EFFECT OF DIADENOSINE POLYPHOSPHATES ON CA2+ ATPASE ACTIVITY

Diadenosine tri-, tetra-, penta-, and hexaphosphate (Ap(3)A, Ap(4)A, A p(5)A and Ap(6)A) have been described as having various effects on vas cular tone depending on the number of phosphate groups. This study exa mined the effect of diadenosine polyphosphates on Ca2+ ATPase activity . The activity of the enzyme was measured spectrophotometrically as th e difference in hydrolysis of ATP in the presence and absence of Ca2with various concentrations of ATP and diadenosine polyphosphates. The diadenosine polyphosphates increased the activity of the Ca2+ ATPase. The effect tended to be stronger with Ap(5)A and Ap(6)A than with Ap( 3)A and Ap(4)A in the order of potency: Ap(3)A approximate to AP(4)A<A p(5)A approximate to AP(6)A. The stimulatory effect of diadenosine pol yphosphates was not competitive with that of ATP, suggesting an allost eric activation of Ca2+ ATPase by diadenosine polyphosphates. This eff ect may be physiologically relevant for limiting the increase in cytos olic free Ca2+ concentration elicited by diadenosine polyphosphates by receptor activation and modulating Ca2+ ATPase function under resting conditions.