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Lumenal Ca 2+ Dissociation from the Phosphorylated Ca 2+ -ATPase of the Sarcoplasmic Reticulum Is Sequential *

Journal of Biological Chemistry, ISSN: 0021-9258, Vol: 270, Issue: 31, Page: 18271-18276
1995
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Once two radioactive Ca 2+ coming from the cytoplasm are bound to the transport sites of the nonphosphorylated ATPase, excess EGTA induces rapid dissociation of both ions, whereas excess nonradioactive Ca 2+ only reaches one of the two bound Ca 2+. This difference has been explained assuming that the two Ca 2+ sites are in a single file channel in which the superficial Ca 2+ is freely exchangeable from the cytoplasm, whereas the deeper Ca 2+ is exchangeable only when the superficial site is vacant. The same experiment was done using phosphorylated ATPase to determine whether Ca 2+ dissociation toward the lumen is sequential as well. Under conditions that allow ADP-sensitive phosphoenzyme to accumulate (leaky vesicles, 5°C, pH 8, 300 mM KCl), we found the same two pools of Ca 2+. Excess EGTA induced dissociation of both ions together with dephosphorylation. Excess nonradioactive Ca 2+ induced the exchange of half the radioactive Ca 2+ without any effect on the phosphoenzyme level. Our results show a close similarity between the transport sites of the nonphosphorylated and the phosphorylated enzymes, although the orientation, affinities, and dissociation rate constants are different.

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