Principles of Biochemistry

5.3.2.2 Purification Steps -a. Precipitation

5.3.2.2 Purification Steps

a)Precipitation

A common second step to purifying a protein from a crude extract is by precipitation in a solution with high osmotic strength (i.e. salt solutions). Nucleic acids in the crude extract can be removed by precipitating aggregates formed with streptomycin sulfate or protamine sulfate. Protein precipitation is usually done using ammonium sulfate as the salt.

Different proteins will precipitate in different concentrations of ammonium sulfate. In general, proteins of higher molecular weight precipitate in lower concentrations of ammonium sulfate. Salt precipitation does not usually lead to a highly purified protein, but can assist in eliminating some unwanted proteins in a mixture and concentrating the sample. Salts in the solution are then removed by dialysis through porous cellulose tubing, filtration, or gel exclusion chromatography.