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Subunit autolysis of mammalian m-calpain upon activation was examined in kinetic terms using a set of antibodies recognizing different portions of the protease. Activation of m-calpain by calcium resulted in no apparent autolysis in the large catalytic subunit, whereas the small regulatory subunit underwent immediate autolysis followed by substrate proteolysis. This profile of subunit autolysis is distinct from that of the other ubiquitous isozyme, μ-calpain, in which autolysis of the large subunit and then of the small subunit precedes substrate proteolysis under the normal conditions. The activation state of m-calpain thus is not reflected by the large subunit autolysis. The mode and role of autolysis may vary among calpain isozymes.