Effect of temperature on the Ca2+ transport ATPase of sarcoplasmic reticulum.

The effect of temperature on sarcoplasmic reticulum vesicles isolated from rabbit skeletal muscle was studied by measuring the Ca2+ uptake, ATP and ITP hydrolysis, Ca2+ efflux, ATP synthesis, ATP Pi exchange reaction, and phosphorylation of the Ca 2+ transport enzyme by either ATP, ITP, or inorganic phosphate. At 0”, the enzyme is phosphorylated by ATP or ITP, but both Ca2+ uptake and P, liberation are impaired. The rate of phosphoenzyme hydrolysis increases progressively as the temperature is raised from 0” to 35”. When ITP is used as substrate, the increment of Pi liberation is accompanied by a decrease of the steady state level of phosphoenzyme. This is not observed when ATP is used as substrate. The phosphoenzyme formed from ATP is more sensitive to ADP at 0” than at 30”. Synthesis of ATP coupled to calcium efflux and ATP -+ Pi exchange reaction are inhibited at 0”. Both the phosphorylation of the enzyme by P, and the transfer of phosphate from the phosphoenzyme to ADP are impaired at 0”. The steady state level of enzyme phosphorylated by 32Pi increases linearly with temperature up to 20”. When the temperature is raised the fraction of enzyme phosphorylated by ITP decreases while the fraction of enzyme phosphorylated by Pi increases. The data presented support the concept of two different conformations of the Ca2+ transport enzyme in equilibrium and indicate that this equilibrium is modified by temperature.