Calpain-υ is activated in hepatocyte mitochondria during ischemia- reperfusion injury

Mitochondrial permeability transition (MMPT), a rapid increase in the permeability of the inner mitochondrial membrane to low molecular weight solutes, plays a predominant role in ischemia-reperfusion injury of hepatocytes. Calpain, a Ca²⁺-dependent neutral proteinase (EC 3, 4, 22, 17) has been considered to be involved in the induction of MMPT, but its activation in mitochondria remains to be proven. By immunoelectron microscopic study with antibodies that specifically recognize the pro-enzyme and intermediate (activated) forms of calpain-υ, its activation at mitochondria was investigated in a 15-min ischemia model of rat liver. Activated calpain-υ was not detected in hepatocytes before ischemia, but was observed primarily in mitochondria 15 min after reperfusion. In contrast, the pro-enzyme form of calpain-υ was detected equally throughout the cytoplasm before ischemia, and no significant change was observed after reperfusion. This data provides the direct evidence that calpain-υ was activated in mitochondria during the ischemia-reperfusion injury of the liver.