The Membrane-Bound Forms of the Mitochondrial ATPase of Turnip (Brassica napusL.) and Mung Bean (Phaseolus aureus): Characterization and Investigation of Factors Controlling Activity

Abstract

Isolated intact plant mitochondria, including those from turnip and mung bean, show low endogenous Mg2 + -ATPase activity and, unlike mammalian mitochondria, lack significant uncoupler-stimulated ATPase activity. In contrast, the rates of respiration driven ATP synthesis are comparable to those in mammalian mitochondria, suggesting the presence of an ATPase inhibitor. Disruption of intact turnip mitochondria only results in limited increases in ATPase activity, indicating that a permeability barrier to ATP transport is not primarily responsible for the low endogenous activity. The ATPase activity of turnip mitochondria and membrane particles can be increased up to 50-fold when assayed under optimum conditions. Time-dependent increases in activity induced by ageing, exposure to salts and trypsin treatment, are all consistent with an inhibitor protein being responsible for the low endogenous activity and lack of uncoupler-stimulation. The ATPase activity of particles under optimum conditions and after ageing is sufficient to account for the rates of ATP synthesis. After activation, turnip mitochondrial ATPase activity is similar to the mammalian enzyme in inhibitor sensitivity, pH optimum, bivalent cation requirement, and sensitivity to 'activating anions'. In mung bean mitochondria, a permeability barrier to ATP is only partly responsible for the low endogenous ATPase activity, together with the inhibitory factor. On the basis of variation in the relative Ca2 + - and Mg2+-ATPase activities after various treatments, a Ca2 +-regulatory site which affects ATPase activity is proposed to exist in the FrATPase complex.