Calcium-dependent hydrophobic interaction chromatography.

Calcium-dependent hydrophobic interaction chromatography has been widely used for the purification of calcium-binding proteins, following the report that calmodulin could be purified using this proce dure (1). The method makes use of the fact that proteins such as calmodulin, undergo a conformational change and expose a hydrophobic region on binding calcium (2). This means that they bind to a hydrophobic resin, such as phenyl Sepharose, in the presence of calcium, and can be eluted with the calcium chelator EGTA. The procedure has been developed to allow separation of calmodulin from other calcium-binding proteins, exploiting differences in affinity for calcium and in hydrophobicity, and hence elution time in EGTA (3,4). Changes in pH in conjunction with EGTA elution have also been used for fractionation of calcium-regulated proteins on phenyl Sepharose (5).