Properties of the Calcium‐Independent ATPase of the Membranes of the Sarcoplasmic Reticulum Delipidated by the Nonionic Detergent Triton X‐100

1 When the sarcoplasmic reticulum membranes are solubilized in Triton X‐100 their ATPases are modified: the calcium‐dependent ATPase is activated whereas the basic ATPase is inhibited. 2 Phospholipids are removed from the solubilized sarcoplasmic membranes completely by chromatography on a sepharose column equilibrated with Triton X‐100. Thereby the activity of the calcium‐dependent ATPase is abolished and cannot be restored by addition of phospholipids. 3 The delipidated ATPase preparation, exhibits only low activity, which is not dependent on calcium but on magnesium alone. This calcium‐independent ATPase is activated to the level of the calcium‐dependent ATPase both by unsaturated and saturated fatty acids, by anionic phospholipids and various synthetic lipids. 4 The enhancement of the ATPase activity produced by lipids is related to the binding of these compounds to the membrane protein in amounts comparable with those of bound Triton as demonstrated by dialysis experiments with dodecylsulfate. When the concentration of Triton in the medium is lowered, the dodecylsulfate/protein binding ratio increases, while Triton is detached from the protein. 5 The activation of the calcium‐independent ATPase by saturated fatty acids is dependent upon the chain‐length of these aliphatic compounds. Only the long‐chain fatty acids are strong activators. Moreover, the hydrophilic group of lipids affects maximal activities. No activation is observed with lecithin or dodecanol, while alkyl‐sulfates or ‐phosphates activate most effectively. 6 The calcium‐independent ATPase differs from the calcium‐dependent ATPase in that it is insensitive to N ‐ethylmaleimide, dinitrodithiobenzoic acid or salyrgan. The enzyme is strongly inhibited by prenylamine, sodium azide and silver ions. Similarly low alkali salt concentrations produce a strong reduction of the activity. 7 The relative rates, with which the nucleoside triphosphates are split by the calcium‐independent ATPase, differ considerably. As observed for the calcium‐activated ATPase. The dependence on the substrate concentration suggests a positive cooperativity concerning Mg · ATP (Hill coefficient 1.46). 50%, of maximal activity is obtained at 0.25 mM Mg · ATP. The enzyme is strongly inhibited by Mg · ADP.