AIMS AND OBJECTIVES
Protein purification has an over 200-year history: the first attempts
at isolating substances from plants having similar properties to “egg
albumen,” or egg white, were reported in 1789 by Fourcroy. Many proteins
from plants were purified in the nineteenth century, though most would
not be considered pure by modern standards. A century later, ovalbumin
was the first crystalline protein obtained (by Hofmeister in 1889). The
year 1989 may not go down in history as a milestone in protein
chemistry, but since then there has been a resurgence of interest in
proteins after more than a decade of gene excitement.
The aims of protein purification, up until the 1940s, were simply
academic. To then, even the basic facts of protein structure were not
fully appreciated, and pure proteins were needed just to study structure
and test the rival theories of the pre-DNA days. During the Second
World War, an acute need for blood proteins led to development of the
Cohn fractionation procedure for purification of albumin and other
proteins from serum (Cohn et al., 1946). This was the inception of
large-scale protein purifications for commercial purposes; Cohn
fractionation continues to be used to this day.
The nature of the proteins studied has also changed substantially.
Whereas enzymes were once the most favored subjects, they have now been
superceded by nonenzymatic proteins such as growth factors, hormone
receptors, viral antigens, and membrane transporters. Many of these
occur in minute amounts in the natural source, and their purification
can be a major task. Heroic efforts in the past have used kilogram
quantities of rather unpleasant starting materials, such as human
organs, and ended up with a few micrograms of pure product. It is now
more usual, however, to take the genetic approach: clone the gene before
the protein has been isolated or even properly identified, and then
express it in a suitable host cell culture or organism. The expression
level may be orders of magnitude higher than in the original source,
which will make purification a relatively simple task. It can be useful
to know beforehand some physical properties of the protein, to
facilitate the development of a suitable purification protocol from the
recombinant source. On the other hand, there are now several ways of
preparing fusion proteins, which can be purified by affinity techniques
without any knowledge of the properties of the target protein. Moreover,
there are ways of modifying the expressed product to simplify
purification further.
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