INHIBITION OF THE (CA-2+)ATPASE FROM SARCOPLASMIC-RETICULUM BY DICYCLOHEXYLCARBODIIMIDE - EVIDENCE FOR LOCATION OF THE CA-2+ BINDING-SITE IN A HYDROPHOBIC REGION

Dicyclohexylcarbodiimide (DCCD) inhibits the (Ca2+) ATPase, Ca2+ uptake by sarcoplasmic reticulum vesicles and Ca2+ binding to the (Ca2+) ATPase from sarcoplasmic reticulum. Ca2+ (at μM concentrations) specifically protects against DCCD inhibition. The inhibition can, therefore, be readily demonstrated only in the presence of Ca2+ chelating agents such as EGTA. In the presence of EGTA, the ion-ophore A-23187 increased the sensitivity to DCCD. The ionophore also increased the phosphorylation of the enzyme by inorganic phosphate in the presence of Mg2+. These results indicate that tightly bound Ca2+ is located in a hydrophobic region of the enzyme which is not accessible to EGTA. Complete inhibition of the (Ca2+)ATPase is accompanied by binding of 4-5 nmol of [l4C]DCCD per mg of ATPase protein in the absence of Ca2+ compared with 2 nmol bound per mg in the presence of Ca2+ with no ATPase inhibition. Assuming a molecular weight of 100000 for the ATPase monomer, about 1 nmol of DCCD inhibits 4 nmol of ATPase. This result suggests that the minimal functional unit of the enzyme is a tetramer. Following trypsin digestion of the [14C]DCCD-labeled ATPase most of the radioactivity appears in the 20000-dalton fragment. We propose that DCCD reacts with the Ca2+-binding site of the ATPase. © 1979, American Chemical Society. All rights reserved.