Enrichment and partial enzyme characterization of ATPase activity associated with the outward-facing membrane complex and inward-facing membrane of the surface epithelial syncytium of Schistosoma mansoni

The outward-facing (OFM) and inward-facing (IFM) membranes of the surface epithelial syncytium of Schistosoma mansoni were separated by sequential exposure to saponin solutions. The OFM, containing both inner and outer bilayers, contained ATPase activity that was stimulated by Mg²⁺ and Na⁺, but not K⁺ or HCO₃^?, and was inhibited by Ca²⁺ and ethacrynic acid. The OFM enzyme was unaffected by ouabain, oligomycin, SCN^? and azide and had a pH optimum of 7.5. The OFM ATPase therefore has properties similar to ATPases characterized from the apical membrane of a variety of epithelial cells where it is thought to augment the regulatory cell volume decreasing function of (Na⁺+K⁺)Mg²⁺?ATPase. The IFM contained ATPase activity that was stimulated by Mg²⁺, Na⁺ and K⁺, and was inhibited by ouabain indicating that the IFM enzyme was the Na⁺-pump ATPase. The results are discussed in terms of the transepithelial transport function of the surface epithelial syncytium and a Ca²⁺-ATPase reported previously from the OFM of S. mansoni.