The hydrolysis of Mg2+-adenosine 5'-triphosphate (ATP) by heavy meromyosin has been studied between +20 and -15 degrees C, especially in the low-temperature range, in a medium containing 30% (v/v) ethylene glycol by fluorometric, spectrophotometric, and potentiometric measurements. The time course of the fluorescence changes of the enzyme during the reaction depends markedly on the temperature in consequence of large differences between the activation energies of the various steps. The observed kinetics have been analyzed according to the simplified scheme of Bagshaw & Trentham [Bagshaw, C. R., & Trentham, D. R. (1974) Biochem. J. 141, 331-349]. The following results have been obtained. (1) The rate-limiting step of the reaction changes in this temperature range; at 20 degrees C M**.ADP.Pi is the predominant steady-state complex, and M*.ADP predominates at -15 degrees C, with a half-life of approximately 10 min. (2) As expected, on the basis that it is the dissociation of the M*.ADP complex which becomes rate limiting at low temperature, one observes, in the pre-steady-state below 0 degrees C, both a proton burst and a lag phase in ADP release. (3) At low temperature, the equilibrium M*.ATP in equilibrium M**.ADP.Pi is displaced to the left All the kinetic data obtained in this study are compatible with a simple pathway for the Mg2+-ATP hydrolysis by myosin and with sequential release of the reaction products.

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