title: Coupling of ATP synthesis to reversal of rat liver microsomal Ca2+-ATPase.
abstract: The reversal of the rat liver microsomal Ca2+-ATPase transport cycle was studied. Microsomes were loaded with 45Ca2+ (approximately 30 nmol/mg of protein) in an ATP-dependent process, and the time dependency of the microsomal 45Ca2+ efflux was determined with various ADP and inorganic phosphate (Pi) concentrations. Pseudo-first-order rate constants (K'e) for 45Ca2+ efflux were determined. Although there was considerable 45Ca2+ efflux in the absence of added ADP or Pi, the addition of ADP or Pi alone had minimal effects upon the K'e; in contrast, a 2.5-fold increase in the K'e was observed in the presence of both ADP and Pi. The apparent Km values for ADP and Pi were 4 microM and 0.22 mM, respectively. Stimulation of 45Ca2+ efflux by ADP and Pi was associated with ATP synthesis. The calcium ionophore A23187 prevented ATP synthesis, which indicates that the Ca2+ gradient facilitates the coupling of ATP synthesis to Ca2+ efflux.
mesh_terms: D000244:Adenosine Diphosphate / Q000378:metabolism; D000255:Adenosine Triphosphate / Q000096:biosynthesis*; D000818:Animals; D001693:Biological Transport, Active; D002118:Calcium / Q000378:metabolism; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D008862:Microsomes, Liver / Q000378:metabolism*; D010710:Phosphates / Q000378:metabolism; D051381:Rats
chemical_list: D010710:Phosphates; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D000252:Calcium-Transporting ATPases; D002118:Calcium