Unique enzymes of purified microsomes from pig fundic mucosa. K+-stimulated adenosine triphosphatase and K+-stimulated pNPPase.

Microsomal fractions from homogenates of pig gastric fundic mucosa showed high levels of K+-stimulated adenosine triphosphatase (ATPase) and K+-stimulated phosphatase. Similar preparations from antral mucosa showed virtually no such activity. Because of mitochondrial contamination the fundic microsomes were further separated by sucrose density gradient centrifugation. A low density band of membranes (peak 1.12 to 1.13 g per ml) possessed all of the K+-stimulated enzyme activities. Morphological features and the abundant glycoproteins of the low density microsomes suggested they might be derived from the tubulovesicles of oxyntic cells. Mitochondrial and ribosomal markers were associated with membranes with much higher densities (greater than 1.22). The K+-stimulated ATPase has a pH optimum of 7.5 and required Mg++, but neither Na+ nor ouabain had any appreciable effect on the activity. Stimulation of basal ATPase by K+ ranged from 1.5 to 3.0-fold with an apparent Ka for activation between 0.2 to 0.4 mM K+. Addition of various K+ ionophoretic substances (e.g., gramicidin) produced further stimulation of K+-ATPase up to 6 times the basal rate. The mean activities for seven separate preparations of purified low density pig fundic microsomes were as follows (micromoles of ATP hydrolyzed per mg protein per hr +/- SEM); basal ATPase, 15.8 +/- 2.8; plus 10 mM K+, 29.3 +/- 4.5; plus 10 mM K+ and 10(-5) M gramicidin, 45.2 +/- 5.2. Neither the basal ATPase nor the K+-stimulated rates were altered by HCO3- or Cl-. The occurrence of these active and unique enzyme activities in the oxyntic region of gastric mucosa suggest some relation with secretory activity. Possible functional roles are discussed.