title: Relationship between calcium ion transport and (Ca2+ + Mg2+)-atpase activity in adipocyte endoplasmic reticulum.
abstract: Calcium uptake by adipocyte endoplasmic reticulum was studied in a rapidly obtained microsomal fraction. The kinetics and ionic requirements of Ca2+ transport in this preparation were characterized and compared to those of (Ca2+ + Mg2+)-ATPase activity. The time course of Ca2+ uptake in the presence of 5 mM oxalate was nonlinear, approaching a steady-state level of 10.8--11.5 nmol Ca2+/mg protein after 3--4 min of incubation. The rate of Ca2+ transport was iM oxalate. The calculated initial rate of calcium uptake was 18.5 nmol Ca2+/mg protein per min. The double reciprocal plot of ATP concentration against transport rate was nonlinear, with apparent Km values of 100 muM and 7 muM for ATP concentration ranges above and below 50 muM, respectively. The apparent Km values for Mg2+ and Ca2+ were 132 muM and 0.36--0.67 muM, respectively. The energy of activation was 23.4 kcal/mol. These kinetic properties were strikingly similar to those of the microsomal (Ca2+ + Mg2+)-ATPase. The presence of potassium was required for maximum Ca2+ transport activity. The order of effectiveness of monovalent cations in stimulating both Ca2+ transport and (Ca2+ + Mg2+)-ATPase activity was K+ greater than Na+ = NH4+ greater than Li+. Ca2+ transport and (Ca2+ + Mg2+)-ATPase activity were both inhibited 10--20% by 6 mM procaine and less than 10% by 10 mM sodium azide. Both processes were completely inhibited by 3 mM dibucaine or 50 muM p-chloromercuribenzene sulfonate. The results indicate that Ca2+ transport in adipocyte endoplasmic reticulum is mediated by a (Ca2+ + Mg2+)-ATPase and suggest an important role for endoplasmic reticulum in control of intracellular Ca2+ distribution.
mesh_terms: D000273:Adipose Tissue / Q000378:metabolism*; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects; D017301:Ca(2+) Mg(2+)-ATPase; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000378:metabolism*; D002414:Cations, Monovalent; D004721:Endoplasmic Reticulum / Q000187:drug effects / Q000378:metabolism*; D007700:Kinetics; D008274:Magnesium / Q000494:pharmacology; D008297:Male; D010070:Oxalates / Q000494:pharmacology; D051381:Rats
chemical_list: D002414:Cations, Monovalent; D010070:Oxalates; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D008274:Magnesium; D002118:Calcium