Mg2+-ATPase as a membrane ecto-enzyme of human granulocytes. Inhibitors, activators and response to phagocytosis

Abstract

(1) The Mg2+-ATPase of purified human granulocytes is located at the plasma membrane. Thus, no additional enzyme activity was detected when the cells were disrupted. Moreover, the Mg2+-ATPase activity of intact cells was inhibited by such poorly permeant reagents as diazotized sulfanilic acid and suramin. Finally, the enzyme activity of cell homogenates was recovered in particulate fractions. (2)The surface Mg2+-ATPase of human granulocytes had an apparent Km of 50 microns for ATP and displayed substrate inhibition. (3) The enzyme was not affected by ouabain, but was inhibited by N-ethyl malemide, sodium meta-periodate, suramin and diazotized sulfanilic acid. The enzyme was activated by cytochalasins B and D and by UDP. Activation by UDP was characterized by changes in the enzyme's apparent Km and V and by belief of substrate inhibition. (4)Internalization of surface membranes subsequent to phagocytosis of suitable particles did not result in depletion of Mg2+-ATPase from the cell surface. The enzyme activity did not decrease after exposure to several varieties of paraffin oil emulsion particles, even if the challenged cells had been pretreated with colchicine of cytochalasin B. (5) Since suramin, which inhibited Mg2+-ATPase, had no effect upon other granulocyte functions such as chemotaxis, superoxide anion generation, or phagocytosis, it is unlikely that the enzyme plays a major role in these functions.