Reversible inhibitions of gastric H+,K(+)-ATPase by scopadulcic acid B and diacetyl scopadol. New biochemical tools of H+,K(+)-ATPase.
Scopadulcic acid B (SA-B), a novel diterpenoid, is a main ingredient of the Paraguayan traditional medicinal herb "Typychá kuratú (Scoparia dulcis L.). SA-B and its debenzoyl derivative, diacetyl scopadol (DAS), specifically inhibit ATP hydrolysis of gastric H+,K(+)-ATPase. Both compounds inhibit the K(+)-dependent dephosphorylation step of the enzyme without any effect on the phosphorylation step. SA-B is a mixed-type inhibitor with respect to the activating cation, K+. SA-B lowers the affinity of H+,K(+)-ATPase to K+ and decreases the maximal velocity of ATP hydrolysis, whereas DAS is an uncompetitive inhibitor with respect to K+. Furthermore, the effects of SA-B and DAS on conformational states of the ATPase were studied by measuring the changes in the fluorescence intensity of the fluorescein isothiocyanate-labeled enzyme. The fluorescence study shows that SA-B primarily binds to the E2K form in the presence of Mg2+ and stabilizes the form and that DAS stabilizes the E2PK form. Therefore, the chemical modification of SA-B, debenzoylation, induced the changes in the pattern of inhibition of H+,K(+)-ATPase. Furthermore, the inhibition mechanisms of SA-B and DAS were different from those of omeprazole, which is an irreversible inhibitor, and SCH 28080, which is a reversible, competitive inhibitor with respect to K+. DAS also inhibited the K(+)-dependent p-nitrophenyl phosphatase activity, and the inhibition was competitive with respect to K+, indicating that the K(+)-dependent p-nitrophenylphosphatase activity does not represent the partial reaction step of H+,K(+)-ATPase.
Asano
S
S
Faculty of Pharmaceutical Sciences, Toyama Medical and Pharmaceutical University, Japan.
Mizutani
M
M
Hayashi
T
T
Morita
N
N
Takeguchi
N
N
D000251:Adenosine Triphosphatases / Q000037:antagonists & inhibitors*; D000818:Animals; D027682:Cation Transport Proteins; D004224:Diterpenes / Q000494:pharmacology*; D005753:Gastric Mucosa / Q000201:enzymology*; D017506:H(+)-K(+)-Exchanging ATPase; D007668:Kidney / Q000201:enzymology; D007700:Kinetics; D008519:Medicine, Traditional; D008962:Models, Theoretical; D015394:Molecular Structure; D000254:Sodium-Potassium-Exchanging ATPase / Q000037:antagonists & inhibitors; D013552:Swine; D014634:Valinomycin / Q000494:pharmacology
D027682:Cation Transport Proteins; D004224:Diterpenes; C058658:scopadulcic acid B; C066773:diacetylscopadol; D014634:Valinomycin; D000251:Adenosine Triphosphatases; C022761:potassium transporting ATPase; D017506:H(+)-K(+)-Exchanging ATPase; D000254:Sodium-Potassium-Exchanging ATPase