Isolation of Endogenous Calpastatin.

We here describe the purification of calpastatin from human erythrocytes. When calpastatin is purified from tissues, it is necessary to measure its inhibitory activity against calpain in the presence of Ca ²⁺ to specifically identify the protein. Thus, the purification steps necessary to obtain the inhibitor protein were originally designed to obtain calpain from the same tissue. For this reason, in addition to calpastatin purification, we also include a method for purifying human erythrocyte calpain and globin. We routinely use these two components for assaying calpastatin inhibition.