MG2-ADP PROTECTS AGAINST INACTIVATION OF SARCOPLASMIC-RETICULUM CA2+,MG2+-ATPASE BY N-CYCLOHEXYL-N'-(4-DIMETHYLAMINO-ALPHA-NAPHTHYL) CARBODIIMIDE()

N-cyclohexyl-N'-(4-dimethylamino-alpha-naphthyl) carbodiimide (NCD-4) inactivates the sarcoplasmic reticulum Ca2+-ATPase by covalent labelli ng at or near the high affinity (transport) Ca2+ sites. Mg2+-ADP prote cts against the inactivation of the Ca2+-ATPase produced by NCD-4, wit h a K-0.5 of Mg2+-ADP of 28 +/- 6 mu M for purified Ca2+-ATPase. With native and solubilized sarcoplasmic reticulum membranes millimolar Mg2 +-ADP concentrations are needed to produce an effective protection of the Ca2+-ATPase against inactivation by NCD-4. These results suggest a tight structural interconnection between catalytic and transport Ca2 sites in the Ca2+-ATPase, modulated by protein-protein interactions i n the SR membrane. (C) 1995 Academic Press, Inc.