Effect of diadenosine polyphosphates on Ca2+ ATPase activity

Abstract

 Diadenosine tri-, tetra-, penta-, and hexaphosphate (Ap₃A, Ap₄A, Ap₅A and Ap₆A) have been described as having various effects on vascular tone depending on the number of phosphate groups. This study examined the effect of diadenosine polyphosphates on Ca²⁺ ATPase activity. The activity of the enzyme was measured spectrophotometrically as the difference in hydrolysis of ATP in the presence and absence of Ca²⁺ with various concentrations of ATP and diadenosine polyphosphates. The diadenosine polyphosphates increased the activity of the Ca²⁺ ATPase. The effect tended to be stronger with Ap₅A and Ap₆A than with Ap₃A and Ap₄A in the order of potency: Ap₃A≈AP₄A<Ap₅A≈AP₆A. The stimulatory effect of diadenosine polyphosphates was not competitive with that of ATP, suggesting an allosteric activation of Ca²⁺ ATPase by diadenosine polyphosphates. This effect may be physiologically relevant for limiting the increase in cytosolic free Ca²⁺ concentration elicited by diadenosine polyphosphates by receptor activation and modulating Ca²⁺ ATPase function under resting conditions.