MECHANISM OF INHIBITION OF THE CA2-ATPASE BY SPERMINE AND OTHER POLYCATIONIC COMPOUNDS()

The ATPase activity of the Ca2+-ATPase of skeletal muscle sarcoplasmic reticulum is inhibited by a variety of polyamines, including spermine , spermidine, and poly(arginine). The effects of spermine on the ATPas e are highly specific. It has no effect on the affinity of the ATPase for Ca2+ or ATP, and no effect on the rate of phosphorylation by ATP. When the ATPase is phosphorylated with P-i in the presence of dimethyl sulfoxide at pH 6.0, and then dephosphorylation is induced by dilutio n in buffer at pH 7.5 in the absence of dimethyl sulfoxide, spermine i s found to have no effect on the rate of dephosphorylation. If the ATP ase is phosphorylated with gamma-P-32ATP and the rate of loss of rad iolabeled phosphoenzyme is measured following the addition of unlabele d ATP, spermine is found to decrease the rate of loss of radiolabel, c onsistent with an effect of spermine on the rate of the Ca(2)E1P --> E 2P step. Direct measurement confirms that spermine decreases the rate of dissociation of Ca2+ from the phosphorylated ATPase (Ca(2)E1P --> E 2P), with the decrease in the rate of this step explaining the inhibit ion of ATPase activity. Spermine also increases the equilibrium consta nt E1/E2 and inhibits phosphorylation of the ATPase by Pi by competiti on with the Mg2+ essential for the reaction. It is suggested that sper mine could bind to the site on the Ca2+-ATPase that interacts with pho spholamban.