Characterization of membrane‐bound MgATPases, purified by aqueous two‐phase partitioning, from young sugar beet roots

Membrane‐bound MgATPase activity from roots of young sugar beet (Beta vulgaris L. cv. Monohill) was investigated in a membrane fraction purified by partition in an aqueous polymer two‐phase system. After two steps of “washing” with fresh bottom phase (rich in dextran), the polyethylene glycol rich top phase (U3) was practically free of mitochondrial membranes (cytochrome oxidase), and the remaining MgATPase activity showed high substrate specificity for ATP. An optimum for the MgATPase activity was found at pH 7. The activation by Na+ or K+ was strongest on the acid side without any observable shift in pH optimum. Oligomycin had no effect, but vanadate strongly inhibited the U3 MgATPase and the K+ activation was lost. The complex activation pattern achieved by varying the Na+/K+ ratio at constant total concentration was interpreted as a synergistic (Na++ K+)‐activation. The U3 fraction MgATP‐ase activity showed a 4‐fold increase in the presence of 0.01% Triton X‐100 implying that the MgATPase activity is located in vesicles of which 75% or more are sealed with the ATP binding site on the inside. Comparison with the properties of plasma membrane. ATPases from other plants indicated that the U3 fraction MgATPase was mainly of plasma membrane origin.