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Four catechins, epigallocatechin-3-gallate, epigallocatechin, epicatechin-3-gallate, and epicatechin, inhibited activity of the Na+,K+-ATPase. The two galloyl-type catechins were more potent inhibitors, with IC50 values of about 1 μM, than were the other two catechins. Inhibition by epigallocatechin-3-gallate was noncompetitive with respect to ATP. Epigallocatechin-3-gallate reduced the affinity of vanadate, shifted the equilibrium of E1P and E2P toward E1P, and reduced the rate of the E1P to E2P transition. Epigallocatechin-3-gallate potently inhibited membrane-embedded P-type ATPases (gastric H+,K+-ATPase and sarcoplasmic reticulum Ca2+-ATPase) as well as the Na+,K+-ATPase, whereas soluble ATPases (bacterial F1-ATPase and myosin ATPase) were weakly inhibited. Solubilization of the Na+,K+-ATPase with a nonionic detergent reduced sensitivity to epigallocatechin-3-gallate with an elevation of IC50 to 10 μM. These results suggest that epigallocatechin-3-gallate exerts its inhibitory effect through interaction with plasma membrane phospholipid.