Biochemical characterization of the hippocampal and striatal Na,K-ATPase reveals striking differences in kinetic properties
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The activities and basic enzymatic properties of Na,K-ATPase were examined in synaptosomal plasma membranes (SPM) prepared from rat hippocampus and striatum. A kinetic analysis showed profound differences in apparent affinities for ATP (K-m) between hippocampal (1.21 mmol/l) and striatal (0.76 mmol/l) enzyme preparations, as well as in the corresponding V-max values. However, physiological efficiencies were almost the same. The complex pattern of dose-response curves to ouabain indicated the presence of two high-affinity forms of Na,K-ATPase in the striatum (very high-:K-i = 3.73 x 10(-8) mol/l and high-:K-i = 4.21 x 10(-5) mol/l), and one high affinity form in the hippocampus (K-i = 6.6 x 10(-7) mol/l). In addition, both SPM preparations contained one low affinity form with similar K-i. The very high-affinity form had positive cooperativity for ouabain inhibition of Na,K-ATPase activity, in contrast to high- and low-affinity forms, which exhibited negative cooperativity. The respectiv...e contributions of ouabain-sensitive forms to the total activity were estimated as 22%, 46%, 19% for the striatum and 36%, 45% for the hippocampus. These data clearly demonstrate striking differences in kinetic properties of the hippocampal and striatal Na,K-ATPase that may be due to the isoenzyme diversity and adaptation to specific physiological demands of the examined rat brain regions.