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Protein Purification The purpose of using cell culture or animal transgenics is to produce a large amount of a specific protein of interest, even if the cell type doesn't naturally make that particular protein. This is possible because much of the machinery used to read genetic codes and to make the respective proteins is the similar among different species. Whether the recombinant protein is produced in bacteria, yeast, mammalian cell culture, or transgenic goat's milk, it must be purified away from contaminating proteins and other contaminants. For example, in mammalian cell culture, the protein of interest must be separated from all other proteins in the cell culture medium. In the case of goat's milk, the protein must be separated from the milk fat and other proteins naturally contained in the milk. There are several methods of purifying recombinant proteins, such as centrifugation, precipitation, filtration and column chromatography. In column chromatography, a resin binds the recombinant protein from the mixture. For example, chromatography columns are generally tall cylindrical objects containing a resin that attracts the specific protein. The contents of the cell or goat's milk are poured through the column and only the protein of interest sticks to the resin. Once the protein is bound to the resin of the column, it can then be released from the resin by eluting the protein from the column. In this way, column chromatography procedures can often extract a protein from a mixture with a high level of purity. The purity required for a protein depends on the application of the protein. For example, a recombinant protein, such as insulin, that is to be used as a pharmaceutical product (also referred to as a biopharmaceutical) would need to be purified to extremely high levels for human consumption. Other applications, for example, spider silk used to produce industrial materials, may not require the same level of purification.
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