Hydrostatic pressure and calcium-induced dissociation of calpains

The dissociation of mu- and m-calpains was studied by fluorescence spectros copy under high hydrostatic pressure (up to 650 MPa). Increasing pressure i nduced a red shift of the tryptophan fluorescence of the calcium-free enzym e. The concentration dependence of the spectral transition was consistent w ith a pressure-induced dissociation of the subunits. Rising temperature inc reased the stability of calpain heterodimers and confirmed the predominance of hydrophobic interactions between monomers. At saturating calcium, the s pectral transition was not observed for native or iodoacetamide-inactivated calpains, indicating that they were already dissociated by calcium. The re action volume was about -150 ml mol(-1) for both isoforms, and the dissocia tion 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. (C) 1999 Elsevier Science B.V. All rights reserved.