] made a thorough study of the e ect of pressure on G-and F-actins. However, all of the measurements in their study were made after the release of pressure. In the present experiment in situ observations were attempted by using eATP to obtain further detailed kinetic and thermodynamic information about the behaviour of actin under pressure. The dissociation rate constants of nucleotides from actin molecules (the decay curve of the intensity of¯uorescence of eATP-G-actin or eADP±F-actin) followed ®rst-order kinetics. The volume changes for the denaturation of G-actin and F-actin were estimated to be )72 mLámol )1 and )67 mLámol )1 in the presence of ATP, respectively. Changes in the intensity of¯uorescence of F-actin whilst under pressure suggested that eADP±F-actin was initially depolymerized to eADP±G-actin; subsequently there was quick exchange of the eADP for free eATP, and then polymerization occurred again with the liberation of phosphate from eATP bound to G-actin in the presence of excess ATP. In the higher pressure range (> 250 MPa), the partial collapse of the three-dimensional structure of actin, which had been depolymerized under pressure, proceeded immediately after release of the nucleotide, so that it lost the ability to exchange bound ADP with external free ATP and so was denatured irreversibly. An experiment monitoring eATP¯uorescence also demonstrated that, in the absence of Mg 2+ -ATP, the dissociation of 1 actin-heavy meromyosin (HMM) complex into actin and HMM did not occur under high pressure.

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