The optimum pH of renal adenosine triphosphatase and its variation with the type of ATP.

The in vitro optimum pH of renal (Na+, K+)-ATPase, assumed to be 7.8, depends on the type of ATP used. The accepted value is obtained with 'grade II' ATP but with purer ATP ('Sigma grade') it lies close to the intracellular pH in rat medulla and cortex (pH 7.0, 7.2). Values were identical in rats subjected to dietary potassium depletion for 2-4 weeks. The optimal Mg2+ concentration was also influenced by the type of ATP but Mg ATPase activity was unaffected. Both types of ATP were hydrolysed by trichloracetic acid. Where the purer ATP is used the assay conditions need to be modified accordingly.