The sources of proteins are plant, animal, or microbial cells. Certain tissues such as pancreas glands or liver are particularly rich in enzymes. However, some of the enzymes in the mixture when cells are disrupted are proteases. They digest proteins. Enzymes are proteins, and proteases can digest themselves. We can take advantage of the change in enzyme activity with pH by changing to a pH where the protease activity is low. The disruption of cells to release enzymes can be carried out at low pH and cold temperatures to minimize losses due to proteolytic activity.
Proteins have great specificity that can be used to attach them to other molecules. Attaching an protein to a solid by exploiting this specificity is called affinity chromatography. Sometimes quite pure enzymes can be recovered from a mixture by affinity chromatography, but more often there is a preliminary separation. Each binding site can attach to only one enzyme molecule, so it can be expensive to provide enough of the binding agent.
A very common preliminary separation comes from adding ammonium sulfate in stages. Different proteins precipitate at different salt concentrations, and this divides the enzymes and other proteins into fractions. There are other salts for precipitation, and polyethylene glycol is an effective organic precipitant.
The fraction with the desired enzyme also contains much salt. Enzymes form colloidal suspensions rather than true solutions, but we make the common mistake of saying that the enzyme is "redissolved" in water. Dialysis with membranes through which the enzyme cannot pass is the popular method for removing the salt.
Chromatographic separation is covered in much detail in other pages and will not be discussed here. New techniques such as displacement chromatography can actually concentrate the enzyme, but conventional chromatography that flushes it through the column causes much dilution. After concentration by means such as forcing the water out through a finely pored membrane, freeze drying is a popular method for producing a stable product.