An endogenous activator of the Ca2+-dependent proteinase of human neutrophils that increases its affinity for Ca2+

An endogenous activator of the Ca2+-dependent proteinase (calpain) has been identified in human neutrophils. In the presence of the activator, the affinity of calpain for Ca2+ is increased by greater than 100-fold and maximum catalytic activity is observed with Ca2+ concentration below 1 microM. The activator is a heat-stable protein having an apparent molecular mass of approximately equal to 40 kDa. It appears to be associated with the cytoskeletal fraction of human neutrophils. Neutrophils also contain an endogenous cytosolic calpain inhibitor (calpastatin), which is readily separated from the activator by size-exclusion chromatography. The effects of the activator and inhibitor appear to be antagonistic and may constitute a physiological mechanism for modulating intracellular calpain activity.