Purification of organelles from mammalian cells.

Organelle purification procedures capitalize on the differences in size, density, and (occasionally) surface charge density of individual types of organelles. Most fractionation procedures that are based on centrifugation involve some combination of procedures that distinguish both size and density. Initially, a homogenate is prepared in isoosmotic (or slightly hyperosmotic) sucrose or some other predominantly nonelectrolyte medium. A wide range of procedures have been used to fractionate tissue homogenates. The protocols in this unit emphasize different fractionation techniques that have been used for rat liver, an abundant tissue that has been a favorite of many investigators and has served as the source of many organelle preparations of excellent purity. For selected procedures, examples have been given using other tissue sources (e.g., glandular tissues that maintain protein storage granules for regulated secretion) or, where particularly favorable, cultured cells.