Localization of intestinal sucrase-isomaltase complex on the microvillous membrane by electron microscopy using nonlabeled antibodies

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Abstract
Microvillous vesicles isolated from rabbit small intestine showed a trilaminar membrane with a rather smooth surface, which was apparently not affected by papain which solubilizes sucrase-isomaltase complex, or by trypsin which is unable to solubilize it. When microvillous vesicles or trypsinized ones were incubated with immunoglobulin G against the sucrase-isomaltase complex or monovalent fragments therefrom, an apparently continuous electron-opaque layer .apprx. 180 .ANG. in width appeared around the external surface of vesicles. Such a layer was not formed on papainized vesicles. Microvillous vesicles and trypsinized ones negatively stained with phosphotungstate showed a great number of particles protruding .apprx. 150 .ANG. from the membrane surface, but papainized vesicles did not. The particles existed close to one another and appeared to form a particulate layer 150 .ANG. in width on the surface. The antibodies, whether they were divalent or monovalent, increased the width of the layer to .apprx. 200 .ANG. and obscured the fine particulate structure of intact and trypsinized vesicles. Papainized vesicles retained their smooth surface upon interaction with the antibodies. These results, together with those with the Triton-solubilized sucrase-isomaltase complex, indicate that sucrase-isomaltase complexes are located close to one another on the membrane, and that they or at least their protein portions protrude .apprx. 150 .ANG. from the surface of the trilaminar membrane.