Ca(2+) transport of Ca(2+)-ATPase of the sarcoplasmic reticulum in an ADP-sensitive phosphoenzyme state.
To understand the energetics of Ca(2+)-transporting adenosine triphosphatase (Ca(2+)-ATPase), it is important to determine the energy consumption step. To do this, we measured the dissociation of Ca(2+) from Ca(2+)-ATPase into Ca(2+)-loaded vesicles. We observed that 45Ca(2+) added to the outside of the vesicles accumulated in the 40Ca(2+)-loaded vesicles after the addition of ADP and ATP. The acceleration of 45Ca(2+) accumulation increased by 1.4-fold after the addition of 1 microM ADP. Under the same conditions, Ca(2+)-dependent phosphate liberation was not observed, and all of the active phosphoenzymes were in the ADP-sensitive phosphoenzyme (E(1)P) state. These results indicated that the ADP stimulated 45Ca(2+) accumulation by the ADP-ATP exchange reaction and that this ADP-ATP exchange reaction did not pass through the ADP-insensitive phosphoenzyme state. Therefore, we demonstrate that one Ca(2+) ion dissociates at the E(1)P state, which does not correspond with the phosphoenzyme conversion, that is the energy consumption step in the E(1)-E(2) model.
Ushimaru
Makoto
M
Department of Chemistry, Kyorin University School of Medicine, Shinkawa, Mitaka, Tokyo, Japan. ushimaru@kyorin-u.ac.jp
Fukushima
Yoshihiro
Y
D000244:Adenosine Diphosphate / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D004734:Energy Metabolism; D066298:In Vitro Techniques; D017136:Ion Transport; D007700:Kinetics; D008954:Models, Biological; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*
D000244:Adenosine Diphosphate; D000252:Calcium-Transporting ATPases; D002118:Calcium