Kinetic Studies of a (Na⁺+ K⁺+ Mg²⁺) ATPase in Sugar Beet Roots

Kinetic studies of a dithiothreitol treated membrane ATPase fraction from sugar beet roots led to the following conclusions: 1) In the presence of MgATP, Na + and K + stimulate the ATPase activity in different ways following simple Michaelis‐Menten kinetics. Thus separate sites for Na + and K + are suggested. 2) In the absence of K + , Na + acts as an uncompetitive modifier raising the apparent K m and V max for MgATP. 3) In the absence of Na + , K + activates non‐competitively with respect to MgATP. Thus K + increases V max but does not affect the apparent affinity constant. 4) K + and Na + double the rate constants. 5) In the presence of Na + or K + , Mg 2+ in excess acts as a weak inhibitor to Na + and/or K + activity. 6) The temperature‐activity dependence in the 5–40°C interval shows biphasic Arrhenius plots with the transition point between 15–18°C. The activation energy is lowered at temperatures > 18°C.