Chemical Modification Reveals Involvement of Different Sites for Nucleotide Analogues in the Phosphatase Activity of the Red Cell Calcium Pump

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Abstract
The calcium pump of plasma membranes catalyzes the hydrolysis of ATP and phosphoric esters like p-nitrophenyl phosphate (pNPP). The latter activity requires the presence of ATP and/or calmodulin, and Ca2+ [22, 25]. We have studied the effects of nucleotide-analogues and chemical modifications of nucleotide binding sites on Ca2+-pNPPase activity. Treatment with fluorescein isothiocyanate (FITC), abolished Ca2+-ATPase and ATP-dependent pNPPase, but affected only 45% of the calmodulin-dependent pNPPase activity. The nucleotide analogue eosin-Y had an inhibitory effect on calmodulin-dependent pNPPase (Ki eosin-Y= 2 μm). FITC treatment increased Ki eosin-Y 15 times. Acetylation of lysine residues with N-hydroxysuccinimidyl acetate inactivates Ca2+-ATPase by modifying the catalytic site, and impairs stimulation by modulators by modifying residues outside this site [9]. Acetylation suppressed the ATP-dependent pNPPase with biphasic kinetics. ATP or pNPP during acetylation cancels the fast component of inactivation. Acetylation inhibited only partially the calmodulin-dependent pNPPase, but neither ATP nor pNPP prevented this inactivation. From these results we conclude: (i) ATP-dependent pNPPase depends on binding of ATP to the catalytic site; (ii) the catalytic site plays no role in calmodulin-dependent pNPPase. The decreased affinity for eosin-Y of the FITC-modified enzyme, suggests that the sites for these two molecules are closely related but not overlapped. Acetimidation of the pump inhibited totally the calmodulin-dependent pNPPase, but only partially the ATP-pNPPase. Since calmodulin binds to E1, the E1 conformation or the E2? E1 transition would be involved during calmodulin-dependent pNPPase activity.

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