Bovine serum albumin

Bovine serum albumin (also known as BSA or "Fraction V") is a serum albumin protein that has numerous biochemical applications including ELISAs (Enzyme-Linked Immunosorbent Assay), immunoblots, and immunohistochemistry. It is also used as a nutrient in cell and microbial culture. In restriction digests, BSA is used to stabilize some enzymes during digestion of DNA and to prevent adhesion of the enzyme to reaction tubes and other vessels. This protein does not affect other enzymes that do not need it for stabilization. BSA is also commonly used to determine the quantity of other proteins, by comparing an unknown quantity of protein to known amounts of BSA. BSA is used because of its stability, its lack of effect in many biochemical reactions, and its low cost, since large quantities of it can be readily purified from bovine blood, a byproduct of the cattle industry.

The nickname "Fraction V" refers to albumin being the fifth fraction of the original Edwin Cohn purification methodology that made use of differential solubility characteristics of plasma proteins. By manipulating solvent concentrations, pH, salt levels, and temperature, Cohn was able to pull out successive "fractions" of blood plasma. The process was first commercialized with human albumin for medical use and later adopted for production of BSA.

Nomenclature, classification and codification
It comes from Ox/cattle, hence the name "bovine".

Properties
The full-length BSA precursor protein is 607 amino acids in length. An N-terminal 18-residue signal peptide is cut off from the precursor protein upon secretion, hence the initial protein product contains 589 amino acid residues. An additional 4 amino acids is cleaved to yield the mature BSA protein that contains 585 amino acids.

Physical properties of BSA:
 * Number of amino acid residues: 585
 * Molecular weight: 66,776 Da
 * isoelectric point in water at 25 °C: 4.7
 * Extinction coefficient of 43,824 M−1cm−1 at 279 nm
 * Dimensions: 140 X 40 X 40 Å3 (prolate ellipsoid where a = b < c)

Function
Useful as a protein concentration standard.