Presence of Two Different Membrane-bound, KCl-stimulated Adenosine Triphosphatase Activities in Maize Roots

Recent publications have indicated that a KCl-stimulated ATPase from cereal roots is specifically associated with plasmalemma-enriched membrane fractions. However, in previous work we found that relatively high specific activities of this enzyme were also associated with a membrane fraction which did not contain plasmalemma. In an attempt to clarify this discrepancy, we have investigated the effect of density gradient composition upon the association of the enzyme with different membrane fractions isolated from the roots of Zea mays L. (WF9 x M14).When roots were fractionated on a single, discontinuous sucrose density gradient the KCl-stimulated ATPase activity was concentrated in membrane fractions of relatively high density (sigma = 1.17-1.22) which were enriched in plasmalemma. When fractions were isolated on both Ficoll and sucrose density gradients, KCl-stimulated ATPase activity was again found associated with plasmalemma and also in a second membrane fraction which did not contain plasmalemma. This fraction was found at relatively low densities (sigma = 1.08) on the Ficoll gradient. The precise identity of the membrane in this fraction could not be determined.A preliminary investigation into the properties of the two KCl-stimulated ATPase activities indicated that both had acid pH maxima and both displayed similar responses to changes in KCl concentration.