Ca2+-ATPase activity in pancreatic acinar plasma membranes. Regulation by calmodulin and acidic phospholipids.
A high degree of ATP hydrolytic activity present in purified rat pancreatic acinar cells was localized to plasma membranes. This activity was stimulated almost equally by Mg2+ or Ca2+. Kinetic analysis revealed that the enzyme had a higher affinity for Ca2+ (Kd = 1.73 microM) than Mg2+ (Kd = 2.98 microM) but a similar maximal rate of activity. A comparison of substrate requirements revealed very similar profiles for the Mg2+- and Ca2+-stimulated activities. Combinations of saturating concentrations of Mg2+ or Ca2+ produced the same degree of maximal activity. Investigation of the partial reactions of the ATPase activity revealed two phosphoprotein intermediates (Mr = 115,000 and 130,000) in the presence of Ca2+ and Mg2+. A significant stimulation of the Ca2+-ATPase activity by calmodulin was observed (Kd = 0.7 microM). Calmodulin increased the Ca2+-sensitivity of this enzyme system; Mg2+ appeared to be required for this effect. The Ca2+-ATPase activity was also stimulated by acidic phospholipids. Using an 125I-labeled calmodulin gel overlay technique, calmodulin was shown to bind in a Ca2+-dependent fashion to 133,000- and 230,000-dalton proteins present in the plasma membrane-enriched fraction. Under conditions that favor Ca2+-dependent kinase activity, calmodulin enhanced the phosphorylation of a 30,000- and 19,000-dalton protein. The major ATP hydrolytic activity in pancreatic acinar plasma membranes was present as an ectoenzyme.
D000251:Adenosine Triphosphatases / Q000378:metabolism; D000818:Animals; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases / Q000302:isolation & purification / Q000378:metabolism*; D002147:Calmodulin / Q000494:pharmacology*; D002413:Cations, Divalent; D002462:Cell Membrane / Q000201:enzymology; D004492:Edetic Acid / Q000494:pharmacology; D005260:Female; D066298:In Vitro Techniques; D007700:Kinetics; D008970:Molecular Weight; D010179:Pancreas / Q000166:cytology / Q000201:enzymology*; D010743:Phospholipids / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism
D002147:Calmodulin; D002413:Cations, Divalent; D010743:Phospholipids; D004492:Edetic Acid; D000251:Adenosine Triphosphatases; D017301:Ca(2+) Mg(2+)-ATPase; D000252:Calcium-Transporting ATPases; D000254:Sodium-Potassium-Exchanging ATPase