Inhibition of the Ca²⁺—Mg²⁺ ATPase of Sarcoplasmic Reticulum by Co—(phen)—ATP

Abstract

Active Ca²⁺ transport in sarcoplasmic reticulum derived from skeletal muscle is shown to be inhibited by an ATP derivative Co(III)-phenanthroline-ATP. The inhibition is by competition for the MgATP binding site on the Ca²⁺—ATPase pump molecule. For preincubation times of 30 min or less, no quasicovalent bonds are established between the inhibitor and the ATPase. The apparent dissociation constant for the enzyme—inhibitor complex is 0.27 mM), a value close to the apparent K^m and K_i valus for MgATP (0.24 mM) and CoATP (0.14 mM). Addition of Co-(phen)—ATP to sarcoplasmic reticulum after maximal Ca²⁺ uptake has been achieved causes the release of the accumulated Ca²⁺. Under the experimental conditions chosen, the release occurs in two phases: one with a half-time of less than 2 sec and one with a half-time of 17 to 50 sec. The implications of these findings to the ATPase active transport mechanism are discussed.