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Does calmodulin regulate the affinity of the human red cell Ca2+ pump for ATP?
A J Caride1, J P Rossi, P J Garrahan
1Instituto de Química y Fisicoquímica Biológicas (UBA-CONICET), Facultad de Farmacia y Bioquímica, Universidad de Buenos Aires, Argentina.
Abstract:
(1) We have reexamined the effects of calmodulin and of the calmodulin antagonist, compound 48/80 on the interaction of ATP at its low-affinity site in the Ca2(+)-ATPase from human red cells. (2) At variance with our earlier proposal (Biochim. Biophys. Acta (1985) 816, 379-386) calmodulin increased the maximum effect of ATP without changing the apparent affinity for ATP at the low-affinity site. Accordingly, ATP increased the maximum activation by calmodulin without altering the apparent affinity of the Ca2(+)-ATPase for calmodulin. (3) Confirming our previous observation (Biochim. Biophys. Acta (1985) 816, 379-386) compound 48/80 lowered the apparent affinity of the Ca2(+)-ATPase for ATP at the low-affinity site. This has to be attributed to a direct effect of this compound on the enzyme rather than to its effect as calmodulin antagonist.