Ion transport ATPases as targets for free radical damage. Protection by an aminosteroid of the Ca²⁺ pump atpase and Na⁺/K⁺ pump ATPase of human red blood cell membranes

Preincubation of red blood cell membranes in the presence of ferrous sulfate and EDTA resulted in both a concentration- and time-dependent inhibition of the Na⁺/K⁺ pump ATPase, basal Ca²⁺ pump ATPase, and the calmodulin- (CaM) activated Ca²⁺ pump ATPase. The IC₅₀, for all three ATPases was approximately 2.5 × 10^-5 M iron. The addition to membranes of ferrous iron and EDTA in an approximately 1:1 ratio resulted in conversion to the ferric iron form in several minutes. However, inhibition of the ion pump ATPases and cross-linking of membrane proteins occurred over the course of several hours. The time course of formation of thiobarbituric acid-reactive substances (TBARS) closely paralleled inhibition of the ion pump ATPases. Inhibition of the ion pump ATPases was prevented by the addition of deferoxamine or Superoxide dismutase but not by mannitol, ethanol, or catalase. Both butylated hydroxytoluene and tirilazad mesylate (U74006F) prevented the formation of TBARS, limited the inhibition of the ion pump ATPases, and reduced cross-linking of membrane proteins. These data may be interpreted to suggest that inhibition of ion pump ATPases in plasma membranes may occur as a result of iron-promoted formation of Superoxide and subsequent lipid peroxidation, which can be prevented by free-radical scavengers including butylated hydroxytoluene and U74006F.