Hydrophobic association of calpains with subcellular organelles. Compartmentalization of calpains and the endogenous inhibitor calpastatin in tissues

A sarcolemma-associated inhibitor is capable of modulating calcium-dependent proteinase activity

Purified bovine myocardial sarcolemma vesicles were shown to contain calcium-dependent proteinase inhibitor protein by direct assay and by immunoblot analysis following gel electrophoresis (Western blotting). Calcium-dependent proteinase (calpain, EC 3.4.22.17) was not detected in the sarcolemma vesicles. The inhibitor protein was not solubilized when the vesicles were ruptured by repetitive freezing and thawing. However, a large amount of latent inhibitor activity was exposed after freezing and thawing the sarcolemma, and the inhibitor was much more susceptible to removal by 1.0 M NaCl or proteolysis following this treatment. Since the vesicles were predominantly right-side-out, the latter observations suggested that the inhibitor was associated with the cytoplasmic face of the sarcolemma. The endogenous inhibitor was capable of protecting sarcolemmal protein kinase C from proteolytic conversion to soluble protein kinase M by type I or type II calcium-dependent proteinase. Thus, the inhibitor is probably important in controlling calcium-dependent proteolysis of sarcolemmal proteins.

Calcium-activated neutral protease and its endogenous inhibitor. Activation at the cell membrane and biological function

The structures of calcium-activated neutral protease (CANP) and its endogenous inhibitor elucidated recently have revealed novel features with respect to their structure-function relationship and enzyme activity regulation. The protease is regarded as a proenzyme which can be activated at the cell membrane in the presence of Ca2+ and phospholipid, and presumably regulates the functions of proteins, especially membrane-associated proteins, by limited proteolysis. Protein kinase C is hydrolysed and activated by CANP at the cell membrane to a cofactor-independent form. These results are reviewed and the possible involvement of CANP in signal transduction is discussed.

Changes in pulmonary calpain activity following treatment of mice with butylated hydroxytoluene

The antioxidant, butylated hydroxytoluene (BHT), causes lung toxicity in mice followed by regenerative repair, and can also modulate the development of carcinogen-induced lung adenomas. We are investigating changes in pulmonary biochemistry following BHT treatment in order to understand the mechanisms of BHT-induced pulmonary regenerative repair. BHT administration lowered cytosolic Ca2+-activated neutral protease (calpain) activity, increased the activity of the endogenous calpain inhibitor, calpastatin, increased the extent of photoincorporation of 8-N3-[32P]cAMP into a Mr 37,000 proteolytic product derived from cAMP-dependent protein kinase regulatory (R) subunits, and increased membrane-associated protease activity. All of these changes were dependent on the BHT dosage; the altered proteolytic activities occurred at a dose lower than that which caused observable lung toxicity as assessed by the lung weight/body weight ratio. Decreased cytosolic calpain activity was detectable within 1 day after BHT administration, was lowest at 4-7 days, and had not returned to control levels by Day 21, a time when normal lung morphology had been regained. The decrease in calpain activity cannot fully be accounted for by increased calpastatin activity; upon separation of these proteins by DEAE chromatography, the amount of calpain activity from BHT-treated mice remained lower than the corresponding peak from control mice. Increased photolabeling of the Mr 37,000 protein began at 1 day and continued to increase up to 4 days after BHT. All of the cytosolic changes preceded the increased particulate proteolytic activity by 1-2 days. R-subunits which have dissociated from their catalytic subunits are more susceptible to degradation by calpain, but BHT treatment did not enhance subunit dissociation as determined by the elution profile of 8-N3-[32P]cAMP-labeled R-subunits following DEAE chromatography. A large percentage of the particulate protease activity was inhibited by calpastatin, leupeptin, and E-64, all of which are known to inhibit calpain activity; this suggested that calpain accounted for most of this activity. Changes in the activities of proteases which catalyze limited proteolysis reactions may play an important role in the repair of acute lung injury.

Isolated bovine myocardial sarcolemma and sarcoplasmic reticulum vesicles contain tightly bound calcium-dependent protease inhibitor

Bovine myocardial sarcolemma and sarcoplasmic reticulum vesicle preparations contained calcium-dependent protease inhibitor protein. No inhibitor was detected in mitochondrial membranes. The membrane-bound inhibitor co-purified with the marker enzymes for sarcolemma and sarcoplasmic reticulum, Na+,K+-ATPase and Ca2+,K+-ATPase respectively, on isopycnic ultracentrifugation through linear sucrose density gradients. Sarcolemma and sarcoplasmic reticulum vesicles contained about 1 mg of inhibitor per g of membrane protein. However, about one-half of the inhibitor in sarcoplasmic reticulum vesicles was not tightly associated with the membrane. The membrane-bound inhibitor may function to modulate calcium-dependent proteolytic cleavage of sarcolemmal or sarcoplasmic reticulum-associated proteins.