Characterization of membrane-bound Mg^<++>-activated ATPase isolated from the lower epidermis of tobacco leaves

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Membrane-bound Mg^<++>-activated ATPase was separated from the lower epidermis of tobacco leaves (Nicotiana tabacum L. Samsun NN) on stepwise sucrose density gradient centrifugation. Membrane-bound epidermal ATPase was localized in the interface of densities in sucrose of 1.12 to 1.16 in the sedimentary fraction between 1,500×g to 10,000×g from the homogenate of the lower epidermis. The epidermal ATPase activity was activated by divalent cations (Mg^<++> > Mn^<++>≧Co^<++> > Fe^<++> > Zn^<++> > Ca^<++>) and further stimulated by KCl by ca. 20%. The pH optimum for Mg^<++>-activation of the epidermal ATPase was ca. 6.0. The enzyme hydrolyzed ATP more rapidly than other nucleoside triphosphates. The optimum temperature for activation of the epidermal ATPase activity was ca. 40℃. 50% of the epidermal ATPase activity was lost in 18 min at 55℃ and in 2.5 days at 2.5℃. The apparent Km value of the epidermal ATPase was 4.7×10^<-4> _M and Vmax was 65.4 nmoles Pi/mg protein/min. The epidermal ATPase was strongly inhibited by N,N'-dicyclohexylcarbodiimide (DCCD) in vitro whereas oligomycin, carbonyl cyanide m-chlorophenylhydrazone (CCCP), indoleacetic acid (IAA) and abscisic acid (ABA) were insensitive to the epidermal ATPase activity.
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