Modification of sarcoplasmic reticulum adenosine triphosphatase by adenosine triphosphate magnesium

Lineweaver-Burk plots of Ca²⁺-activated adenosine triphosphatase from rabbit muscle sarcoplasmic reticulum have been determined for a wide range of substrate concentrations. The plots measured at constant Mg²⁺ concentrations are normally nonlinear, but approach linearity either as the sarcoplasmic reticulum ages, or when small quantities of Triton-X100 are added. Titration with N-ethylmaleimide has the same effect on the activity of the ATPase measured either at high or low substrate concentrations. Lineweaver-Burk plots measured under conditions where the Mg²⁺ concentration is varied so as to be always equal to the ATP concentration are linear. These results have been interpreted as evidence that the adenosine triphosphatase has a single active site which uses MgATP as its substrate and which can be modified by free Mg²⁺.