Biology Reference
In-Depth Information
biochemicals and are ideally suited for column chromatography separations. Both applica-
tions work best for plasma membrane vesicles (microsomes). As discussed in Chapters 5
and 7, carbohydrates are attached to outer leaflet plasma membrane sphingolipids (cerebro-
sides, globosides and gangliosides). Most plasma membrane proteins are also heavily glyco-
sylated (Chapter 6), with the sugars always facing outside of the cell. In fact, carbohydrates
are a characteristic of the plasma membrane and so are obvious targets for membrane puri-
fication schemes.
Lectin-Affinity Chromatography
Lectins play a central role in membrane studies. Their history and function are thoroughly
discussed in several excellent reviews by Nathan Sharon (for example see
[33,34]
). The term
lectin is derived from the Latin word
legere
, 'to select', and indeed that is what lectins do. Lec-
tins are proteins of non-immune origin that reversibly bind to specific sugars without modi-
fying them. They are ubiquitous in nature, being found in animals, plants, insects,
microorganisms, and even humans. Each lectin has two or more separate binding sites
and so lectins are bifunctional reagents that have the ability to clump or agglutinate cells.
It is this property that led to their 1888 discovery by Herrmann Stillmark (
Figure 12.12
)at
the University of Dorpat in Estonia. In his Ph.D. thesis Stillmark described the agglutination
of erythrocytes by the lectin ricin isolated from castor bean (
Ricinus communis
).
Countless lectins have now been identified making their classification difficult, but
increasing their importance in membrane isolations. For example, the Sigma-Aldrich
catalog offers almost 500 lectins from dozens of sources! Lectins vary from those that
FIGURE 12.12
Hermann Stilmark (1860
e
1923).
Courtesy of Tartu University
