A Comparison Between the ATPase and Proton Pumping Activities of Plasma Membranes Isolated from the Stele and Cortex ofZea maysRoots

Abstract

Plasma membrane vesicles of high purity, determined by marker enzyme assays, were obtained by phase partitioning microsomal fractions from stelar and cortical tissues of Zea mays (cv. LG11) roots. ATP hydrolytic activities in both of the plasma membrane fractions were inhibited by vanadate, SW26 and erythrosin B, but were insensitive to nitrate. Activity in both fractions exhibited a marked pH optimum of 6·5 and displayed typical Michaelis-Menten kinetics. A high substrate specificity was apparent in both the stele and cortex plasma membrane fractions, while the lower fractions, after phase partitioning, showed lower specificity for nucleotide substrates. Specific activities of the stele (67·8 μmol Pi mg⁻¹ h⁻¹) and cortex (78·4 μmol Pi mg⁻¹ h−¹) plasma membrane H⁺ -ATPases were very similar. Proton pumping activities in microsomal membrane fractions from stele and cortex were inhibited by nitrate and insensitive to vanadate. Homogenization of stele and cortex tissue in the presence of 250 mol m⁻³ KI resulted in microsomal fractions exhibiting vanadate-sensitive, nitrate-insensitive proton pumping activity, suggesting a plasma membrane origin for this activity. SW26 was also an effective inhibitor of proton pumping activity, although results indicated an interaction between SW26 and the fluorescent probes quinacrine and acridine orange. The results are discussed in relation to models for the transport of ions into the stele and are consistent with a role for the H⁺ -ATPase activity in this process.