Effects of curcumin, a major constituent of turmeric, on ecto-nucleotidases have not been clarified. Here, we investigated whether curcumin affects ecto-nucleotidase activities in human hepatocellular carcinoma HepG2 cells. In the cells, high levels of Mg(2+)-dependent activity of ecto-nucleotidases were observed in the presence of 1 mM adenosine triphosphate (ATP). The activity was inhibited by ecto-ATPase inhibitors such as suramin, ZnCl(2) and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. On the other hand, the activity was significantly decreased at alkaline pH (pH 9) and was not inhibited by levamisole, an inhibitor of alkaline phosphatase. In the presence of ATP, curcumin inhibited the activity in a concentration-dependent manner (IC(50) = 6.2 μM). In contrast, curcumin had no effects on ecto-nucleotidase activity in the presence of ADP (1 mM) or AMP (1 mM). The K (m) value for ATP hydrolysis of curcumin-sensitive ecto-ATPase was similar to the value of NTPDase2, an isoform of ecto-nucleoside triphosphate diphosphohydrolase. These results suggest that curcumin is a potent inhibitor of ecto-ATPase and may affect extracellular ATP-dependent responses.