Uncoupled ATP hydrolysis and thermogenic activity of the sarcoplasmic reticulum Ca2+-ATPase -: Coupling effects of dimethyl sulfoxide and low temperature

The sarcoplasmic reticulum. Ca2+-ATPase transports Ca2+ using the energy derived from ATP hydrolysis. During catalysis, part of the energy is used to translocate Ca2+ across the membrane, and part is dissipated as heat. At 35 degreesC the heat released during the hydrolysis of each ATP molecule varies depending on the formation of a Ca2+ gradient across the membrane. With leaky vesicles (no gradient) the heat released varies between 9 and 12 kcal/mol of ATP cleaved, and with intact vesicles (gradient), the heat released increases to 20-24 kcal/mol of ATP. After Ca2+ accumulation, 82% of the Ca2+-ATPase activity is not coupled to Ca2+ transport, and the ratio between Ca2+ transported and ATP cleaved is 0.3. The addition of 20% dimethyl sulfoxide (v/v) to the medium or decreasing the temperature from 35 to 20 degreesC abolishes the difference of heat produced during ATP hydrolysis in the presence and absence of a gradient. This is accompanied by a simultaneous inhibition of the uncoupled ATPase activity and an increase of the Ca2+/ATP ratio from 0.3 to 1.3-1.4. It is concluded that the uncoupled Ca2+-ATPase is responsible for both the low Ca2+/ATP ratio measured during transport and the difference of heat produced during ATP hydrolysis in the presence and absence of a gradient.