Specific inhibition of Na+,K+-ATPase activity by atractylon, a major component of Byaku-jutsu, by interaction with enzyme in the E(2) state

Atractylon, a major component of the crude drug ''Byaku-jutsu'' (rhizomes of Atractylodes japonica), strongly inhibited Na+,K+-ATPase activity with an I-50 value of 8.9 x 10(-6) M. It also inhibited Mg2+-ATPase, H+,K+-ATPase, H+-ATPase and Ca2+-ATPase activities, but less potently. No effects on alkaline and acid phosphatase activities were observed. The inhibition of Na+,K+-ATPase activity by atractylon was noncompetitive with respect to ATP and was greater with increasing KC concentration, whereas it was not affected by Na+ concentration. The activity of K+-dependent p-nitrophenyl phosphatase, a partial reaction of Na+,K+-ATPase, was inhibited noncompetitively with respect to substrate (I-50 value of 1.8 x 10(-5) M), and the inhibition rate was independent of the K+ concentration. Furthermore, atractylon increased the K-i value for Na+ from 130 to 190 mM, but did not alter the K-i value for ATP. Inhibition of the phosphoenzyme formation by atractylon was greater at 0.1 M than at 1 M NaCl. K+-dependent dephosphorylation (E(2)-P to K . E(2)) was inhibited by atractylon, whereas ADP-sensitive (Na . E(1)-P to Na . E(1)) and non-specific dephosphorylation steps were not affected. These results suggest that atractylon, a specific inhibitor of Na+,K+-ATPase, interacts with enzyme in the E(2) State and inhibits the reaction step from E(2)-P to K . E(2).