title: ATPase and phosphatase activities from human red cell membranes: I. The effects of N-ethylmaleimide.
abstract: In human red cell membranes the sensitivity to N-ethylmaleimide of Ca2+-dependent ATPase and phosphatase activities is at least ten times larger than the sensitivity to N-ethylmaleimide of (Na+ + K+)-ATPase and K+-activated phosphatase activities. All activities are partially protected against N-ethylmaleimide by ATP but not by inorganic phosphate or by p-nitrophenylphosphate. (ii) Protection by ATP of (Na+ + K+)-ATPase is impeded by either Na+ or K+ whereas only K+ impedes protection by ATP of K+-activated phosphatase. On the other hand, Na+ or K+ slightly protects Ca2+-dependent activities against N-ethylmaleimide, this effect being independent of ATP. (iii) The sensitivity to N-ethylmaleimide of Ca2+-dependent ATPase and phosphatase activities is markedly enhanced by low concentrations of Ca2+. This effect is half-maximal at less than 1 micron Ca2+ and does not require ATP, which suggests that sites with high affinity for Ca2+ exist in the Ca2+-ATPase in the absence of ATP. (IV) Under all conditions tested the response to N-ethylmaleimide of the ATPase and phosphatase activities stimulated by K+ or Na+ in the presence of Ca2+ parallels that of the Ca2+-dependent activities, suggesting that the Ca2+-ATPase system possesses sites at which monovalent cations bind to increase its activity.
mesh_terms: D000251:Adenosine Triphosphatases / Q000097:blood*; D000255:Adenosine Triphosphate / Q000097:blood; D002118:Calcium / Q000097:blood; D004910:Erythrocyte Membrane / Q000187:drug effects / Q000201:enzymology*; D004912:Erythrocytes / Q000201:enzymology*; D005033:Ethylmaleimide / Q000494:pharmacology*; D006801:Humans; D010744:Phosphoric Monoester Hydrolases / Q000097:blood*; D011188:Potassium / Q000097:blood; D012964:Sodium / Q000097:blood
chemical_list: D000255:Adenosine Triphosphate; D012964:Sodium; D010744:Phosphoric Monoester Hydrolases; D000251:Adenosine Triphosphatases; D005033:Ethylmaleimide; D011188:Potassium; D002118:Calcium