title: Does a calmodulin-dependent Ca2+-regulated Mg2+-dependent ATPase contribute to hepatic microsomal calcium uptake?
abstract: Solubilization of microsomal proteins followed by calmodulin affinity chromatography resulted in the separation of two distinct Ca2+-Mg2+-ATPases (Ca2+-regulated Mg2+-dependent ATPases), one being insensitive to calmodulin (ATPase-1), the other being stimulated about 5-fold by calmodulin (ATPase-2). ATPase-2 accounts for only 8% of total microsomal Ca2+-Mg2+-ATPase-activity. ATPase-1 and -2 can also be distinguished by different pH optima, different sensitivity towards inhibition by vanadate and LaCl3, and different apparent Mr values of the phosphoenzyme intermediates (115,000 and 150,000 for ATPase-1 and ATPase-2 respectively). ATPase-1 from liver co-migrated with Ca2+-Mg2+-ATPase from rat skeletal-muscle sarcoplasmic reticulum, whereas ATPase-2 from liver co-migrated with calmodulin-dependent Ca2+-Mg2+-ATPase derived from rat skeletal-muscle sarcolemma. After separation of parenchymal and nonparenchymal liver cells, a calmodulin-dependent Ca2+-Mg2+-ATPase of Mr 150,000 was found only in the non-parenchymal cells. The kinetic parameters of ATPase-2 and the similarity of the apparent Mr of its phosphoenzyme intermediate to that of skeletal-muscle sarcolemma Ca2+-Mg2+-ATPase makes it likely that the calmodulin-sensitive Ca2+-Mg2+-ATPase found in rat liver microsomal fractions reflects a contamination with plasma membranes (possibly from non-parenchymal cells) rather than a true location in the endoplasmic reticulum of parenchymal liver cells.
mesh_terms: D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase / Q000302:isolation & purification / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000302:isolation & purification / Q000378:metabolism*; D002147:Calmodulin / Q000494:pharmacology*; D002846:Chromatography, Affinity; D004591:Electrophoresis, Polyacrylamide Gel; D004789:Enzyme Activation / Q000187:drug effects; D007811:Lanthanum / Q000494:pharmacology; D008297:Male; D008862:Microsomes, Liver / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D014638:Vanadates / Q000494:pharmacology
chemical_list: D002147:Calmodulin; D014638:Vanadates; D007811:Lanthanum; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D002118:Calcium