SPECIFIC-INHIBITION OF NA-ATPASE ACTIVITY BY ATRACTYLON, A MAJOR COMPONENT OF BYAKU-JUTSU, BY INTERACTION WITH ENZYME IN THE E(2) STATE(,K+)

Atractylon, a major component of the crude drug ''Byaku-jutsu'' (rhizo mes of Atractylodes japonica), strongly inhibited Na+,K+-ATPase activi ty 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 potentl y. No effects on alkaline and acid phosphatase activities were observe d. The inhibition of Na+,K+-ATPase activity by atractylon was noncompe titive with respect to ATP and was greater with increasing KC concentr ation, 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 o f the K+ concentration. Furthermore, atractylon increased the K-i valu e 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 a t 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 affe cted. These results suggest that atractylon, a specific inhibitor of N a+,K+-ATPase, interacts with enzyme in the E(2) State and inhibits the reaction step from E(2)-P to K . E(2).