Inhibition of the Ca2+-Mg2+ ATPase of sarcoplasmic reticulum by Co-(phen)-ATP.

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 Km and Ki values 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.