Density gradient media for the purification of cells – a perspective

1. Iodinated density gradient media a historical perspective 1a. Metrizoate and diatrizoate In the nineteen-sixties, Nyegaard (later Nycomed) in Oslo began producing X-ray imaging agents that were derivatives of tri-iodobenzoic acid. Metrizoate (produced under the trade name IsopaqueTM) was one of the early versions of these tri-iodobenzoic acid derivatives (Figure 1). At about the same time, cell biologists were trying to transfer the density gradient technology developed by Christian de Duve, Albert Claude and George Palade for the purification of subcellular organelles, to the fractionation of different cell types from blood and from tissues. Many reagents were investigated. Sucrose, the compound favoured for membrane organelle purifications (and still widely used for certain subcellular particles) is toxic to most mammalian (and non-mammalian) cells; moreover all its solutions above approx. 1.03 g/ml are hyperosmotic. Other polyhydric alcohols such as mannitol and glycerol were much more cell-friendly than sucrose, but the osmolality of their solutions was even higher. High molecular weight polymers (e.g. polysaccharides, plant glycans and proteins) solved the osmolality problem, but made the viscosity problem far worse; furthermore these compounds tend to adhere to the surface of cells. Solution formation of some of these polymers is also not easy.