Kinetic studies of membrane (NA+-K+-Mg2+)-ATPase

Abstract Kinetic studies on a microsomal (Na + -K + -Mg 2+ )-ATPase (ATP phosphohydrolase EC 3.6.1.3) from rat brain are reported. The results indicate that MgATP is the real substrate of the reaction and ATP is a weak competitive inhibitor. Kinetic studies in which MgATP is varied at several Na + and K + concentrations reveal that the order of addition of cations and substrate to the enzyme is random. Product inhibition experiments suggest that the release of products is ordered, with P i being released before ADP. A double reciprocal plot of 1/v vs 1/s[S] at temperatures ranging from 7–37° produced a linear Arrhenius plot with an energy of activation of 23150 cal/mole. Studies of the effect of pH on the kinetic constants show that two ionizing groups with p K values of 7.1 and 7.6 control the catalysis by (Na + -K + -Mg 2+ )-ATPase.