Characterization of ATPase Activity Associated with Plasma Membrane from Vigna Hypocotyls

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A plasma membrane fraction was isolated from the hypocotyls of cowpea (Vigna unguiculata) by a combination of differential centrifugation and sucrose density gradient centrifugation. The ATPase activity of this fraction was dependent on divalent cations (Mn^<2+> > Mg^<2+> > Co^<2+> > Ca^<2+> > Fe^<2+> > Zn^<2+> > Ni^<2+>) but was not further stimulated by monovalent cations (K^+ and/or Na^+). The pH optimum for the activation of ATPase by Mg^<2+> was 7.0. This fraction hydrolyzed ATP or UTP as a substrate and the ATPase activity obeyed a Michaelis-Menten type ofkinetics. The K_m for MgATP ranged from 0.65 to 1.1 mM. The ATPase activity was inhibited by inhibitors such as N,N'-dicyclohexylcarbodiimide, diethylstilbestrol and triphenyltin chloride, all of which are reported to block proton (H^+) transport in plant cells, but was insensitive to those of mitochondrial ATPase such as oligomycin and sodium azide. The ATPase activity was not stimulated by treatment with ionophores (e.g., carbonyl cyanide p-trifluoromethoxy-phenylhydrazone, 3,5-di-tert-butyl-4-hydroxybenzilidenemalononitrile and valinomycin +KCl) which would be expected to dissipate the electrochemical potential difference of H^+ or the membrane potential difference. The characteristics of the ATPase are compared with those of plasma membrane ATPases of other plants and its possible role in H^+-transport is discussed.
日本植物生理学会の論文