Ligand binding properties of the sarcoplasmic reticulum (Ca2+ + Mg2+)-ATPase labelled with N-cyclohexyl-N'-(4-dimethylamino-alpha-naphthyl)carbodiimide.

The (Ca2+ + Mg2+)-ATPase of rabbit sarcoplasmic reticulum, when labelled at two Ca2+-protected sites with N-cyclohexyl-N'-(4-dimethylamino-alpha-naphthyl)carbodiimide (NCD-4) retains Ca2+ binding capacity at the sites with Kd values of approx. 3 microM and 0.12 mM as assessed by fluorescence titration. The sites correspond to the two high-affinity Ca2+ binding sites present in the native ATPase. The NCD-4 labelled ATPase exhibits slow conformational changes at each site on addition of Ca2+. It retains the ability to form phosphoenzyme, and can most likely translocate Ca2+.