The effects of chemical modification of calmodulin on Ca2+-induced exposure of a hydrophobic region. Separation of active and inactive forms of calmodulin.

Native calmodulin binds four calcium ions per molecule and exhibits strong Ca2+-dependent binding to phenyl-Sepharose. In contrast, calmodulin inactivated by oxidation of methionine residues or by deamidation binds fewer calcium ions (two per molecule) and shows relatively weak interaction with phenyl-Sepharose. Calmodulin inactivated by modification of lysine residues still is able to bind four calcium ions per molecule and shows strong binding to phenyl-Sepharose similar to native calmodulin. The results suggest that complete exposure of calmodulin's hydrophobic region occurs only after the binding of four ions of calcium to the calmodulin molecule. Thus, phenyl-Sepharose hydrophobic interaction chromatography might be used to separate active calmodulin from inactive forms of calmodulin obtained by oxidation or heat treatment for prolonged periods. As an example, phenyl-Sepharose chromatography can be used to separate free iodide and inactivated species of calmodulin readily from the active, iodinated form of calmodulin following iodination.