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albumin

albumin

[al-byoo-muhn]
albumin [Lat.,=white of egg], member of a class of water-soluble, heat-coagulating proteins. Albumins are widely distributed in plant and animal tissues, e.g., ovalbumin of egg, myogen of muscle, serum albumin of blood, lactalbumin of milk, legumelin of peas, and leucosin of wheat. Separation of serum albumins from other blood proteins can be carried out by electrophoresis or by fractional precipitation with various salts. Albumins normally constitute about 55% of the plasma proteins. They adhere chemically to various substances in the blood, e.g., amino acids, and thus play a role in their transport. Albumins and other proteins of the blood aid significantly in regulating the distribution of water and maintenance of proper osmotic pressure in the body. Albumins are also used in textile printing, in the fixation of dyes, in sugar refining, and in other important processes.

Any of a diverse class of proteins historically defined by their ability to dissolve in water and in a half-saturated (see saturation) solution of ammonium sulfate. They are readily coagulated by heating. Examples include serum albumin, a major component of plasma; α-lactalbumin, found in milk; ovalbumin, which makes up about half the proteins of egg white; and conalbumin, another egg-white protein. Ovalbumin is used commercially in the food, wine, adhesives, paper coatings, pharmaceutical, and other industries and in research.

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Albumin (Latin: albus, white) refers generally to any protein with water solubility, which is moderately soluble in concentrated salt solutions, and experiences heat coagulation (protein denaturation). Substances containing albumin, such as egg white, are called albuminoids.

Types

Serum albumin

The most well-known type of albumin is the serum albumin in the blood.

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 70% of human plasma protein; all other proteins present in blood plasma are referred to collectively as globulins.

Serum albumins are important in regulating blood volume by maintaining the osmotic pressure of the blood compartment. They also serve as carriers for molecules of low water solubility, including lipid soluble hormones, bile salts, bilirubin, free fatty acids (apoprotein), calcium, iron (transferrin), and some drugs. 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:

Low albumin (hypoalbuminaemia) may be caused by liver disease, nephrotic syndrome, burns, protein-losing enteropathy, malabsorption, malnutrition, late pregnancy, artefact, posture, genetic variations and malignancy.

High albumin is almost always caused by dehydration. In some cases of retinol (Vitamin A) deficiency the albumin level can become raised to borderline High-normal values. This is because retinol causes cells to swell with water (this is also the reason too much Vitamin A is toxic)

Normal range of human serum albumin in adults (> 3 y.o.) is 3.5 to 5 g/dL. For children less than three years of age, the normal range is broader, 2.5-5.5 g/dL.

Other types

Other types include the storage protein ovalbumin in egg white, and different storage albumins in the seeds of some plants. Note that albumin is spelled with an "i" while "albumen" with an "e" is the white of an egg.

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