Sucrose and ouabain effects on the kinetic properties of a membrane‐bound (Na++ K++ Mg2+) ATPase in sugar beet roots

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Abstract
Kinetic studies of a microsomal (Na++ K++ Mg2+)ATPase from sugar beet roots (Beta vulgaris L. cv. Monohill) show that sucrose influences the MgATPase in different ways depending on the presence of K+ and/or Na+ 1) In the presence of the substrate MgATP and Na+ the effect of sucrose follows simple Michaelis‐Menten kinetics. 2) In the presence of substrate together with K+ or (K++ Na+), sucrose has little effect on the ATPase activity. 3) In the presence of Na+, onabain acts as an uncompetitive inhibitor with respect to MgATP. 4) In the presence of K+ or (K++ Na+), the inhibition by ouabain is somewhat depressed and shows non‐linearity when 1/v is plotted versus 1/MgATP. 5) Sucrose and Na+ activate in a competitive way, so that a successive increase of the Na+ level decreases the activation by sucrose. Both Km and V‐values are thereby changed. 6) The sucrose activation in the presence of Na+ is also influenced by ouabain. It is, therefore, suggested that Na+ may regulate the interference between the Na+/K+ pump and a sucrose sensitive system.
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