An ATPase was extracted and purified from castor bean endosperm mitochondria. The enzyme is stable at 60°C only in the presence of ATP in the incubation medium. It is less stable at 0°C than at 30°C but is stabilized by ammonium sulfate or glycerol. Activity is dependent on the presence of Mg++, and in the presence of Mg++is enhanced by 2,4-dinitrophenol, but is not inhibited by oligomycin. The enzyme hydrolyzes ITP in addition to ATP, but ITPase activity is hardly enhanced by 2,4-dinitrophenol. This preparation has many properties in common with the ATPase (coupling factor 1) from beef heart mitochondr
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