An ecto-ATPase of thyroidal cell membrane

The isolated cells were obtained from hog thyroid glands treated with dispase. More than 95% of the cells obtained were intact and viable immediately after preparation, and the cell viability did not change during incubation in the experimental conditions. ATP added to the external medium of whole cell suspensions was hydrolyzed in the presence of various divalent cations, especially Mg, and the rate of hydrolysis of ATP was not significantly different between the Mg-ion system and the completed ion system (Mg+Na+K). When whole cell suspensions were disrupted with homogenizer, the hydrolysis of ATP was markedly increased by adding Na plus K. But there was no difference in the Mg-ion system between cell homogenates and whole cell suspensions. ADP, AMP and adenosine as reaction products were found in the reaction mixture which resulted from the hydrolysis of ATP by whole cell suspensions. Our data suggest that Mg-ATPase in the thyroidal isolated cells is an ectoenzyme whose active site(s) are exposed to the external surface of plasma membrane, and that ATP is finally hydrolyzed to adenosine via ADP and AMP by the enzyme(s).