Cation accumulation by microsomal (Na+ + K+)-activated ATPase

Abstract

• 1.

  1. Possible cation accumulation by microsomal (Na⁺ + K⁺)-activated, ouabain-sensitive ATPase was examined by an electron microscopical method.

• 2.

  2. Densitometry measurements of membrane opacity were performed on the electron micrograph plates obtained after incubation of the ATPase preparation under a variety of experimental conditions.

• 3.

  3. To aid visualization of possible cation accumulation, the more electron opaque Cs⁺ was substituted for K⁺ in the incubation system of many experiments.

• 4.

  4. Increase in membrane density of the microsomal ATPase was observed in the presence of sodium and caesium only when ATP hydrolysis was inhibited by ouabain.

• 5.

  5. This phenomenon was not observed when a ouabain-insensitive mitochondrial ATPase system was examined.

• 6.

  6. This increase in density is interpreted as selective Cs⁺ accumulation by the microsomal enzyme membrane fragments.

• 7.

  7. The significance of these results is discussed in terms of reaction mechanism for (Na⁺ + K⁺)-activated ATPase.