Proteases calcium-activated

Figure 3. Schematic illustrating the possible role of a calcium-activated protease in the degradation of the Z-line of sacromere of muscle and the dissociation of the proteins of the thick and thin filaments (21)...

Calcium-activated protease in muscle The enz)one catalyzes the hydrolysis of troponin to tropomyosin. The enzyme is thought to be used in the degradation and turnover of muscle fibers. 

Suzuki and associates (Suzuki et al., 1978 Suzuki and Nonami, 1982) carried out an interesting reconstitution experiment Z lines almost disappeared on prolonged extraction with a low ionic strength solution, but could be reconstituted by treatment with the proteins released from Z lines by calcium-activated protease. The active principal contained three kinds of proteins 400-, 100- (a-actinin), and 34-kDa proteins. The 400-kDa protein, named Z-nin, has been shown in the interior of Z lines. It would be desirable to characterize Z-nin in detail. 

In keeping with their proposed cytoskeletal nature, IFPs initially were thought to serve a purely structural role in muscle cells. It was hypothesized that the function of these proteins was to keep other cytoplasmic proteins in proper relationship to one another, as well as to anchor the cytoplasmic contractile apparatus to the cell membrane. Flowever, subsequent developments in cell biology cast considerable doubt on this premise." The intermediate filaments are now known to serve a nucleic acid-binding function moreover, they are susceptible to processing by calcium-activated proteases and are substrates for cyclic adenosine monophosphate-dependent protein kinases. Thus, it has been proposed that all IFPs serve as modulators between extracellular influences governing calcium flux into the cell (and subsequent protease activation) and nuclear function at a transcriptional... 

Calpains are calcium-activated proteases that participate in turnover of proteins in the cell. 

Additionally, energy deficits lead to ionic imbalance, and excito-toxic glutamate efflux and buUd up of intracellular calcium. Downstream pathways ultimately include direct free radical damage to membrane lipids, cellular proteins, and DNA, as well as calcium-activated proteases, plus caspase cascades that dismantle a wide range of homeostatic, reparative, and cytoskeletal proteins (courtesy of DaLkara et al. 2003)... 

Ishiura, S., Murofushi, H., Suzuki, K. Imahori, K. (1978). Studies of a calcium-activated protease from chicken skeletal muscle. ]. Biochem., 84, 225—30. 

If reperfusion is established after short periods of ischaemia, there is usually a prolonged period of impaired contraction, even though eventual recovery can be almost complete. This is referred to as stunned myocardium . The mechanism of this impairment of contractile function appears to be decreased calcium sensitivity of the myofilaments (Hofmann et al. 1993). Reperfusion followed by recovery is associated with moderately elevated Cai (Allen etal. 1989, Lee and Allen 1992), and it is thought that this switches on calcium-activated proteases (Barry 1991). 

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