| Summary: | The endogenous Mg<sup>2+ </sup>-ATPase activity of isolated intact turnip and mung bean mitochondria is very low relative to that of animal mitochondria, and is not stimulated by uncouplers. Results indicate that the endogenous ATPase activity is not low due to either the existence of a permeability barrier to ATP, or to a reduced content of the ATPase enzyme in turnip mitochondria. The absence of uncoupler-stimulation with mung bean mitochondria is partly due to the transport and/or permeability characteristics of the membrane. However, in both cases, an (additional) 'ATPase-inhibitory' factor is implicated. The ATPase activity of turnip mitochondria and membrane particles can be 'activated' (up to 50-fold) kinetically, and in a time-dependent manner, evidence which points to the existence of a proteinaceous ATPase-inhibitor, like that in mammalian mitochondria, which is responsible for the low ATPase activity and lack of uncoupler-stimulation in turnip mitochondria. However, alternate inhibitory ligands cannot be dismissed at present. The 'activated' ATPase activity is similar to that of the F<sub>O</sub>F<sub>1</sub>-ATPase with respect to inhibitor sensitivity, optimum pH, bivalent cation requirement, and sensitivity to 'activating anions'. The F<sub>1</sub> sector has been solubilized from turnip mitochondria by dichloromethane-extraction, and further purified to yield a preparation containing the five polypeptides (α, β, γ, δ, ε) characteristic of MF_1, BF_1 and CF_1, plus an additional sixth polypeptide (δ') which has been found in certain plant (and animal) MF<sub>1</sub> preparations, but whose identity has yet to be determined. The M<sub>r</sub> values of the δ and ε polypeptides are closer to those of BF<sub>1</sub> and CF<sub>1</sub> than MF<sub>1</sub>. The specific activity of the purified enzyme is higher than previously reported for a plant F<sub>1</sub>-ATPase. Other notable features include apparent negative cooperativity for ATP hydrolysis, even with 'activating anions' present, reduced specificity for adenine nucleotides, high Ca<sup>2+ </sup>-ATPase activity, and slight stimulation by the chloride anion.
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