Uncoupled ATP hydrolysis and thermogenic activity of the sarcoplasmic reticulum Ca2+-ATPase: coupling effects of dimethyl sulfoxide and low temperature.
The sarcoplasmic reticulum Ca(2+)-ATPase transports Ca(2+) using the energy derived from ATP hydrolysis. During catalysis, part of the energy is used to translocate Ca(2+) across the membrane, and part is dissipated as heat. At 35 degrees C the heat released during the hydrolysis of each ATP molecule varies depending on the formation of a Ca(2+) 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 Ca(2+) accumulation, 82% of the Ca(2+)-ATPase activity is not coupled to Ca(2+) transport, and the ratio between Ca(2+) 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 degrees C 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 Ca(2+)/ATP ratio from 0.3 to 1.3-1.4. It is concluded that the uncoupled Ca(2+)-ATPase is responsible for both the low Ca(2+)/ATP ratio measured during transport and the difference of heat produced during ATP hydrolysis in the presence and absence of a gradient.
Barata
Hosana
H
Departamento de Bioquimica Médica, Instituto de Ciências Biomédicas, Centro de Ciências da Saúde, Universidade Federal do Rio de Janeiro, Cidade Universitária, Rio de Janeiro 21941 590, Brazil.
de Meis
Leopoldo
L
D000251:Adenosine Triphosphatases / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000737:chemistry* / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D006868:Hydrolysis; D008956:Models, Chemical; D018482:Muscle, Skeletal / Q000201:enzymology; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*; D013696:Temperature; D013816:Thermodynamics; D013997:Time Factors
D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases; D000252:Calcium-Transporting ATPases; D002118:Calcium