Localization by immunofluorescence and histochemical labeling of aminopeptidase N in relation to its biosynthesis in rabbit and pig enterocytes.

Antibodies raised against highly purified rabbit intestinal brush border aminopeptidase N were found in certain rabbits or pigs to crossreact with human blood-grouplike substances present in goblet cells and at the surface of the basolateral membrane of enterocytes. To obtain antibodies strictly specific for aminopeptidase, depletion of antibodies with a water-insoluble, mucus-rich fraction or human A erythrocytes was necessary. By using ultrathin frozen sections of rabbit and pig jejunum mucosa, both the brush border and a structure at the upper pole of the nucleus were labeled by the specific antiaminopeptidase antibodies. This structure, tentatively identified as the Golgi apparatus in which aminopeptidase is glycosylated, escaped detection by histochemistry, thus suggesting that the enzyme is not yet active at this stage of its intracellular processing. By contrast, histochemistry revealed active aminopeptidase molecules in another structure located under the terminal web and in the upper part of the lateral membrane.