Drug-sensitive ecto-ATPase in human leukocytes

An ecto-ATPase has been described in human leukocytes. The enzyme is present in both granulocytes and lymphocytes, exhibiting higher activity and substrate affinity in the latter cells, K_m values ranged from 0.06 mM in lymphocytes to 0.3 mM in the unresolved leukocyte fraction. Enzymatic activity had a pH optimum in the physiological range, required Mg²⁺, and was insensitive toward ouabain and Ca²⁺. Potent inhibitors of ecto-ATPase were tricyclic antidepressants and substituted phenothiazines, to which the enzyme in lymphocytes was most susceptible. These drugs also decreased the phagocytic ability of leukocytes. However, while drug inhibition of the enzyme was reversible by washing, the effect on phagocytosis was not.