Albumin is the protein most labs use without thinking about it. It blocks membranes, stabilizes enzymes, dilutes antibodies and calibrates protein assays. A few of its properties are worth knowing when you're choosing between bovine (BSA) and human (HSA) albumin, or between two grades of the same one.
The protein itself
Serum albumin is a single 66 kDa polypeptide folded into three domains. The liver makes it and secretes it into blood, where it accounts for roughly half of the plasma protein. It is very soluble, it tolerates heat and pH swings that would unfold most proteins, and it carries a net negative charge at physiological pH. In the body its main job is oncotic, holding water in the circulation. Its second job is transport: two well-characterized pockets, Sudlow sites I and II, bind fatty acids, bilirubin, hormones and a long list of drugs, which is why albumin is the reference protein in drug-binding studies (NCBI Bookshelf). Cells take albumin up through receptors such as GP60 on endothelium and SPARC in tissue, a route that is now used to deliver albumin-bound drugs (PMC).
BSA or HSA
Bovine and human albumin are about 76% identical in sequence and behave alike in most assays. BSA is cheaper and more abundant, so it does the routine work. HSA is for the cases where the species is the point: the matrix or calibrator for a human diagnostic, binding studies of drugs that will go into people, and cell culture where a bovine protein would raise a regulatory or immunogenicity question.
What separates one BSA from another
The first thing to look at is how it was made. Cohn Fraction V is albumin precipitated by cold-ethanol fractionation; heat-shock albumin is recovered by heating plasma until the other proteins denature. Both work for blocking and as a diluent, and fractionation gives the cleaner product. The methods are compared in albumin purification methods and fractions.
Then the pH of the powder. Our BSA Cohn Fraction V is offered at pH 7 and at pH 5.2. The acidic form dissolves to a slightly acidic solution and is the traditional choice for some enzyme and diagnostic formulations.
Endotoxin only matters once cells are involved. Standard BSA is fine on a membrane, but in cell culture or in vivo the endotoxin that rides along with the protein can activate cells, so use the low-endotoxin grade there.
Fatty-acid-free and IgG-free grades exist for binding studies and for immunoassays where trace bovine IgG would cross-react.
Choosing
Pick BSA unless the species matters. Match the grade to how sensitive the assay is rather than defaulting to the most purified one, and check the lot Certificate of Analysis for the tests your application actually depends on. We supply BSA, human serum albumin and albumins from rabbit, mouse and other species as lyophilized powder. If you need a grade or species that isn't listed, ask us.