Basal‐lateral and brush border membranes from pig kidney cortex were prepared by differential centrifugation followed by free‐flow electrophoresis. In each type of membrane, azide‐insensitive, low‐affinity Ca2+‐ATPase and Mg2+‐ATPase activities are demonstrated. A comparative study for both membranes further reveals the following analogies between these ATPase: (a) they show maximal activity between pH 8 and 8.5; (b) they exhibit Km values for Ca‐ATP or Mg‐ATP in the millimolar range and have a comparable low substrate specificity; (c) they are insensitive to 10 μM of vanadate, N,N′‐dicyclohexylcarbodiimide, diethylstillbestrol, quercetin, harmaline and amiloride. The partial inhibition by 1 mM of the various compounds is rather aspecific. In view of these similarities it is concluded that only one enzyme entity is responsible for the activity which is measured in both membrane types. The HCO3−‐stimulated Mg2+‐ATPase activity in pig kidney cortex was also studied. This enzyme, however, is clearly of mitochondrial origin since the HCO3−‐stimulation coincides with the distribution profile of succinate dehydrogenase, a mitochondrial marker; and since it is inhibited by azide.

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