Inhibition of Gastric H⁺K⁺-ATPase by Quercetin

The effects of the naturally occurring flavonoid, quercetin, on gastric H+, K(+)-ATPase were investigated. Quercetin inhibited hog gastric H+, K(+)-ATPase and K(+)-stimulated p-nitrophenyl phosphatase (K(+)-pNPPase) activity in a dose dependent manner with IC50 values of 2.3 microM, and 6.0 microM respectively. The inhibition of H+, K(+)-ATPase by quercetin is competitive with ATP and is noncompetitive with K+. The steady-state phosphorylation level of the enzyme was also dose-dependently reduced by quercetin with an IC50 value of 4.5 microM. These results suggest that quercetin reduces the phosphorylated enzyme level by competition with ATP, and thereby inhibits the H+, K(+)-ATPase activity.