PHOSPHORYLATION OF H+/K+-ATPASE BY INORGANIC-PHOSPHATE - THE ROLE OF K+ AND SCH 28080

The effects of K+ on the phosphorylation of H+/K+-ATPase with inorganic phosphate were studied using H+/K+-ATPase purified from porcine gastric mucosa. The phosphoenzyme formed by phosphorylation with P(i) was identical with the phosphoenzyme formed with ATP. The maximal phosphorylation level obtained with P(i) was equal to that obtained with ATP. The P(i) phosphorylation reaction of H+/K+-ATPase was, like that of Na+/K+-ATPase, a relatively slow reaction. The rates of phosphorylation and dephosphorylation were both increased by low concentrations of K+, which resulted in hardly any effect on the phosphorylation level. A decrease of the steady-state phosphorylation level was caused by higher concentrations of K+ in a noncompetitive manner, whereas further increase in the dephosphorylation rate was observed. The decreasing effect was caused by a slow binding of K+ to the enzyme. All above-mentioned K+ effects were abolished by the specific H+/K+-ATPase inhibitor SCH 28080 (2-methyl-8-[phenylmethoxy]imidazo-[1-2-a]pyrine-3-acetonitrile). Additionally, SCH 28080 caused a 2-fold increase in the affinity of H+/K+-ATPase for P(i). A model for the reaction cycle of H+/K+-ATPase fitting the data is postulated.