S68, S344), and glycosylation (N292). It has three isoforms, A1, A2, and A3, which vary based on the number of bound phosphate residues. It is secreted from the cell, targeted by an internal signal sequence (residues 21–47), rather than the N-terminal signal sequence commonly found in other secreted proteins. Ovalbumin’s signal sequence is not cleaved off, but remains as part of the mature protein.

Change upon heating

When heated, ovalbumin undergoes a conformational change from its soluble, serpin structure into an insoluble all-β-sheet structure with exposed hydrophobic regions. This causes the protein to aggregate and cause the solidification associated with cooked egg white.

See also

Influenza vaccines are created by injecting a live virus into fertilized chicken eggs. The viruses are harvested, killed and purified, but a residual amount of egg white protein remains.

The measles and mumps parts of the “MMR vaccine” (for measles, mumps, and rubella) are cultured on chick embryo cell culture and contain trace amounts of egg protein.

References

External links

Serpins
Globular proteins

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