Abstract
Gastric microsomes do not contain any significant Ca 2+-stimulated ATPase activity. Trypsinization of pig gastric microsomes in presence of ATP results in a significant (2–3-fold) increase in the basal (with Mg 2+ as the only cation) ATPase activity, with virtual elimination of the K +-stimulated component. Such treatment causes unmaksing of a latent Mg 2+-dependent Ca 2+-stimulated ATPase. Other divalent cations such as Sr 2+, Ba 2+, Zn 2+ and Mn 2+ were found ineffective as a substitute for Ca 2+. Moreover, those divalent cations acted as inhibitors of the Ca 2+-stimulated ATPase activity. The pH optimum of the enzyme is around 6.8. The enzyme has a K m of 70 μM for ATP and the K a values for Mg 2+ and Ca 2+ are about 4 · 10 −4 M and 10 −7 M, respectively. Studies with inhibitors suggest the involvement of sulfhydryl and primary amino groups in the operation of the enzyme. Possible roles of the enzyme in gastric H + transport have been discussed.
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