Biochemical Characterization of the Hippocampal and Striatal Na,K-ATPase Reveales Striking Differences in Kinetic Properties

Abstract

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 × 10 -8 mol/l and "high-": K i = 4.21 × 10 -5 mol/l), and one high affinity form in the hippocampus (K i = 6.6 × 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 respective 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.