Membrane anchors of alkaline phosphatase and trehalase associated with the plasma membrane of larval midgut epithelial cells of the silkworm, Bombyx mori

Abstract

The larval midgut epithelial cell of the silkworm, Bombyx mori, has two forms of alkaline phosphatase and trehalase, soluble and membrane-bound. Alkaline phosphatase and trehalase of the latter form are found in the brush border membrane and the basolateral membrane, respectively. In this work we studied the membrane anchors of these membrane-bound enzymes. Alkaline phosphatase was solubilized by phosphatidyl-inositol-specific phospholipase C, but not by papain. Conversely, trehalase was released from the membrane by papain, but not by phosphatidylinositol-specific phospholipase C. Both enzymes were solubilized in an amphiphilic form with 0.5% Triton X-100 plus 0.5% sodium deoxycholate (pH 7.0). The detergent-solubilized alkaline phosphatase and trehalase were converted to hydrophilic form on incubation with phosphatidylinositol-specific phospholipase C and papain, respectively. The effects of papain on solubilization and conversion of trehalase were completely inhibited by leupeptin. These results suggest that, in the silkworm larvae, alkaline phosphatase is anchored in the brush-border membrane via a glycosyl-phosphatidylinositol, while trehalase is associated with the basolateral membrane through a hydrophobic segment of the polypeptide.