Organic acids prevent aluminum-induced conformational changes in calmodulin.

At a molar excess of 10:1 for [citrate]/[calmodulin], citrate can prevent aluminum binding to calmodulin when present in the protein solution in micromolar concentration, as determined by fluorescence and circular dichroism spectroscopy. In contrast, citrate is only partially effective in restoring calmodulin to its native structure once the aluminum-calmodulin complex (3:1) is formed, as measured by the alpha-helix content of the protein. Considering the magnitude of the stability constant of the citrate-aluminum chelate, citrate and perhaps other carboxylic acids may protect calmodulin, and thus cells, from toxic aluminum ions.