Ammonium, sodium, and magnesium sulfate have been used as agents for precipitation and crystallization of proteins for more than half a century. Na2SO 4 and (NH4)2SO4, being on the salting out end of the Hofmeister series, are good protein precipitants; MgSO4 is a weaker precipitant but is useful when Mg ions interact with the protein. (NH 4^SO4 is the most frequently used salt, due to its great solubility.
The usual procedure in the use of these salts in the purification of a protein is that of fractional precipitation by a step-wise increase in salt concentration (1). To a crude protein extract, one adds (NH4) 2SO4 up to a certain salt concentration. The precipitated material is centrifuged off, and more salt is added to the supernatant, precipitating more protein. In such manner, a step is reached that precipitates the protein of interest, usually detected by a specific bioassay. The fractionation is then refined. The active precipitate is dissolved in dilute buffer, and (NH4)2SO4 is added in much narrower steps, with the isolation of the active fraction.
In protein fractionation, ammonium sulfate is used most frequently because of its high solubility in water (~4 M). It would be preferable to use Na2SO4 because of possible problems with subsequent nitrogen analysis and interference of the NH+4 ion with some biological assays. The drawback to Na2SO4 is its lower solubility (~2 M) that frequently is insufficient for the desired fractionation.
