Demonstration of three calpains in the matrix of rat liver mitochondria.

Three distinct Ca(2+)-activated proteolytic activities could be proven in rat liver mitochondria. The proteolytic activities detected in the presence of Ca2+ are different in the two mitochondrial soluble compartments, the matrix and the intermembrane space. Strikingly three Ca(2+)-activated proteolytic activities (M1, M2 and M3) appear in the matrix whereas the intermembrane space contains only two such activities. These proteolytic activities are similar to the calpains already described in the cellular cytosol with regard to their optimal pH and inhibition profiles. The Ca2+ requirements for activation of M1 and M2 correspond to those of the micromolar and millimolar Ca(2+)-requiring proteinases, whereas the concentration of Ca2+ required to activate M3 is an intermediate value.