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Characterization of Calcium‐Activated and Magnesium‐Activated ATPases of Brain Nerve Endings
Author(s) -
Lin Shihchia C.,
Way E. Leong
Publication year - 1984
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1984.tb12761.x
Subject(s) - synaptic vesicle , atpase , ouabain , vesicle , chemistry , synaptosome , magnesium , gtp' , biophysics , enzyme , free nerve ending , calcium , biochemistry , atp hydrolysis , stimulation , membrane , sodium , biology , endocrinology , organic chemistry
The properties of Ca 2 + ‐activated and Mg 2 + ‐activated ATPases of nerve endings from mouse brain were investigated. Ca 2 + and Mg 2 + each can activate ATP hydrolysis in synaptosomes and its subfractions. Both Ca 2 + ‐ATPase and Mg 2 + ‐ATPase exhibit high and low affinity for their respective cations. At millimolar concentrations of Ca 2 + or Mg 2 + , several nucleoside triphosphates could serve as substrate for the two enzymes and their specific activities were about three to four times higher in synaptic vesicles than in synaptosomal plasma membranes (SPM). Both in SPM and in synaptic vesicles the relative activity in the presence of Ca 2 + was in the order of CTP > UTP > GTP = ATP, but with Mg 2 + the activity was higher with ATP than with the other three triphosphates. Mg 2 + ‐ATPase was more active than Ca 2 + ‐ATPase in SPM, but in synaptic vesicles the two enzymes exhibited similar activity. Kinetic studies revealed that Mg 2 + ‐ATPase was inhibited by excess ATP and not by excess Mg 2 + . The simultaneous presence of Na + + K + stimulated Mg 2 + ‐ATPase and inhibited Ca 2 + ‐ATPase activity in intact synaptosomes and SPM. The stimulation of Mg 2 + ‐ATPase by Na 2+ + K + was further increased by increasing Mg 2 + concentration and was inhibited by Ca 2 + and by ouabain. When Ca 2 + and Mg 2 + are present together in SPM or synaptic vesicles, the total P i liberated by the two cations may either increase or decrease, depending on their relative concentrations. Kinetic analyses indicate that Ca 2 + and Mg 2 + bind independently to the enzyme alone or together at different sites. The results suggest that Ca 2 + ‐ATPase and Mg 2 + ‐ATPase in SPM or synaptic vesicles may be separate and distinct systems.