title: Uncoupled ATPase activity and heat production by the sarcoplasmic reticulum Ca2+-ATPase. Regulation by ADP.
abstract: Sarcoplasmic reticulum vesicles of rabbit skeletal muscle accumulate Ca2+ at the expense of ATP hydrolysis. The heat released during the hydrolysis of each ATP molecule varies depending on whether or not a Ca2+ gradient is formed across the vesicle membrane. After Ca2+ accumulation, a part of the Ca2+-ATPase activity is not coupled with Ca2+ transport (Yu, X., and Inesi, G. (1995) J. Biol. Chem. 270, 4361-4367). I now show that both the heat produced during substrate hydrolysis and the uncoupled ATPase activity vary depending on the ADP/ATP ratio in the medium. With a low ratio, the Ca2+ transport is exothermic, and the formation of the gradient increases the amount of heat produced during the hydrolysis of each ATP molecule cleaved. With a high ADP/ATP ratio, the Ca2+ transport is endothermic, and formation of a gradient increased the amount of heat absorbed from the medium. Heat is absorbed from the medium when the Ca2+ efflux is coupled with the synthesis of ATP (5.7 kcal/mol of ATP). When there is no ATP synthesis, the Ca2+ efflux is exothermic (14-16 kcal/Ca2+ mol). It is concluded that in the presence of a low ADP concentration the uncoupled ATPase activity is the dominant route of heat production. With a high ADP/ATP ratio, the uncoupled ATPase activity is abolished, and the Ca2+ transport is endothermic. The possible correlation of these findings with thermogenesis and anoxia is discussed.
authors.affiliation: Instituto de Ciências Biomédicas, Departamento de Bioquimica Médica, Universidade Federal do Rio de Janeiro, Cidade Universitária, RJ 21941-590, Brazil. demeis@bioqmed.ufjr.br
authors.forename: L
authors.initials: L
authors.lastname: de Meis
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000096:biosynthesis / Q000378:metabolism; D000818:Animals; D001833:Body Temperature Regulation; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002384:Catalysis; D006358:Hot Temperature; D008956:Models, Chemical; D018482:Muscle, Skeletal / Q000201:enzymology; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*
chemical_list: D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases; D002118:Calcium