Abstract
1. 1.|Calcium binding to ( Na + + K +)- ATPase (ATP phosphohydrolase, EC 3.6.1.3) preparations from beef and pig heart preparations of varying degrees of purity was measured. 2. 2.|Binding was inhibited by Mg 2+, Na + and K +. Inhibition by Na + and K + appeared to be due to an ionic strength effect. 3. 3.|Four classes of binding sites were identified with K d values for calcium of about 0.03, 1, 15 and 200 μM. 4. 4.|Cyclic AMP-dependent phosphorylation of the enzyme by protein kinase (ATP : protamine O- phosphotransferase , EC 2.7.1.70) had no effect on (Na + + K +)- ATPase activity. 5. 5.|Phosphorylation also had no effect on either K d or B max for calcium binding at any of the four sites whether measured in the presence or absence of NaCl or KCl. 6. 6.|It is concluded that previous reports of an effect of phosphorylation on calcium binding to a (Na + + K +)- ATPase preparation may have been due to the presnece of membrane material not directly associated with ( Na + + K +)- ATPase .
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