THE K⁺‐STIMULATED ATPase SYSTEM OF MICROSOMAL MEMBRANES FROM GASTRIC OXYNTIC CELLS*

Numerous studies, using cells and tissues that are predominantly transporters of Na+, have been directed toward identifying the Na+, r-ATPase with the pump.Although the evidence is largely indirect, there is overwhelming support for this identity.However, many other ion transport mechanisms are considerably less established with respect t o the specific enzyme pathways responsible for ion translocation.In the case of HCl secretion by gastric mucosa, an anion sensitive, microsomal ATPase has been suggested as providing a primary basis for the membrane transport Although the microsomes have been thought to be derived from the characteristic tubular membrane system of the oxyntic cells, 4-6 a question regarding contamination by other organelles, especially mitochondria, recurs throughout the literature.Recent work in our laboratory has addressed itself to these problems in gastric mucosa.We have directed our efforts toward the segregation and purification of gastric microsomal fractions and toward the detailed study of the ATP-utilizing systems therein.In this presentation we show that a mitochondrial-free fraction of oxyntic cell microsomal vesicles has a unique K+-regulated ATPase.This ATP-utilizing system consists of ATP phosphotransferase and phosphatase activities that demonstrate some membrane asymmetry with respect t o K+ sensitivity.This ATPase system is of special interest as a possible pathway in HCl secretion since (1) it occurs in microsomal fractions from stomachs of several species; (2) it occurs only in oxyntic cell regions of the stomach; and (3) K+ is thought to be an important regulator of secretion. Methods and MaterialsStomachs of bullfrog, rabbit, and pig were used for the studies reported here.In the case of the former two species the animals were killed in the laboratory and the stomachs immediately taken for use.Pig stomachs were obtained from a local abbatoir and stored in ice-cold saline solution during the trip back to the laboratory.Frog oxyntic cell homogenates were prepared according to previously described methods.'For pig and rabbit stomachs the glandular mucosa from the main body, or fundic area, of the stomach was scraped and homogenized as previously d e ~c r i b e d .~The homogenates of all tissues were subjected to a series of differential centrifugation steps in order to provide the crude cell fractions that were subsequently used.The mitochondria1 fraction was taken as that particulate material which sedimented between 600 x g for 10 min and 12,000 X g for 15 min.Frog microsomes were taken as the postmitochondrial fraction that sedimented between 12,000 x g for 15 min and 40,000 x g for 90 min.