Kinetic Studies of a (Na++ K++ Mg2+) ATPase in Sugar Beet Roots |
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Authors: | SYLVIA LINDBERG |
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Affiliation: | Botanical Institute, University of Stockholm, S-10405 Stockholm, Sweden |
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Abstract: | 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 Km and Vmax for MgATP. 3) In the absence of Na+, K+ activates non-competitively with respect to MgATP. Thus K+ increases Vmax but does not affect the apparent affinity constant. 4) K+ and Na+ double the rate constants. 5) In the presence of Na+ or K+, Mg2+ 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. |
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