Ca2+ translocation and catalytic activity of the sarcoplasmic reticulum ATPase. Modulation by ATP, Ca2+, and Pi.
The ratio between Ca2+ uptake and Ca(2+)-dependent ATP hydrolysis measured in sarcoplasmic reticulum vesicles of rabbit skeletal muscle was found to vary greatly depending on the concentrations of oxalate or Pi used. In the presence of 5 mM oxalate, 20 mM Pi, and 1 mM Pi, the ratios found were in the range of 1.4-2.3, 0.6-0.8, and 0.01-0.10, respectively. The rates of Ca2+ exchange and ATP synthesis were measured at the steady state by adding trace amounts of 45Ca and 32Pi, after the vesicles had been loaded with Ca2+. In the presence of 1 mM Pi, 10 mM MgCl2, and 0.2 mM CaCl2, the ratio between Ca2+ exchange and ATP synthesis varied from 9 to 14. This ratio approached two when Ca2+ in the medium was reduced to a very low level, or when in the presence of Ca2+, dimethyl sulfoxide was added to the assay medium, or when the Pi concentration was raised from 1 to 20 mM. A ratio of two was also measured when the steady state was attained using ITP instead of ATP. In all the conditions that led to a ratio close to two, there was an increase in the fraction of enzyme phosphorylated by Pi. It is proposed that the coupling between Ca2+ translocation and ATP hydrolysis or synthesis is modulated by the phosphorylation of the ATPase by Pi.
Galina
A
A
Departamento de Bioquimica, Universidade Federal do Rio de Janeiro, Brasil.
de Meis
L
L
D000251:Adenosine Triphosphatases / Q000378:metabolism*; D000255:Adenosine Triphosphate / Q000096:biosynthesis / Q000378:metabolism*; D000818:Animals; D001692:Biological Transport; D002118:Calcium / Q000378:metabolism*; D002384:Catalysis; D006868:Hydrolysis; D010758:Phosphorus / Q000378:metabolism*; D010766:Phosphorylation; D011817:Rabbits; D012519:Sarcoplasmic Reticulum / Q000201:enzymology*
D010758:Phosphorus; D000255:Adenosine Triphosphate; D000251:Adenosine Triphosphatases; D002118:Calcium