Properties of an ATP-dependent Ca2+ pump and (Ca2+ + Mg2+)-ATPase in brain synaptosome membrane vesicles from the bertha armyworm,Mamestra configurata Wlk |
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Authors: | Ma Luo Robert P Bodnaryk |
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Abstract: | Biochemical and kinetic properties under identical substrate and reaction conditions were obtained for an ATP-dependent Ca2+ pump and (Ca2+ + Mg2+)-ATPase in synaptosome membrane vesicles prepared from the brain of the moth, Mamestra configurata. Both the ATP-dependent Ca2+ pump and (Ca2+ + Mg2+)-ATPase had single, high-affinity binding sites for ATP (Km = 14 and 116 μM, respectively), Ca2+free (Km = 0.13 nM and 0.072 nM, respectively), and Mg2+ (Km = 1.1 mM and 0.07 mM, respectively). Both systems were relatively little affected by K+ and were insensitive to ouabain, an inhibitor of (Na+ + K+)-ATPase. The results indicate that the ATP-dependent Ca2+ pump and (Ca2+ + Mg2+)-ATPase are functionally coupled in synaptic membranes and constitute a mechanism for Ca2+ transport in the brain of M. configurata. Although moth brain (Ca2+ + Mg2+)-ATPase is maximally active at nanomolar concentrations of free calcium ion, the enzyme retains at least one-half of its maximal activity at micromolar calcium concentrations, indicating either that the enzyme has two binding sites for calcium (a high-affinity site at nanomolar Ca2+free and a low-affinity site at micromolar Ca2+free), or that there are two enzymes with high and low affinity for calcium, respectively. Calcium extrusion from brain neurones of M. configurata may operate in a two-stage, concentration-dependent process in which a first stage, low-affinity pump reduces intraneuronal calcium to a concentration at which a second stage, high-affinity pump becomes activated. |
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Keywords: | synaptosome brain calcium transport magnesium |
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