Chemical modification of the Ca2+ -dependent ATPase of sarcoplasmic reticulum from skeletal muscle. II. Use of 2, 4, 6-trinitrobenzenesulfonate to show functional movements of the ATPase molecule.

1. By changing the concentrations of Mg2+ and Ca2+ ions and ATP, the Ca2+, Mg2+-dependent ATPase [EC 3.6.1.3] of SR was converted to various enzymatic states, E, MgE, MgECa, MgEATP' CaECa, and CaECap at pH 8.0 and 0 degrees (cf. Eq. 1). 2. SR vesicles were allowed to react with 0.5 mM 2,4,6-trinitrobenzenesulfonate (TBS) at pH 8.0 and 0 degrees, keeping the ATPase in one of the enzymatic states listed above. Trinitrophenyl (TNP)-protein and TNP-lipid were separated by gel-filtration, and the amounts of TNP incorporated into protein and lipid were determined. 3. In all the enzymatic states of ATPase tested, the amount of TBS bound with SR protein increased exponentially with time, and reached a maximum level 10 min after starting the reaction. On the other hand, the amount of TBS bound to lipid increased with time, and did not reach a maximum level for at least 20 min. 4. The SR ATPase activity and the amount of EP formed decreased only slightly, even when the amount of TBS bound to SR protein reached the maximum level. 5. The maximum amount of TBS bound to SR protein varied on changing the enzymatic state of SR ATPase. Namely, about 2,3,1,3, or 4, and 3 moles of TNP were incorporated per mole of SR ATPase, when SR was allowed to react with TBS in the enzymatic states MgE, MgECa, MgEATP' CaECa, and CaECap, respectively. 6. When the enzymatic state was changed from MgE to MgECa 10 min after starting the reaction with TBS, about 4 moles of TBS was bound per mole of ATPase within 10 min, while the maximum levels of TBS bound in states MgE and MgECa were about 2 and 3 moles per mole of ATPase, respectively, as mentioned above. 7; When the enzymatic state was changed from MgE to MfEATP 10 min after starting the reaction with TBS, about 3 moles of TBS was bound per mole of ATPase within 10 min, while the maximum levels of TBS bound in states MgE and MgEATP were about 2 and 1 moles per mole of ATPase, respectively, as mentioned above. 8. When the enzymatic state was changed from CaECa to CaECap 10 min after starting the reaction with TBS, about 4 moles of TBS was bound per mole of ATPase, while the maximum levels of TBS bound in states CaECa and CaECap were about 3 or 4 moles per mole of ATPase, respectively. 9. A diagrammatic model of functional movements of the ATPase molecule coupled with elementary steps in the ATPase reaction is proposed on the basis of these results.