Characterization of anion binding sites of sarcoplasmic reticulum vesicles by 35Cl-NMR

Sarcoplasmic reticulum vesicles were prepared from rabbit skeletal muscle. It could be demonstrated that the anion binding sites on this membrane can be studied by 35Cl-NMR spectroscopy. Titration of sarcoplasmic reticulum vesicles with the sulfate exchange inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) revealed specific binding of this compound to the sarcoplasmic reticulum membrane. A new inhibitor, pyridoxalphosphate-6-azophenyl-2'-sulfonic acid (PPAPS) was introduced and proved to displace chloride equally well from its binding sites. Two binding sites could be distinguished by titration with inorganic phosphate in the presence and absence of the inhibitors. Because of the insensitivity of 35Cl-NMR spectroscopy these anion binding sites have to be located on a protein being present in considerable amount in the sarcoplasmic reticulum membrane.