Factors influencing calcium release from the ADP-sensitive phosphoenzyme intermediate of the sarcoplasmic reticulum ATPase.

Calcium release from the ADP-sensitive phosphoenzyme intermediate of the sarcoplasmic reticulum ATPase was investigated at 6 degrees C under a variety of conditions using the purified ATPase protein and the rapid membrane filtration system. The rate of calcium release measured in the presence of [ethylene bis-(oxyethylenenitrilo)]tetraacetic acid increased monotonically with increasing pH of the medium, the time at which 50% of the bound calcium was released being reduced to one third when the pH was raised from 5.5 to 9.0. Dimethyl sulfoxide at 10 or 20% (v/v) also was very effective in accelerating the calcium release. ATP at a millimolar concentration range also was stimulatory, but millimolar concentrations of Mg2+ were found to be inhibitory. Using an indirect method, i.e. by measuring the overall rate of calcium transport by the reconstituted vesicles under conditions where calcium release from the ADP-sensitive phosphoenzyme was presumably rate-limiting, the calcium release was shown to be accelerated up to 1.5-fold by the inside-negative potential imposed across the membrane using the K+-valinomycin system. As evidence was presented suggesting that the observed calcium release primarily reflects the phosphoenzyme isomerization which leads to reduction in calcium affinity of the phosphoenzyme, the results strongly suggest that this phosphoenzyme isomerization was affected significantly by each of the factors described above.