EFFECTS OF POLYCATIONS ON CA2-ATPASE( BINDING TO THE CA2+)

Spermine and polyarginine have been shown to increase the rate of diss ociation of Ca2+ from the Ca2+-ATPase of skeletal-muscle sarcoplasmic reticulum. They also decrease the affinity of the ATPase for Mg2+ as d etected by changes in the fluorescence intensity of the ATPase labelle d with 4-(bromomethyl)-6,7-dimethoxycoumarin (DMC). Polyarginine itsel f also decreases the fluorescence intensity of DMC-labelled ATPase. Th ese results are consistent with binding of spermine and polyarginine t o a gating site controlling the rate of access of Ca2+ to its binding sites on the ATPase. A basic peptide PLN-(1-25) corresponding to resid ues 1-25 of phospholamban had no effect on the rate of dissociation of Ca2+ or on the fluorescence of DMC-labelled ATPase. Spermine, polyarg inine and PLN-(1-25) all increased the equilibrium constant E1/E2, and spermine and polyarginine increased the rate of Ca2+ binding to the A TPase, consistent with an increase in the rate of the E2 --> E1 transi tion. Spermine displaced Tb3+ and Ruthenium Red from the ATPase, consi stent with binding in the stalk region of the ATPase. Polyarginine and PLN-(1-25), however, had no effect on Tb3+ or Ruthenium Red binding, suggesting a greater specificity in binding basic peptides to the ATPa se than spermine.