The albumins white; dried egg white") are a family of globular proteins, the most common
of which is serum albumin.
The albumin family consists of all proteins that are water-soluble, are moderately soluble
in concentrated salt solutions, and experience heat denaturation.
Albumins are commonly found in blood plasma, and are unique from other blood proteins in
that they are not glycosylated.
Substances containing albumins, such as egg white, are called albuminoids.
A number of blood transport proteins are evolutionarily related, including serum albumin, alpha-fetoprotein,
vitamin D-binding protein and afamin.
Function Albumin is the main protein of human blood
plasma.
It binds water, cations, fatty acids, hormones, bilirubin, thyroxine and pharmaceuticals - its
main function is to regulate the colloidal osmotic pressure of blood.
Alpha-fetoprotein is a fetal plasma protein that binds various cations, fatty acids and
bilirubin.
Vitamin D-binding protein binds to vitamin D and its metabolites, as well as to fatty
acids.
The biological role of afamin has not yet been characterised.
Structure The 3D structure of human serum albumin has
been determined by X-ray crystallography to a resolution of 2.5 Å.
Albumin is a 65-70 kDa protein.
Albumin comprises three homologous domains that assemble to form a heart-shaped molecule.
Each domain is a product of two subdomains that possess common structural motifs.
The principal regions of ligand binding to human serum albumin are located in hydrophobic
cavities in subdomains IIA and IIIA, which exhibit similar chemistry.
Structurally, the serum albumins are similar, each domain containing five or six internal
disulfide bonds, as shown schematically below:
Types Serum albumin
Serum albumin is the most abundant blood plasma protein and is produced in the liver and forms
a large proportion of all plasma protein.
The human version is human serum albumin, and it normally constitutes about 50% of human
plasma protein.
Serum albumins are important in regulating blood volume by maintaining the oncotic pressure
of the blood compartment.
They also serve as carriers for molecules of low water solubility this way isolating
their hydrophobic nature, including lipid soluble hormones, bile salts, unconjugated
bilirubin, free fatty acids, calcium, ions, and some drugs like warfarin, phenobutazone,
clofibrate & phenytoin.
For this reason, it's sometimes referred as a molecular "taxi".
Competition between drugs for albumin binding sites may cause drug interaction by increasing
the free fraction of one of the drugs, thereby affecting potency.
Specific types include: human serum albumin
bovine serum albumin or BSA, often used in medical and molecular biology labs.
Low albumin may be caused by liver disease, nephrotic syndrome, burns, protein-losing
enteropathy, malabsorption, malnutrition, late pregnancy, artefact, genetic variations
and malignancy.
High albumin is almost always caused by dehydration.
In some cases of retinol deficiency, the albumin level can be elevated to high-normal values.
This is because retinol causes cells to swell with water.
In lab experiments it has been shown that All-trans retinoic acid down regulates human
albumin production Normal range of human serum albumin in adults
is 3.5 to 5 g/dL.
For children less than three years of age, the normal range is broader, 2.9-5.5 g/dL.
Albumin binds to the cell surface receptor Albondin.
Other types Other types include the storage protein ovalbumin
in egg white, and different storage albumins in the seeds of some plants.
Note that the protein 'albumin' is spelled with an "i", while "albumen" with an "e",
is the white of an egg, which contains several dozen types of albumin, mostly ovalbumin.
Medical uses For patients with low blood volume, there
is no evidence that albumin reduces mortality when compared with cheaper alternatives such
as normal saline, or that albumin reduces mortality in patients with burns and low albumin
levels.
Therefore, the Cochrane Collaboration recommends that it not be used, except in clinical trials.
In acoustic droplet vaporization, albumin is sometimes used as a surfactant.
ADV has been proposed as a cancer treatment by means of occlusion therapy.
See also Cohn process
Serum albumin Bovine serum albumin
Human serum albumin
References
External links Albumins at the US National Library of Medicine
Medical Subject Headings The Albumin website
Albumin binding prediction This article incorporates text from the public
domain Pfam and InterPro IPR014760