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The dissociation of mu- and m-calpains was studied by fluorescence spectroscopy under high hydrostatic pressure (up to 650 MPa). Increasing pressure induced a red shift of the tryptophan fluorescence of the calcium-free enzyme. The concentration dependence of the spectral transition was consistent with a pressure-induced dissociation of the subunits. Rising temperature increased the stability of calpain heterodimers and confirmed the predominance of hydrophobic interactions between monomers. At saturating calcium, the spectral transition was not observed for native or iodoacetamide-inactivated calpains, indicating that they were already dissociated by calcium. The reaction volume was about -150 ml mol-1 for both isoforms, and the dissociation constants at atmospheric pressure are approximately 10-12 M and 10-15 M for mu- and m-calpains, respectively. This result indicates a tighter interaction in the isoform that requires higher calcium concentration for activity.

In previous studies, we isolated and identified a μ-calpain-PKCα complex from rabbit skeletal muscle. At the same time we pointed out that an association between μ-calpain and PKCα could occur at the level of the plasma membrane of muscle cells, and that PKCα could thus be considered as a potential μ-calpain substrate. In the present study, using the μ-calpain-PKCα complex as a model, we report that μ-calpain is activated in the combined presence of physiological calcium concentrations (less than 1 μM) and phosphatidylserine. Furthermore our data also show that: (1) there exists a correlation between the appearance of autolyzed μ-calpain forms and PKCα hydrolysis which leads to the formation of PKMα; (2) in certain experimental conditions, autolyzed μ-calpain forms are able to hydrolyze PKMα independently of the presence of diacylglycerol.

Two muscle thiol proteinases causing partial degradation of myofibrillar constituents were isolated and purified from skeletal muscle. The two proteinases that differ significantly in calcium requirements were designated respectively high- and low-Ca2+-requiring proteinase. Both are inhibited, in vitro, by a specific inhibitor which is a protein also isolated from skeletal muscle. Experiments using carboxymethylated monomeric proteinases and inhibitor-conjugated Sepharose were carried out in order to understand the mechanism of control of the proteinases by the inhibitor. The results using increasing inhibitor concentrations show a non-competitive inhibition for both enzymes. The Ki value for the low-Ca2+-requiring form was 0.3 microM, while the Ki value for the high-Ca2+-requiring form was 0.9 microM. Likewise, the low-Ca2+-requiring form needs about 3-fold more inhibitor than the high-Ca2+-requiring form for the same per cent inhibition.