Enzyme calpain

Systematic cell degradation and death, apoptosis, on the other hand, is thought to be necessary to avoid cell damage accumulating so as to cause incorrect differentiation. The process is very complicated involving activation of a number of destructive enzymes where once more increase in cell calcium often initiates the hydrolyses using special internal calcium-dependent enzymes, calpains (see Demaurex in Further Reading). 

Shields, D.C., Schaecher, K.E., Saido, T.C., Banik, N.L., 1999, A putative mechanism of demyelination in multiple sclerosis by a proteolytic enzyme, calpain, Proc. Natl. Acad. Sci. USA 96,11486-11491 Shollmeyer, J., 1986, Possible role of calpain I and calpain II in differentiating muscle, Exp. Cell. Res., 163, 413 122... 

Under ischemic conditions, accumulation of pathological levels of glutamate is accompanied by an early and robust activation of the proteolytic, calcium-dependent, enzyme calpain (Branca, 2004) that is maintained over 24-h period of reperfusion (Oka et al., 2006 Sakamoto et al., 2000) and is prevented by treatment with the NMDA receptor antagonist MK801 (Russo and Morrone, personal communication). 

Miller AC, Schattenberg DG, Malkinson AM, et al. 1994. Decreased content of the il 1 alpha processing enzyme calpain in murine bone marrow-derived macrophages after treatment with the benzene metabolite hydroquinone. Toxicol Lett 74(2) 177-184. 

In certain types of brain injury, such as diffuse axonal injury, spectrin is irreversibly cleaved by the proteolytic enzyme calpain. This destroys the cytosketelon, causing the membrane to form blebs, irregular bulges in the plasma membrane of a cell caused by localised decoupling of the cytoskeleton from the plasma membrane, ultimately leading to degradation and usually death of the cell. 

These findings provide a direct link between AMPA-induced excitotoxicity in the cerebellum and the activation of the Ca -dependent enzyme calpain. Based upon... 

Calpains are proteolytic enzymes with cysteine in the catalytic site. In general, calpains are found as intracellular proteases. These proteases are associated with many physiological basic processes of the cell. 

During the last ten years, it has become apparent that calcium-dependent papain-like peptidases called calpains (EC 3.4.22.17) represent an important intracellular nonlysosomal enzyme system [35][36], These enzymes show limited proteolytic activity at neutral pH and are present in virtually every eukaryotic cell type. They have been found to function in specific proteolytic events that alter intracellular metabolism and structure, rather than in general turnover of intracellular proteins. Calpains are composed of two nonidentical subunits, each of which contains functional calcium-binding sites. Two types of calpains, i.e., /i-calpain and m-calpain (formerly calpain I and calpain II, respectively), have been identified that differ in their Ca2+ requirement for activation. The activity of calpains is regulated by intracellular Ca2+ levels. At elevated cytoplasmic calcium concentrations, the precursor procal-pain associates with the inner surface of the cell membrane. This interaction seems to trigger autoproteolysis of procalpain, and active calpain is released into the cytoplasm [37]. 

A large and diverse group of proteins, including enzymes, cytoskeleton, contractile proteins, and receptors, have been shown to be modified by calpains. Thus, a number of enzymes such as tyrosine hydrolase, tryptophan hydrolase, transglutaminase, protein kinase C, and membrane Ca2+-ATPase are activated by calpain proteolysis [38]. Several receptor proteins, in particular receptors for steroid hormones, growth factors, and adrenaline, are modulated by calpains, which participate also in platelet activation, cell fusion, and mitosis [39], Although the physiological roles of calpains continue to be un-... 

Both the 26S proteasome and the RC hydrolyze all four nucleotide triphosphates, with ATP and CTP preferred over GTP and UTP [58]. Although ATP hydrolysis is required for conjugate degradation, the two processes are not strictly coupled. Complete inhibition of the peptidase activity of the 26S proteasome by calpain inhibitor I has little effect on the ATPase activity of the enzyme. The nucleotidase activities of the RC and the 26S proteasome closely resemble those of E. coli Lon protease, which is composed of identical subunits that possess both proteolytic and nucleotidase activities in the same polypeptide chain. Like the regulatory complex and 26S proteasome, Lon hydrolyzes all four ribonucleotide triphosphates, but not ADP or AMP [18]. 

Calpains are enzymes that consist of a proteolytic subunit and a calcium binding subunit. In the cytosol, these enzymes are inactive due to binding of the inhibitory protein, calpastatin. Attachment to the cell membrane removes this inhibition and activation occurs at low concentrations of Csi ions. The enzymes hydrolyse proteins as far as peptides complete hydrolysis requires peptidases, which are also present in the cytosol. 

This enzyme [EC 3.4.22.17] is an intracellular, nonlyso-somal member of the peptidase family C2. The enzyme catalyzes the calcium ion-dependent hydrolysis of peptide bonds with preference for Tyr-Xaa, Met-Xaa, or Arg-Xaa with a leucyl or valyl residue at the P2 position. There are two main types of calpain. One has a high calcium sensitivity in the micromolar range and is called (,-calpain or calpain I. The other calpain has a low calcium sensitivity in the millimolar range and is called m-calpain or calpain II. Forms of calpain exhibiting intermediate calcium sensitivity also exist. 

Mammalian Cell Protease Inhibitor CocktaiL These should contain AEBSF, pepstatin A, E-64, bestatin, leupeptin, and aprotinin. (Metal chelators can be added to suppress the activity of calcium ion-dependent proteases such as calpain. Again, one must determine whether the protein or enzyme being purified does not require a divalent metal cofactor for stabihty or activity.)... 

The protease [APP secretase) responsible for the normal cleavage of APP through the PA4 segment is unknown, but a number of proteases, including calpain [D. H. Small et al. 1992), a serine protease, and cathepsin B [Cataldo et al. 1991), have been proposed. It remains to be determined whether therapy should be directed at enhancing the function of the secretase to accelerate the breakdown of the PA4 segment or at inhibiting the enzyme to reduce APP turnover. 

Amplification Enzymatic groups such as protein kinase C, calmodulin, and calpaine-regulating enzymes (e.g., phospholipase) are activated. These changes give way to the long-term activation of neuronal circuits, which increase excitotoxicity. 

The cysteinyl proteases include papain calpains I and II cathepsins , H, and L proline endopeptidase and interleukin-converting enzyme (ICE) and its homologs. The most well-studied cysteinyl protease is likely papain, and the first x-ray crystallographic structures of papain [193] and a peptide chloromethylketone inhibitor-papain complex [194] provided the first high resolution molecular maps of the active site. Pioneering studies in the discovery of papain substrate peptide-based inhibitors having P, electrophilic moieties such as aldehydes [195], ketones (e.g., fluoromethylketone, which has been determined [196] to exhibit selectivity for cysteinyl proteases versus serinyl proteases), semicarbazones, and nitriles are noteworthy since 13C-NMR spectro-... 

Ca2+-activated proteases. These enzymes, also known as calpains, are extra-lysosomal. They are involved in normal cell functions such as enzyme activation and membrane remodeling. When activated by increases in cytosolic Ca2+, these proteases especially target cytoskeletal and membrane proteins, including receptors, which are vulnerable proteins. This results in cytotoxicity. 

Sustained cytosolic Ca2+ overload usually results in a different route leading to cell death. It mainly relies on the activation of the calcium/calmodulin (CaM)-dependent phosphatase, calcineurin. Calcineurin-catalyzed dephosphorylation promotes apoptosis by regulating the activity of a number of downstream targets, including the pro-apoptotic Bcl-2 family member, Bad (Wang, et al., 1999), and transcription factors of the NFAT (nuclear factor of activated T cells) family (Rao, et al., 1997). There are also other Ca2+-dependent enzymes contributing to the apoptotic events, and they include several DNA-degrading endonucleases (Robertson, et al., 2000) and Ca2+-activated cystein proteases of the calpain family essential for the enzymatic activation of the crucial pro-apoptotic effectors (Altznauer, et al., 2004). 

---