pH and temperature resolve the kinetics of two pools of calcium bound to the sarcoplasmic reticulum Ca2+ -ATPase

Calcium bound to the sarcoplasmic reticulum Ca2+ -ATPase was removed by chelating free calcium ion with EGTA. The kinetic calcium binding reaction to the calcium-unbound ATPase was studied by varying the pH (6.0-8.8) and temperature (0-20 degrees C) at a saturating concentration of 50-100 microM [Ca2+]. At pH 6.0 and 0 degrees C, calcium sites of the enzyme at a rate of t1/2 approximately 10 s. By increasing the pH from 6.0 to 8.8, about half of the total calcium sites were converted from a slow binding state to a rapid binding state (less than 2s). The maximum level was reached at about pH 7.4, and the midpoint of the conversion was observed at about pH 6.7. On the other hand, the slow binding reaction to the other sites was not significantly affected by the pH increase. At pH 7.0 and 20 degrees C, about 90% of the total calcium sites rapidly (less than 2s) bound calcium. The present results suggest that pH and temperature resolve the kinetics of two pools of calcium bound to the Ca2+-ATPase.