J. Biol. Chem., Vol. 259, Issue 11, 7061-7066, Jun, 1984
High affinity, calcium-stimulated adenosine triphosphatase activity in the particulate fraction of rat pancreatic acini
TW Hurley, JK Becker and JR Martinez
Adenosine triphosphatase activity which is Mg2+-dependent and stimulated by
submicromolar concentrations of Ca2+ (as Ca . ATP) was identified in the
total particulate fraction of rat pancreatic acini. Half-maximal activity
(V0.5) is obtained at 100.1 +/- 6 nM Ca . ATP with a Hill coefficient of
2.2 +/- 0.1 (mean +/- S.E.; n = 4). Maximal activity was 75 +/- 19 pmol of
Pi released from ATP minute-1 microgram of membrane protein-1 (mean +/-
S.E.; n = 7). High affinity Ca2+-ATPase activity was unaffected by ouabain,
Na+, K+, La3+, and added calmodulin. Activity was slightly reduced by
ruthenium red (0.1 mM) and by oligomycin (80 micrograms/ml) but was reduced
almost 50% by the phenothiazine derivative fluphenazine in a dose-related
and Ca2+- dependent manner. Hydrolysis of p-nitrophenyl phosphate was 9% of
the rate of ATP hydrolysis and was independent of Ca2+ concentration.
However, ADP, GTP, UTP, and ITP were hydrolyzed at 76-93% the rate that ATP
was hydrolyzed with V0.5 values and Hill coefficients similar to those of
Ca . ATP. We conclude that rat pancreatic acini contain an enzyme for
active Ca2+ translocation: ATPase activity that is Mg2+- dependent and
stimulated by submicromolar concentrations of Ca . ATP. Substrate
hydrolysis appears to involve positive cooperative interactions of multiple
ligand-binding sites and may be regulated in part by calmodulin.
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