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Kinetic Relationships between Phosphate-Oxygen Exchange and Hydrolytic Cleavage of Adenosine Triphosphate Catalyzed by Myofibrils

From the ¹ From the Departments of Biochemistry and of Laboratory Medicine, University of Minnesota Medical School, Minneapolis, Minnesota 55455

We studied the time course of ¹⁸O exchange between water and inorganic orthophosphate catalyzed by rat skeletal myofibrils during the hydrolysis of adenosine triphosphate. A lag period of 3 to 4 min occurred, during which P_i liberated from ATP contained only the amount of ¹⁸O expected from the hydrolytic reaction. Following this lag period, an exchange of ¹⁸O between water and P_i was noted, the rate of this exchange paralleled the rate of the hydrolysis of ATP. The lag period was eliminated in the presence of 1,2-bis(2-dicarboxy methylaminoethoxy)ethane. An exchange of ¹⁸O between water and P_i of the medium was detectable in the absence of ATP and, during ATP hydrolysis, at high concentrations of medium P_i (above 30 mm). The predominant exchange reaction observed is one occurring between water and an intermediate in the hydrolysis of ATP by myofibrils. The lag in ¹⁸O exchange may be related to the formation of an activated myosin-phosphate intermediate.

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