Calcium-stimulated ATPase of dog submandibular gland

A Ca^2?-stimulated ATPase present in a microsomal fraction prepared from canine submandibular glands was investigated. The Ca²⁺ concentration for half maximal activation of the enzyme was about 0.3 mM. Addition of Mg²⁺ to incubation media containing Ca²⁺ decreased the ATPase activity. The presence of neither Na⁺ nor K⁺ is required for Ca²⁺-activation of the enzyme. Also, Ca²⁺ will not substitute for Mg²⁺ in the Mg²⁺-dependent (Na⁺ + K⁺)-ATPase reaction. The Ca²⁺-activation was not appreciably affected by ouabain (10^?4M), but was inhibited by about 50 per cent by 5 × 10^?3M ethacrynic acid. These studies provide a possible enzymatic basis for the calcium uptake by salivary gland microsomes that has been reported by other workers.