SODIUM ACTS AS A POTASSIUM ANALOG ON GASTRIC H,K-ATPASE

The effects of Na+ on gastric H,K-ATPase were investigated using leaky and ion-tight H,K-ATPase vesicles. Na+ activated the total ATPase act ivity in the absence of K+, reaching levels of 15% relative to those i n the presence of K+. The Na+ activation, which takes place at the lum inal side of the membrane, depended on the ATP concentration and the t ype of buffer used. The steady-state ATP phosphorylation level, studie d with leaky vesicles, was reduced by Na+ due to both activation of th e dephosphorylation reaction and a shift to E(2) in the E(1)<->E(2) eq uilibrium. By studying this equilibrium in ion-tight H,K-ATPase vesicl es, it was found that Na+ drives the enzyme via a cytosolic site to th e nonphosphorylating E(2) conformation. No H+-like properties of cytos olic Na+ could be detected. We therefore conclude that Na+ behaves lik e K+ rather than like H+ in the H,K-ATPase reaction.