Detection of different adenosine triphosphatases in human placental brush border membranes

The microvillous membrane of human placental syncytiotrophoblast cells contains a high ATPase activity. The purpose of this study was to characterize this activity and to investigate the presence of vacuolar type H+ ATPase in this membrane. Intact brush border membrane vesicles strongly hydrolyzed ATP, reflecting the presence of ATPase on the external side of the membrane. The ATPase activity was entirely Mg2+ dependent and increased with pH. At pH 7.5, Vmax was 31.0 +/- 1.7 mumol/mg/20 min and Km 0.18 +/- 0.03 mM ATP. Hydrolysis of ATP was not influenced by the presence of bicarbonate or alkaline phosphatase inhibitors, but at pH 8 it decreased by half following addition of 100 microM dicyclohexylcarbodiimide (DCCD). At pH 7.5, 1 mM N-ethylmaleimide (NEM) depressed this activity by less than 5%. Opening the membrane vesicles with 0.1% desoxycholate (DOC) or Triton-X neither revealed any additional ATPase activity nor altered the low sensitivity to NEM. Treatment of these membranes with 1% cholate decreased the ATPase activity by more than 70% and did not enhance the sensitivity of ATP hydrolysis to NEM. 10(-7) M Bafilomycin, which reduced by 56 +/- 9% the ATPase activity in dog kidney brush border membranes treated with 0.1% DOC, had no effect on placental brush border membranes subjected to the same procedure. Finally, neither immunocytochemical staining using monoclonal antibody to the M(r) 31000 subunit of V-type H+ ATPase, nor electron microscopic examination detected the presence of H(+) ATPase in placental membranes. In conclusion, the placental brush border membrane is the site of a strong "ecto" ATPase activity which is partially DCCD sensitive.(ABSTRACT TRUNCATED AT 250 WORDS)