Characterization of plasma membrane bound cation-stimulated ATPase isolated from the roots ofCucumis sativus

摘要

A membrane fraction enriched with Mg2+- or Mn2+-dependent, monovalent cation stimulated ATPase was isolated from cucumber roots (Cucumis sativusL.) by an aqueous two-polymer phase system of Dextran T500 (6.5, w/w) and polyethylene glycol (PEG) 3350 (6.5, w/w) at pH 7.8. The ATPase activity associated with the upper PEG-rich fraction (plasma membrane) was characterized. The optimum pH for the activation by Mg2+and Mn2+was in the range 5.8ndash;6.0. The activity was substrate specific for ATP. Kinetics with Mg2+or Mn2+followed a simple Michaelisndash;Menten relationship. The apparentKmfor Mg2+activation (0.60ensp;mM) of the ATPase was about twice that of the apparentKmfor Mn2+(0.38ensp;mM). ATPase was stimulated by monovalent cations and showed an order of cation preference of. Calcium inhibited the plasma membrane ATPase, apparently by a direct interaction with ATPase rather than by disrupting the MgATP2minus;complex.