Topological studies on the hydrolases bound to the intestinal brush border membrane. II. Interactions of free and bound aminopeptidase with a specific antibody.

The position of the intestinal brush border aminopeptidase with respect to the lipid bilayer has been investigated with the aid of right side out vesicles prepared from the brush border and an immunological technique using an unlabelled or peroxidase-labelled antibody specific for aminopeptidase. The finding that the bound form of the enzyme was almost as readily inhibited and agglutinated as the free form during incubation with the antibody was consistent with the view that the majority of the aminopeptidase surface emerged from the bilayer. This finding was entirely corroborated by the observation that only a few antigenic determinants were not free to react with the antibody in bound aminopeptidase. This immunological technique may be applied to other membrane proteins provided that preparations of the pure proteins and of specific antibodies are available.