Interaction of beta-eudesmol with Na+,K(+)-ATPase: inhibition of K(+)-pNPPase activity.

The effect of beta-eudesmol, one of the major components in So-jutsu (Atractylodis Lanceae Rhizoma), on K(+)-dependent p-nitrophenyl phosphatase (K(+)-pNPPase) activity was studied. It inhibited K(+)-pNPPase activity with an I50 value of 4.1 x 10(-4) M. The inhibition rate decreased as the K+ concentration was increased, whereas greater inhibition was observed with high concentrations of either Na+ or ATP. The Ki values for Na+ in the presence of 0, 0.1 and 1 mM ATP were 140, 260 and 310 mM, respectively, but with the addition of beta-eudesmol, these values decreased to 90 mM regardless of the ATP concentration. This study on K(+)-pNPPase activity supports the conclusion obtained from the study on Na+,K(+)-ATPase activity (Satoh K et al., Biochem Pharmacol 44: 373-378, 1992) that is, beta-eudesmol interacts with enzyme in the Na.E1 form and inhibits the reaction step Na.E1----Na.E1-P. Furthermore, in the study of the effects of K+ and beta-eudesmol on K(+)-pNPPase activity, it was confirmed that beta-eudesmol prevents the conformational change of Na.E1----K.E2.